evaluation of soil fertility benefits of hairy vetch ......there is soil fertility decline in maize...
TRANSCRIPT
EVALUATION OF SOIL FERTILITY BENEFITS OF HAIRY VETC H (Vicia
Villosa Roth) IN SMALLHOLDER MAIZE BASED CROPPING SYSTEMS OF
CENTRAL ZIMBABWE
Akinson Tumbure
A Thesis submitted in partial fulfillment of the requirements of the Degree of
Master of Philosophy in Agriculture
Department of Soil Science and Agricultural Engineering
Faculty of Agriculture
University of Zimbabwe
Mt. Pleasant, Harare
September 2015
ii
Abstract
There is soil fertility decline in maize (Zea mays L.)-based smallholder farming areas as a result of insufficient or lack of application of soil ameliorants. While some options can be employed such as manuring, cover cropping, intercropping and planting leguminous trees, these are often limited in terms of adaptability to very poor infertile soils and ability to give high returns on investment to encourage farmer adoption. This study sought to evaluate the potential soil fertility and maize yield benefits of hairy vetch (Vicia villosa L.) when used in rotation or intercropped with maize. Field experiments on maize-legume rotations and relay intercropping were conducted from the 2009/2010 to 2011/2012 cropping seasons in Wedza and Chihota communal areas, and on-station at the Grasslands Research Station in Marondera. Greenhouse and field experiments were also setup during the 2011/2012 season to evaluate the effect of soil acidity and Rhizobium leguminosarum strain on growth and N2 fixation of hairy vetch. Results from the maize-legume rotation experiment showed that when legumes were basal fertilized with 300 kg ha -1 NPK fertilizer (7N : 14 P2O5 : 7 K2O), sunnhemp produced the highest above-ground biomass (7.7 t ha -1) followed by cowpea (5.9 t ha -1) and hairy vetch (2.5 t ha -1) during the first season (2009/2010). Sunnhemp also fixed more N2 (161.5 kg N ha -1) than cowpea (84.7 kg N ha -1) and hairy vetch (25 kg N ha -1). There were no significant differences (P > 0.05) in maize grain yield between basal fertilized maize that followed basal fertilised legumes and maize that was fertilised with ammonium nitrate (34.5% N) and NPK fertiliser. When relay intercropped and provided with 300 kg ha -1 NPK, sunnhemp was also superior in improving maize grain yields by at least 50% compared to the control. Leaching tube incubations revealed that residues of hairy vetch, cowpea and sunnhemp mineralized about 18% more N than when they were mixed with maize residues. They were however, no significant differences in net N mineralized between the sole legumes residues for the 56 day incubation period. The liming and inoculation experiment revealed that there were no significant differences in biomass production of hairy vetch between limed and unlimed (acidic) soil, implying that soil acidity (pH 4.7) was not a significant problem to hairy vetch. The R. leguminosarum strain had a significant effect on biomass production and when hairy vetch was inoculated with the strain MAR833 it fixed up to 66.5 kg N ha -1. Inoculating hairy vetch with the strain MAR833 improved its biomass production to 3.1 t ha -1 compared to the control (0.37 t ha -1). In low soil fertility conditions coupled with erratic rainfall distribution it might be worthwhile to include sunnhemp in maize cropping systems as compared to cowpea and hairy vetch. However the potential of hairy vetch still needs to be assessed with new inoculant strains and more multi-location experiments should be done to identify a strain that can further boost its biomass production.
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DECLARATION
I, Akinson Tumbure (R036288E) declare that this study is a result of my research,
investigations and findings. Sources of information other than my own have been
acknowledged and a reference list has been appended. This work has not been previously
submitted to any other university for any award of any type of academic degree.
Signature:___________________________ Date: 22 September 2015
SUPERVISORS
I declare that I have supervised Mr. TUMBURE AKINSON, student registration number
R036288E, during his MPhil study. I have looked at his thesis and assisted him with
necessary corrections. His thesis is ready for examination.
Date:………………………………………. Signed:………………………………………..
(Dr Menas Wuta)
Date:………………………………………. Signed:………………………………………..
(Dr Farai Mapanda)
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Dedication
This work is dedicated to the poor farmer toiling in the field,
Going unnoticed - the scorch of the sun on his half-covered back,
Nostrils full of dust - as he thrusts his hoe into the hardened earth,
Who despite all odds keeps holding onto the hope of a bumper harvest.
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Acknowledgements
This study was funded by the Regional Universities Forum (RUFORUM) GRANT:
RU/CGS/GRG/30/06/09. I am grateful for the visionary guidance and supervision from
Dr. Menas Wuta and Dr. Farai Mapanda. I would also like to thank the late Dr. David,
K.C. Dhliwayo for his support and encouragement especially at a time when I was
recovering from a traffic accident incurred during my studies. I wish to thank Catherine
Mushangwe, Togarasei Mazengera, Levi Gokova, Anesu Manjengwa, Teki Matapira
(belated), Simba Chipungare and other staff at the Soil Productivity Research Laboratory
for their assistance in on-station field experiments, greenhouse experiments and
laboratory work. Many thanks to the farmers I worked with for letting me use their fields.
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Table of Contents
Abstract ............................................................................................................................... ii
DECLARATION ............................................................................................................... iii
Dedication .......................................................................................................................... iv
Acknowledgements ............................................................................................................. v
LIST OF TABLES .............................................................................................................. x
LIST OF FIGURES ........................................................................................................... xi
LIST OF PLATES ............................................................................................................ xii
LIST OF ABBREVIATIONS .......................................................................................... xiii
CHAPTER 1 ....................................................................................................................... 1
INTRODUCTION .............................................................................................................. 1
1.1 Background ................................................................................................................... 1
1.2 Research problem.......................................................................................................... 3
1.3 Justification ................................................................................................................... 4
1.4 Hypotheses .................................................................................................................... 5
1.5 Specific objectives ........................................................................................................ 5
CHAPTER 2 ....................................................................................................................... 7
LITERATURE REVIEW ................................................................................................... 7
2.1 Soil fertility challenges in communal farming areas .................................................... 7
2.2 Past and present soil fertility management technologies in communal areas ............... 8
2.3 Legume-cereal intercrops and rotations in smallholder agriculture ........................... 11
2.4 Potential applications of hairy vetch ........................................................................... 14
2.5 Cowpea and Sunnhemp use in smallholder areas ....................................................... 16
2.6 Challenges with legume technology in smallholder farming areas ............................ 18
CHAPTER 3 ..................................................................................................................... 20
GENERAL MATERIALS AND METHODS .................................................................. 20
3.1 Study sites ................................................................................................................... 20
3.1.1 Initial soil sampling and preparation........................................................................ 21
3.1.2 Soil Texture determination ...................................................................................... 21
3.1.3 Soil pH determination .............................................................................................. 22
3.1.4 Determination of exchangeable cations in soils ....................................................... 22
3.1.5 Available soil phosphorus determination................................................................. 24
3.1.6 Determination of soil total nitrogen ......................................................................... 25
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3.1.7 Determination of soil mineral N .............................................................................. 26
3.2.1 Determination of legume biomass, maize grain and stover yields. ......................... 27
3.2.2 Initial plant preparation ............................................................................................ 28
3.2.3 Determination of total plant nitrogen ....................................................................... 29
3.2.4 Estimation of N2 fixation of legumes....................................................................... 29
CHAPTER 4 ..................................................................................................................... 31
BIOMASS PRODUCTION AND RESIDUAL N CONTRIBUTION OF SELECTED LEGUMES TO A MAIZE CROP IN A LEGUME-CEREAL ROTATIONAL SYSTEM........................................................................................................................................... 31
4.1 Introduction ................................................................................................................. 31
4.2 Materials and Methods ................................................................................................ 33
4.2.1 Study sites ................................................................................................................ 33
4.2.2 Experimental design, treatments and management .................................................. 34
4.2.3 Statistical analysis .................................................................................................... 39
4.3 Results ......................................................................................................................... 39
4.3.1 Soil fertility status of study sites .............................................................................. 39
4.3.2 Legume biomass production and biological nitrogen fixation (BNF) ..................... 39
4.3.3 Effect of legume rotations on maize grain and stover yield. ................................... 44
4.3.3.1 Maize grain yield .................................................................................................. 44
4.3.3.2 Maize stover yields ............................................................................................... 47
4.3.4 Effect of legume crops (mono-cropped and in cereal rotations) on soil available P and soil N .......................................................................................................................... 48
4.3.4.1 Mono-cropped legumes ........................................................................................ 48
4.3.4.2 Legume-cereal rotations........................................................................................ 49
4.4 Discussion ................................................................................................................... 51
4.5 Conclusions ................................................................................................................. 58
CHAPTER 5 ..................................................................................................................... 59
MAIZE GRAIN YIELD BENEFIT OF LEGUME RELAY INTERCROPS IN LOW FERTILITY SOILS IN ZIMBABWE .............................................................................. 59
5.1 Introduction ................................................................................................................. 59
5.2 Materials and methods ................................................................................................ 61
5.2.1 Study sites ................................................................................................................ 61
5.2.2 Determination of the effect of legume inter-crop N benefit on maize yield ............ 61
5.2.3 Determination of mineralization of sunnhemp, cowpea and hairy vetch residues .. 63
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5.2.3.1 Determination of nitrate N in leachates ................................................................ 65
5.2.3.2 Determination of ammonium N in leachates ........................................................ 65
5.2.3.4 Determination of P in leachates ............................................................................ 66
5.3.4 Statistical analysis .................................................................................................... 68
5.4 Results ......................................................................................................................... 68
5.4.1 Rainfall ..................................................................................................................... 68
5.4.2 Intercropped maize grain and stover yields ............................................................. 68
5.4.3 Intercropped legume biomass yields ........................................................................ 70
5.4.4 Soil available phosphorus and nitrogen after legume intercrops. ............................ 72
Plate 5.1 Biomass production of sunnhemp when intercropped with maize in Wedza communal area during the 2010/11 season. The intercrop: (a) received NPK fertiliser (b) was unfertilised ................................................................................................................. 73
5.4.5 Third season intercropped maize grain N accumulation at GRS ............................. 76
5.4.6 N and P mineralization of legume residues ............................................................. 78
5.5 Discussion ................................................................................................................... 80
5.6 Conclusions ................................................................................................................. 84
CHAPTER 6 ..................................................................................................................... 86
EFFECTIVENESS OF Rhizobium leguminosarum bv. Viceae STRAINS IN NODULATING HAIRY VETCH (Vicia villosa Roth) IN THE SANDY SOILS OF ZIMBABWE ..................................................................................................................... 86
6.2 Introduction ................................................................................................................. 86
6.3 Materials and methods ................................................................................................ 89
6.3.1 Study site and rainfall .............................................................................................. 89
6.3.2 Culture preparation .................................................................................................. 89
6.3.3 Greenhouse experiment ........................................................................................... 91
6.3.4 Field experiment ...................................................................................................... 92
6.3.5 Data analysis ............................................................................................................ 93
6.4 Results ......................................................................................................................... 94
6.4.2 Hairy vetch nodulation and biomass production in the greenhouse ........................ 94
6.4.3 N content of hairy vetch in the greenhouse ............................................................. 95
6.4.4 Hairy vetch nodulation and biomass production in the field ................................... 97
6.4.5 N content and N2 fixation of hairy vetch in the field ............................................... 99
6.5 Discussion ................................................................................................................. 100
6.6 Conclusions ............................................................................................................... 104
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CHAPTER 7 ................................................................................................................... 105
OVERALL DISCUSSION, CONCLUSION AND RECOMMENDATIONS .............. 105
7.1 Introduction ............................................................................................................... 105
7.2 Biomass production of sole legumes ........................................................................ 105
7.3 Benefits of legume rotations and legume relay intercrops to a maize crop .............. 108
7.4 Recommendations ..................................................................................................... 109
7.5 Areas for further research ......................................................................................... 109
REFERENCES ............................................................................................................... 110
APPENDIX ..................................................................................................................... 123
LIST OF PUBLICATIONS AND CONFERENCE PROCEEDINGS FROM THIS STUDY ........................................................................................................................... 123
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LIST OF TABLES
Table 4.1 Treatments (first and second season) for rotation experiments conducted in Wedza, Chihota communal area and at GRS .................................................................... 36 Table 4.2 Physical and chemical characteristics of soils at the legume-cereal rotation study sites. ......................................................................................................................... 39 Table 4.3 Above-ground biomass production of three fertilized and unfertilized legumes at 45, 60 and 75 days after planting across three cropping seasons (2009/10, 2010/11 and 2011/12) in Zimbabwe ...................................................................................................... 40 Table 4.4 Maize stover yields after a legume-cereal rotation during the 2010/2011 and 2011/2012 seasons in Wedza and at Grasslands Research Station (GRS). ...................... 47 Table 4.5 Available soil P and total N after legume mono-cropping in Wedza, Chiota and GRS during the 2010/11 and 2011/12 seasons. ................................................................ 48 Table 4.6 Available soil P, mineral N and total N after basal fertilised and unfertilised legume-cereal rotations over two seasons (2010/11 and 2011/12). .................................. 50
Table 5.1 Maize grain and stover yields at Wedza, Chihota and Grasslands Research Station (GRS) between the 2009/10 and 2011/2012 seasons. .......................................... 71 Table 5.2 Biomass production of legumes relay intercropped into maize in Wedza, Chiota and at Grasslands Research Station during the 2009/2010 and 2010/2011 seasons........................................................................................................................................... 72 Table 5.3 Soil available P and total N after maize/legume relay intercrops in Wedza, Chihota and at Grasslands Research Station (GRS) during the 2009/10 to 2011/12 seasons. ............................................................................................................................. 75 Table 5.4 Soil mineral N after a maize-legume intercrop in Wedza, Chihota and at Grasslands Research Station (GRS).................................................................................. 76 Table 6.1 Nodule weight per unit root mass and biomass production of hairy vetch from the greenhouse pots in acidic and limed soil at 80 days after planting at the Grassland Research Station................................................................................................................ 94 Table 6.2 Nodule counts per plant, nodule weight per plant and aboveground biomass of hairy vetch in the field at Grasslands Research Station (GRS) in Marondera at 80 days after planting ..................................................................................................................... 98
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LIST OF FIGURES
Figure 4.1 Rainfall distribution at Grasslands Research Station and at Wedza and Chihota communal areas. .................................................................................................. 38 Figure 4.2 Nitrogen uptake of legumes grown during the 2009/2010 season at (a) GRS, in (b) Chihota and (c) Wedza. ........................................................................................... 43 Figure 4.3 N2 fixed by legumes during the 2009/2010 season at (a) GRS (b) Chihota and (c) Wedza. ......................................................................................................................... 43 Figure 4.4 Maize grain yields after crop rotations during the 2010/2011 season at (a) Wedza and (b) GRS sites. ................................................................................................. 45 Figure 4.5 Maize grain yields during the 2011/2012 season after crop rotations at Marondera (GRS).............................................................................................................. 46 Figure 5.1 N content of maize seed at GRS in the 2011/2012 season.. ........................... 77 Figure 5.2 Grain N uptake of maize at GRS in the 2011/2012 season. ........................... 78 Figure 5.3 Net N mineralization patterns of cowpea, sunnhemp and hairy vetch residues when incubated solely and in combination with maize residues. ..................................... 79 Figure 5.4 Net P mineralisatiom patterns of cowpea, sunnhemp and hairy vetch residues when incubated soley and in combination with maize residues…………………………81 Figure 6.1 Daily rainfall distribution and the times of crop planting and harvesting during 2011/2012 cropping season (1 October 2011 to 5 May 2012) at the experimental site at Grasslands Research Station in Marondera……………………………………………...89 Figure 6.2 Total N content of hairy vetch (a) above ground parts and (b) belowground parts (roots) grown on acidic soil (pH 4.7) and limed soil (pH 6.5) in the greenhouse at 80 DAP................................................................................................................................... 97 Figure 6.3 Above-ground dry mass of hairy vetch grown in the field at 120 DAP. ........ 98 Figure 6.4 Total N concentration of hairy vetch (above-ground parts) grown in the field at GRS and harvested at 120 D.A.P. ................................................................................. 99 Figure 6.5 N2 fixed by hairy vetch when grown in an acidic sandy soil at GRS, Marondera. ...................................................................................................................... 100
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LIST OF PLATES
Plate 4.1 a and b Growth of hairy vetch (HV), sunnhemp (SH) and cowpea (CP) in Marondera at 53 DAP. Unfertilised and uninoculated hairy vetch shows stunted growth and severe purpling of leaves and stems ........................................................................... 42 Plate 5.1 Biomass production of sunnhemp when intercropped with maize in Wedza communal area during the 2010/11 season. The intercrop: (a) received NPK fertiliser (b) was unfertilised ................................................................................................................. 73
Plate 6.1 Un-inoculated hairy vetch in the greenhouse with yellow and purple colouration suggesting critical deficiencies of nitrogen and phosphorus. ........................................... 96
Plate 7.1 Hairy vetch still green and flowering in June 2011 (at 170 DAP) in Marondera after cowpea, sunnhemp and maize have reached physical maturity and dried up. ....... 107
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LIST OF ABBREVIATIONS
DAP Days after planting GRS Grasslands Research Station SPRL Soil Productivity Research Laboratory FAO Food and Agricultural Organisation of the United Nations SOC Soil Organic Carbon LSD Least Square Differences SEM Standard Error of Mean ANOVA Analysis of Variance MZ Maize SH Sunnhemp CP Cowpea AN Ammonium Nitrate NPK / D Compound D basal fertiliser (7 N: 14 P2O5:7 K2O) USDA United States Department of Agriculture YEM Yeast Extract Mannitol nifTAL nitrogen fixation by Tropical Agricultural Legumes center, University of Hawaii RCBD Randomised Complete Block Design
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CHAPTER 1
INTRODUCTION
1.1 Background
Soil fertility decline is a major challenge to the Zimbabwean crop production system.
Rampant nutrient mining in low external input maize-based cropping systems
exacerbates the already inherently low soil fertility (Mtambanengwe and Mapfumo
2006). Some studies have shown that maize fails to yield anything without fertiliser
addition on some depleted soils in communal areas (Kumwenda et al. 1995). All the
effort that communal farmers put in to produce maize for their subsistence has
generally been rewarded by low grain yields that compound their poverty and forces
them to work for the few better resource endowed farmers. Neglecting their fields
means a continual progression of poverty and with seemingly few resources to
improve fertility of their soil, the poor households remain trapped. This situation
would however vary with agro-ecological regions with the worst affected farmers
being found in the dry regions.
Possible solutions to soil fertility decline have to take into consideration factors such
as lack of adequate nutrient resources, limited soil moisture and shortage of labour.
Numerous Zimbabwean researchers have identified these factors as worsening the low
levels of production (Mapfumo et al. 2005; Zingore et al. 2009). With this in mind,
Mapfumo et al. (2005) pointed out that only mineral fertilisers and N2-fixing legumes
offer a realistic chance for raising agricultural production in Zimbabwe. However,
farmers have limited opportunities for increasing availability of resources and mineral
fertiliser use remains on an average, very low because smallholder farmers have low
disposable incomes to purchase fertilisers (Sanchez 2002). Consequently, the
2
cultivation of cereals in smallholder low-input cropping systems requires the
development of sustainable farming systems that increasingly rely on fixed nitrogen
input from legumes (Hardarson and Atkins 2003; Rusinamhodzi et al. 2006; Mahieu
et al. 2009). The use of crops capable of symbiotic nitrogen (N2) fixation is a
reasonable way of addressing agricultural sustainability given that most smallholder
farmers are less resource endowed (Vance et al. 2000).
Nitrogen is a key nutrient limiting cereal production in Zimbabwe (Tanner and
Mugwira 1984) and research efforts have tried to look at ways of improving soil N
availability to crops. Legumes have taken centre stage with more research looking at
grain legumes because besides the potential of soil improvement they also provide an
immediate grain benefit to the farmer. Soil N enrichment after growing legumes is
attributed to residual effects on soil fertility because of legume decomposition and N
mineralization (Kasasa et al. 1999). However, some studies have also shown that most
high yielding grain legumes such as soyabean have high nitrogen harvest indexes and
are net removers of soil N (Toomsan et al. 1995). The case is different with legumes
grown specifically for soil fertility improvement where generally little is harvested
and removed from the field; such systems add the largest amount of N to the soil
(Vanlauwe and Giller 2006). On the other hand, researchers attribute agro-forestry
technologies to be labour intensive and management sensitive (Kumwenda et al.
1995). Some researchers have also noted that competition between the trees and
associated crops can occur for moisture, nutrients and light (Ong 1994). Therefore
screening for more suitable legumes is required to gain advantages from legumes, for
soil conservation and N2 fixation (Shobeiri et al. 2010).
3
Hairy vetch (Vicia villosa Roth) is an annual leguminous crop, which produces high-
quality forage and can be cultivated in most climates rainfed or irrigated (Shobeiri et
al. 2010). Hairy vetch is the most commonly used cover crop in the United States,
because it is widely adapted and forms a dense groundcover (Baldwin and Creamer
1999). In a study done in Iran (Shobeiri et al. 2010), sole hairy vetch produced dry
mass yields of up to 6.14 t ha-1 and in a subtropical region in Japan it yielded 4.47 t
ha-1 (Anugroho and Kitou 2011). Benefits of growing hairy vetch include weed
control, provision of significant amounts of N, as a mulch to retard soil moisture loss
(Sadeghi and Insensee 2001) and to reduce surface runoff and soil erosion (Czapar et
al. 2002). The potential of hairy vetch for use in a tropical climate like Zimbabwe is
high since it is drought tolerant and is adapted to sandy soils. In the Americas,
nitrogen fixed by hairy vetch under field conditions is about 110 kg N ha-1 yr-1
(Burton 1984).
Various legumes have been researched for their soil fertility benefits in Zimbabwean
soils but only a few of them are adapted to poor soil fertility conditions and dry
regions. These include Acacia angustissima, Sesbania sesban (Nyamadzawo et al.
2008), Mucuna pruriens, Cajanus cajan, Crotalaria paulina (Chikowo et al. 2004c),
Vigna unguiculata, (Ncube et al. 2009) and Crotalaria juncea L. (Jeranyama et al.
1998). However, there is limited research on the benefits of hairy vetch (Vicia villosa
Roth) in maize based systems of Zimbabwe.
1.2 Research problem
While it is known that biological nitrogen fixation (BNF) under legume cropping can
improve productivity of the subsequent maize crop in Zimbabwean smallholder
4
farming areas, possible contributions are limited by the soil, rainfall, plant varieties,
lack of appropriate rhizobial inoculations and limited use of inorganic fertilisers.
Communal smallholder farming areas are characterised by acidic and infertile sandy
soils and are subject to erratic rainfall and prolonged mid-season droughts. Legumes
that are suited to such areas with multiple constraints are limited. Hairy vetch might
be better suited than other legumes to contribute meaningful amounts of fixed N2 to
cereal cropping systems due to its low soil nutrient demand and its adaptation to
acidic soils and low rainfall conditions.
1.3 Justification
Hairy vetch is adapted to sandy soils and has been shown to produce considerable
biomass in temperate and subtropical climates (Shobeiri et al. 2010; Anugroho and
Kitou 2011). Due to its growth habit, hairy vetch can cover the ground leading to
restricted weed growth and soil moisture conservation. This attribute would make it
more suitable for resource constrained farmers because once it is established the
second weeding can be foregone. In maize intercrops this would mean reduced
weeding labour requirements.
While the legume has great potential to contribute considerable amounts of fixed N2 it
is also drought tolerant and can be a better an option to include in cereal cropping in
light of climate change in places where rainfall becomes more erratic and mid-season
droughts are prolonged. Hairy vetch has a low harvest index (ratio of seed weight to
biomass) of approximately 24 % (Kendir 1999), as a result it has potential to
contribute more fixed N2 (110 kg N ha -1 (Burton 1984)) to maize based cropping
systems compared to legumes such as cowpea that partition more N to the grain which
5
is then harvested for human consumption. This will result in improvement of maize
yields and food security in smallholder farming areas. Improvement of the soil
fertility status will also result in yield benefits of other crops grown by smallholder
farmers and thereby improve farm income.
However information about hairy vetch’s performance in tropical climates, especially
in Zimbabwe is limited. There is therefore a need to field test hairy vetch against
some grain and forage legumes for soil fertility benefits to a maize crop under specific
biophysical limitations common in smallholder farming areas.
1.4 Hypotheses
It was hypothesised that hairy vetch has higher biomass production compared to
cowpea and sunnhemp on impoverished low fertility soils and as result has greater N
contributions in maize cropping systems.
The specific hypotheses of the study were;
1. Hairy vetch has higher plant biomass production potential on granitic sandy
soils and contributes significantly greater amounts of fixed nitrogen towards a
succeeding maize crop compared to cowpea and sunnhemp.
2. Hairy vetch residues decompose and release nutrients (N and P) faster than
sunnhemp and cowpea residues.
1.5 Specific objectives
The overall objective of this study was to evaluate the potential of hairy vetch (Vicia
villosa Roth) in terms of its nitrogen fixation, biomass production and yield effects on
maize when rotated or intercropped. The specific objectives were;
6
1. To determine above-ground biomass production of hairy vetch (Vicia villosa
Roth) on granitic sandy soils and compare it with cowpea (Vigna unguiculata)
and sunnhemp (Crotalaria juncea L.)
2. To determine the residual N contribution of hairy vetch to a succeeding maize
crop in comparison with the N contribution of cowpea and sunnhemp.
3. To determine the effect of hairy vetch intercropping on maize yields and
compare it with cowpea and sunnhemp.
4. To determine the N and P mineralization pattern of hairy vetch, cowpea and
sunnhemp residues.
7
CHAPTER 2
LITERATURE REVIEW
2.1 Soil fertility challenges in communal farming areas
Smallholder farming areas in Zimbabwe are located in low rainfall areas and on sandy
soils of granite origin that constitutes about two thirds of the country’s soils
(Nyamapfene 1991). The soils contain mainly the 1:1 lattice clay minerals, are acidic
and have a low cation exchange capacity. Pioneering research in the early 1970s
showed that multiple macro-nutrient deficiencies of nitrogen (N), phosphorus (P),
sulphur (S), magnesium (Mg) and of micronutrients such as zinc (Zn), boron (B) were
common (Grant 1970). Saunder and Grant (1962) described these soils as difficult to
crop with sustained productivity suggesting that they can only be cultivated for a
limited period and be put under grass or ley systems to maintain their fertility. Recent
research has also supported these early findings (Mtambanengwe and Mapfumo 2008,
Mapfumo 2011).
Soils provide the sustenance for communal households and hence they have been in
constant low external input cereal cultivation. Kumwenda et al. (1995) and
Waddington et al. (2004) reported that in up-land fields, soil nutrient levels are now
so low that maize fails to yield anything without fertiliser addition in some northern
parts of Zimbabwe. Low pH is also an important constraint in these soils (Dhliwayo et
al. 1998) and early research findings showed that liming is necessary to increase the
availability of molybdate and phosphate (Tanner and Grant 1977; Tanner 1978;
1982).
8
Communal farmers have tried to use various mineral and organic fertilisers in-light of
worsening infertility of the granite derived sands. However, use of mineral fertilisers
is minimal (one-fifth of total fertiliser demand) because of various factors which
include economic, political, technical and institutional factors (FAO 2006).
Communal farmers have limited access to mineral fertilisers and have limited
disposable income to purchase fertilisers (Murray 1997; FAO 2006). As a result,
mineral fertilisers are often purchased in amounts inadequate to replace those
nutrients lost annually in harvested produce (Mtambanengwe and Mapfumo 2003).
Waddington et al. (2004) showed that nutrient depletion rates far exceeded
replenishment in communal areas by nutrient balance studies conducted from single
farm to national scales. In fact soil fertility is on the decline in communal areas
(Smaling 1993; Kumwenda 1995; Mafongoya et al. 2006). It is therefore imperative
that soil amelioration options be considered that can be easily adopted by farmers
given that they are resource limited.
2.2 Past and present soil fertility management technologies in communal areas
Communal farmers are aware that soil fertility is declining as shown in a study by
Cobo et al. (2009) where 50 – 80% of communal farmers, independent of wealth
class, perceived that soil fertility has declined. Communal farmers, having noticed
that maize yields were declining have been innovative in trying to address this
problem. This has led to the use of various traditional and relatively new soil fertility
management technologies in communal areas. To various degrees, communal farmers
use mineral fertilisers, animal manure, composts, leaf litter, incorporation of legumes
into cropping systems and grass fallowing (Nhamo et al. 2003). Whatever technology
is used and to what extent farmers use it, is generally determined by availability of
9
local resources (Mtambanengwe and Mapfumo 2003). From a survey done by Chuma
et al. (2000) it was observed that winter ploughing is practiced by 82% of the
households in Mangwende and 68% in Chivi, most of these having access to draught
power. The same authors highlighted that very few farmers in either area rotated their
crops because of shortage of land and that cereals predominated.
One of the most preferred traditional methods that of fallowing land is now
impracticable because of high land pressure resulting in very short fallowing periods
usually with grasses that do not significantly improve soil fertility (Nezomba et al.
2007). Chuma et al (2000) showed that fallowing was used by 35% of farmers in
Mangwende and 37% in Chivi, and explains that it is often the farmers’ last resort and
it is usually the result of lack of inputs such as labour, draught and fertilizers, and not
a deliberate way of improving soil fertility.
Livestock manure is another traditional source of plant nutrients and can be one of the
cheapest sources of organic fertiliser in smallholder communities. However this is
only an option for the few farmers with cattle (Mtambanengwe and Mapfumo 2003).
The Zimbabwean government has promoted policies that encourage food production
through programs such as the Agricultural Sector Productivity Enhancement Facility
(ASPEF) that aims to rebuild the national herd (Mudzonga and Chigwada 2009).
Other policy interventiontions being currently pursued include the support of
programs that provide sufficient long term concessionary finance for beef producers
to purchase breeding stock and other beef husbandry products (Mavedzenge et al.
2006). The effects of such policies are however yet to translate to increases in
livestock manure that can be used in farmers’ fields for crop production. It should
10
also be noted that the quantities of useful manure that can be obtained are highly
dependent on factors such as how much the cattle are fed, pen rearing, penning the
animals at night or free range and the manure collection efficiency (FAO 2006; Wuta
and Nyamungafata 2012). The manure quality or composition is also of major
concern as this determines the amount of nutrients that will be added to the soil.
Pioneering research on cattle manure use in communal areas by Tanner and Mugwira
(1984) showed that cattle manure used in communal areas usually had a low N
content that ranged from 0.5 to 1.4 percent of dry matter. The authors cite the poor
quality of grazing in many communal areas coupled with low-fertility inputs into the
cropping areas as major causes of low nutrient content of manure.
As Mugwira and Murwira (1997) explained, the effectiveness of cattle manure is also
limited by lack of a peak N release period during mineralization and prolonged
immobilization, which makes it difficult to synchronize crop demand and N release.
Various approaches have been suggested to improve manure quality such as planting
legume pastures, appropriate handling of manures while in cattle pens to minimise
nutrient loss, storing manure in covered and plastic lined pits and anaerobic treatment
of the manure. However, these approaches require more labour, resources and
technical knowhow which the communal farmer seldom has. Therefore manure
applied alone produces low crop yields and needs supplementing with other nutrient
sources such as inorganic fertilisers for optimal yields (Kumwenda et al. 1995; FAO
2006).
Other nutrient sources such as composts are required in large quantities that are not
achievable given that the farmers concerned are less resource endowed when it comes
11
to livestock, with some farmers not having any cattle at all. The collection of leaf
litter is labour intensive for the poor farmer with no cattle for transportation also
making this option impracticable and unsustainable. Severe deforestation in
communal areas has also rendered the collection of woodland litter unfeasible
(Mtambanengwe and Mapfumo 2003). On the other hand legume trees are rarely
worth including simply for their effect on soil fertility and farmers will only willingly
include these if they have multi-purposes such as fruit, fuel-wood or timber (Rowe
and Giller 2003).
Snapp et al. (1998) reported that adoption of organic matter technologies by
communal farmers has been nonexistent because of constraints such as high labour
requirements, limited income and the imperative need to provide immediate calorie
production. For every technology that shows potential in improving soils, a varied set
of constraints the smallholder faces forms the decision making factor. As mineral
fertilisers and organic residues are often not available or affordable in sufficient
quantities or qualities to be used alone, integrated soil fertility management (ISFM) is
currently promoted as a management approach that optimizes the use of all available
resources within each target environment (Gentile et al. 2009). This approach is very
promising and when leguminous trees and plants that fix nitrogen are included with
minimal addition of basal fertilisers the yield benefits to cereal crops can be quite
substantial.
2.3 Legume-cereal intercrops and rotations in smallholder agriculture
Legumes constitute an important protein and food source throughout the world
(Wollum 1998) supplementing or supplanting expensive meat protein (Carr et al.
12
1998). In Zimbabwe, legumes are often rotated or intercropped with cereals and
mixed cropping is referred to as one of the traditional agricultural practices in
Zimbabwe (Reid 1977; Foti et al. 2003). Shumba et al. (1990) notes that intercropping
maize with non-cereal crops such as pumpkins or cowpea randomly planted at low
plant densities could still be seen in the 1990s’ despite discouragement by government
agencies in pre-Independence Zimbabwe. In assessing legume intercrops in the
communal areas Nhamo et al. (2003) noted that the fertiliser types and amounts on the
maize-cowpea intercrop were the same with ones for a sole maize intercrop. However,
both basal and top dressing fertilisers were in some cases applied in very limited
amounts at rates of less than 50 kg ha -1.
The inclusion of a legume affords greater yield stability in sub-optimal growth
conditions such as poor soil fertility and droughts and legumes benefit companion or
the following non-leguminous crops (Sanginga 2003). Communal farmers intercrop to
ensure food production by avoiding risk of single crop failure, and thereby effectively
utilise limited land and labour (Mucheru-Muna et al. 2010). Biological nitrogen
fixation through legume inclusion in cereal-based cropping systems is an efficient
source of fixed N2, which plays an important role in soil fertility restoration (Zahran
2001).
Yield increases of crops planted after harvesting of legumes are often equivalent to
those expected from an application of 30 – 80 kg of N ha-1 (Zahran 1999). Mucheru-
Muna et al. (2010) explains that when crops have mutually exclusive growth patterns
such as different above-ground canopies and rooting systems, and are complementary
in their water and nutrient demands, intercropping results in a more efficient
13
utilisation of available resources (sunlight, moisture and soil nutrients). This can
result in higher yields than when the crops are monocropped. Results of an
experiment by Pypers et al. (2007) confirmed that the introduction of a legume
supplied with phosphate rock into a maize-based cropping system increases yield and
P-uptake by a subsequent maize crop, compared to maize following a first maize crop
supplied with phosphate rock. He also suggests that the legume in the rotation system
has other positive, possibly soil-microbiological effects which enhance maize growth
and production. Other benefits of legumes, which should not be ignored, include
reduced pest and weed occurrence and improved soil quality (Van Kessel and Hartley
2000).
Vanlauwe and Giller (2006) explained that there were usually large grain legume N-
harvest indices and only creeping varieties of groundnut (Arachis hypogaea) and
cowpea (Vigna unguiculata) tend to have low harvest indices for N. High yielding
varieties of soyabean (Glycine max) usually have high N-harvest indices and often are
net removers of soil N. In a study by Jeranyama et al. (1998) it was shown that
cowpea and sunnhemp relay-intercropped into maize replaced up to 36 kg N ha-1 of
the N requirements in the subsequent maize crop. Results from Domboshava showed
that when maize without inorganic fertiliser was rotated with groundnut the maize
grain yield doubled from 2.46 t ha -1 to 4.61 t ha -1 (Waddington et al. 1998). The
groundnut in that study was also not fertilised and most of the groundnut haulms and
maize stover were grazed by animals. Waddington et al. (1998) attributed this to leaf
fall and the root system of groundnut. The sparing of soil N by the legumes as they
acquire most of their N requirements from fixation in the preceding season is also
14
another factor contributing to higher maize dry matter and grain yields compared with
the control plots (Kasasa et al. 1999).
2.4 Potential applications of hairy vetch
Hairy vetch is native to the temperate zones of Europe and west Asia (Undersander et
al. 1990). It is often suggested as a preferred legume cover crop to supply nitrogen
and suppress weeds in a maize production system in these areas (Czapar et al. 2002).
Hairy vetch is a straggling, climbing, prostrate plant that can grow as an annual,
biennial, or perennial herb (Jost 1998). It is tolerant to cold, drought and extreme soil
pH (4.9-8.2), but grows best when the pH ranges from 6.0 - 7.0. It is more tolerant to
acid soil conditions than most legume crops (Henson and Schoth 1955). Hairy vetch
does well without any N additions if it is provided with supplemental phosphorus and
sulfur on sandy and sandy loam soil with good drainage, even in infertile soil.
Hairy vetch is used for improving soil fertility. In temperate climates hairy vetch is
generally considered to be the best of the legume winter cover crops for nitrogen
production (Jost 1998). It is considered superior because of its ability to be productive
in low soil fertility or acid soils (Dastikaite et al. 2009). In a 6-year cotton-based study
in Australia, Rochester and Peoples (2005) found that hairy vetch added 33% more N
to the soil than faba beans (Vicia faba minor) and increased the crop yield potential by
up to 13% relative to the comparative non-legume system. Hairy vetch is also said to
facilitate the availability of potassium to other, shallower-rooted, crops (Undersander
et al. 1990). Hairy vetch nodulates with Rhizobium leguminosarum strains that
nodulate pea (Pisum Sativum L).
15
Hairy vetch protein content ranges from 12 to 20 % (2.5 - 3.5 % N) depending on the
stage of development and makes good hay, silage, pasture and green manure (Henson
and Schoth, 1955). Recent studies by Lanyasunya et al. (2007) in Kenya showed that
hairy vetch has adequate mineral content for dairy and beef cattle except for Cu and
Mg. The same authors attributed the Mg and Cu deficiencies to the high soil pH (7.4)
of the soil in which hairy vetch was grown in Kenya. Hairy vetch can reduce weed
infestations by forming a dense groundcover in fields when planted as a cover crop
(Baldwin and Creamer 1999). In a study by Lybecker et al. (1988) in temperate
America, hairy vetch cover crops reduced weed biomass by 96% and its residues have
been shown to inhibit the establishment of the weed common lambsquarter
(Chenopodium album L.) without affecting maize establishment. Common
lambsquarter is found in Zimbabwean farming areas (Holm et al. (1997)
Other studies by Teasdale and Daughtry (1993) in America where hairy vetch has
been successfully domesticated showed that hairy vetch reduced weed density by 70-
78% and reduced weed biomass by 52 – 70% compared with a fallow treatment. The
authors showed that an average of 87% of sites beneath the cover crop received < 1%
of unobstructed sunlight and decreased the red (660 nm) to near infra-red (730 nm)
ratio of transmitted light by 70%. Reductions in weed density through growing hairy
vetch can be attributed to light extinction and changes in light quality and soil thermal
regime (Liebman and Davis 1999). Though this was under temperate conditions the
potential of hairy vetch in tropical areas is high due to its ability to adapt to various
environments.
16
In order to obtain high biomass hence soil fertility benefits from growing hairy vetch,
it must be properly nodulated by an efficient R. leguminosarum strain for N2 fixation.
Many failures with vetch are directly attributable to lack of inoculation (Henson and
Schoth 1955). In a study done in subtropical Iran and Japan, sole hairy vetch produced
dry mass yields of up to 6.14 t ha -1 (Shobeiri et al. 2010) and 4.47 t ha -1 respectively
(Anugroho and Kitou 2011). The hairy vetch in Iran was supplied with 20 kg P ha -1
and 23 kg N ha −1 and the hairy vetch in Japan was not fertilized but the soil in which
the hairy vetch was grown in Japan was fertile (Total C = 11.6 g kg -1, Total N = 1.3 g
kg -1, Total P = 32.1 g kg -1, mineral N = 8 mg kg -1). In tropical Kenya when
irrigated, hairy vetch yielded dry matter yields of yielded 9.5 t ha -1 (Lanyasunya et al.
2007). Hairy vetch therefore, shows great potential in improving cereal yields through
N2 fixation and weed suppression in tropical systems.
2.5 Cowpea and Sunnhemp use in smallholder areas
Cowpea (Vigna unguiculata Walp.) is a multi-purpose tropical legume and in
Zimbabwe it is used as a vegetable (leaves and flowers), grain, fresh forage, hay and
silage (Mapiye et al. 2007). Woodend (1995) describes cowpea as one of the most
important drought tolerant legumes grown in Zimbabwe. It is considered a cheap
legume to grow because its fertility and rainfall demands are low and it has the
advantage of a deep rooting system that makes it adaptable to different agro-
ecological zones (Nhamo et al. 2003). Mapiye et al. (2007) reported that cowpea was
successfully being grown by some smallholder farmers of Zimbabwe as a dairy feed
supplement. These dairy farmers produce it over an estimated area of about 2 000
hectares (Woodend 1995).
17
Cowpea is traditionally intercropped with cereals such as millet, sorghum and maize
(Woodend 1995) and survey results by Nhamo et al. (2003) in Chihota, Zimuto and
Shurugwi communal areas revealed that cowpea was mostly grown on homestead
fields. Cowpea can produce 3 - 10 t ha -1 dry mass in 8 - 12 weeks with grain
production of 250 - 4000 kg ha -1 (Mapiye et al. 2007). Cowpea has high nutritive
value and serves as an important source of vegetable protein in the diets of communal
farmers with the boiled beans as a side dish and relish and fresh leaves as vegetables
(Woodend 1995; Nhamo et al. 2003; Mapiye et al. 2007).
Sunnhemp (C. Juncea L.) is a tropical forage legume that has an upright, bushy
growth. It has been researched on in the smallholder areas but adoption is limited by
shortage of seed and farmers are reluctant to plant non-food crops (Steiner 2002). It
produces considerable biomass of around 5.1 t ha -1 (Ngongoni et al. 2007) and like
cowpea fixes atmospheric N. Sunnhemp is used as a pasture legume and for soil
fertility improvement in some smallholder farming areas of Zimbabwe (Ngongoni et
al. 2007). It is drought tolerant, fast growing and is less demanding on soil fertility
than cowpea (Miller et al. 1989).
Studies done by Jeranyama et al. (1998) showed that sunnhemp and cowpea relay
intercropped into maize at 28 days after planting and basal fertilised produced dry
masses of up to 2.03 and 3.1 t ha -1, respectively. In that study sunnhemp added 55 kg
N ha -1 and cowpea added 111 kg N ha -1. However in the same study it is revealed
that relay intercropping of the legumes in unfertilised maize reduced maize grain yield
by 7 - 12% and total above ground maize biomass by 8%. This was attributed to
competition for mainly water and nutrients between the legumes and the maize crop.
18
Shumba et al. (1990) also found that cowpea-maize intercrops can greatly reduce the
grain yield of maize in dry areas and thus plant densities need to be reduced. In wetter
areas however, the yield benefits of intercropping cowpea are significant as shown by
Mupangwa et al. (2003).
In a study done in Wedza (Zimbabwean agro-ecological region II) by Mupangwa et
al. (2003) maize-cowpea intercropping had higher maize yield (4.64 t ha -1) compared
with maize monocropping (2.27 t ha -1) during the 2000/01 season. The same authors
report that in natural region III neither intercropped nor sole cowpea, lablab and
velvet bean had any significant effect on maize yields. In another study by Perin et al.
(2006) in tropical Brazil, maize cropped after a sunnhemp monocrop had in the above
ground biomass 25 kg ha -1 of N derived from BNF, corresponding to 15% of the
initial 174 kg ha -1 of fixed nitrogen present in the sunnhemp aboveground biomass. In
that study, it was reported that in the presence of N-fertiliser, sunnhemp intercropped
with millet (Pennisetum glaucum L.) resulted in maize grain yields 56 and 67% higher
than maize crop grown after weeds and millet, respectively.
2.6 Challenges with legume technology in smallholder farming areas
Legumes contribute far less than their potential amounts because there is under-
utilisation as a result of poor market development of legume products and because the
legumes are grown on very poor soils where growth and N2 fixation are limited and
perceived benefits are small (Rowe and Giller 2003). Nhamo et al. (2003) attributes
little use of legumes for soil fertility management to the low perception about minor
crops, little biomass from the small areas planted, seed availability problems, lack of
information on production and soil fertility benefits. On the other hand bio-physical
19
constraints such as low soil fertility and drought compound to form formidable
challenges. Chivenge et al. (2003) indicated that the biophysical conditions under
which different legumes perform need to be ascertained.
The soil environment presents some challenges towards a legume-Rhizobium
symbiotic association. Production constraints arise from soil acidity, aluminium
toxicity, high phosphorus fixation and low nutrient reserves or other combination of
factors (Wollum 1998). Acidity is very common in Zimbabwean communal areas
since the soils are granite derived, highly weathered and leached (Hungria and Vargas
2000). Legumes often fail to nodulate in soils of pH < 5.0. Legumes are also slower to
form nodules when phosphorus is limiting and P is the most limiting nutrient after N
in the predominantly granite-derived soils of Zimbabwe (Mapfumo et al. 2005).
Effects of low pH can be mitigated by incorporation of organic matter and liming the
soil to increase Ca levels in the soil.
High soil temperatures also affect N2-fixation by reducing nodulation and the
rhizobium count in the soil. This can be mitigated through mulching. Soil moisture
aspects also influence BNF. Moisture relationships are affected by soil texture,
aggregation, organic matter content and to a lesser extent, soil fertility relationships
(Wollum 1998). There is also a need to breed plant cultivars that nodulate well and fix
N2 effectively. Indigenous rhizobia bacteria that are sometimes less effective in N2
fixation may compete with the more effective inoculums for nodulation and reduce N2
fixation. However even the best inoculants will not perform satisfactorily if they are
improperly handled (Wollum 1998). Lack of knowledge on how to use inoculants
properly may adversely affect the potential benefits of inoculation.
20
CHAPTER 3
GENERAL MATERIALS AND METHODS
3.1 Study sites
The study was conducted over 2 cropping seasons, i.e. 2009/2010 and 2010/2011 in
Wedza (31o30’E, 18o46’S) and Chihota (31o05’E, 18o11’S) smallholder farming
areas, and on-station at Grasslands Research Station (GRS) (18o10’E, 31o29’S) in
Marondera. At GRS, experiments were continued through the 2011/2012 season. The
Chihota site had been under maize in 2008/2009 and groundnut in 2007/2008 and in
Wedza the site had been under maize monocropping with a history of manure
application. Seven years before cropping was done, the Wedza site had been a brick
burning furnace. The site at GRS had been a natural fallow for five years.
At all sites rainfall follows a unimodal pattern with rain falling between November
and March. The soils at all sites are granite-derived sands to loamy sands typical of
the majority of smallholder farming areas. These soils show catenary association with
upper slopes comprising of moderately well-drained sands with a depth greater than
0.5 m passing down to a vlei edge, yellow over very pale brown coarse sand (>1 m)
and then into usually medium textured imperfectly to poorly drained vlei soils
(Anderson et al. 1993). The soils at all sites are classified as fersiallitic using the
Zimbabwean soil classification system (Anderson et al. 1993).
Sites selected in Wedza (Ushe ward) and Chihota (Chiwanzamarara) fall in Agro-
ecological Region III and IIa of Zimbabwe respectively and were selected because
they show a contrast in agricultural production potential. Total annual rainfall ranges
21
from 650 to 750 mm yr-1 in Wedza and is greater than 800 mm yr-1 in Chihota and
Marondera.
3.1.1 Initial soil sampling and preparation
Soils were sampled in November 2009 from maize fields selected for the field
experiments in Wedza, Chiota and at Grasslands Research Station (GRS) using augers
to a 20 cm depth. Five sub-samples were obtained from random points within each
intended experimental area and thoroughly mixed in a clean bucket to make a
composite sample. Approximately 500 g was extracted from the composite sample for
laboratory analysis. Samples were then air dried in wooden trays in an area without
direct sunlight, crushed and sieved through a 2 mm sieve and packaged in standard
laboratory soil cartons. Packaged soil samples were then characterised for texture, pH,
exchangeable bases, available P and mineral N.
3.1.2 Soil Texture determination
A 600 ml beaker was placed on a digital balance and 50 g of air dried soil was
measured into it using a clean spatula. About 100 ml of 5% sodium
hexametaphosphate (calgon) was then added and the mixture was stirred with a clean
glass rod and left to stand overnight. After standing overnight, the mixture was
transferred into a dispersing cup of an electric mixter and stirred for 5 minutes. The
mixture was then transferred to a 1 L graduated cylinder and brought to the 1 L mark
by adding de-ionised water. The suspension was stirred thoroughly using a weighted
disc shaped bar for 1 minute. A hydrometer and thermometer were immediately
placed into the cylinder and the hydrometer and temperature reading were recorded at
40 seconds after stirring. The suspension was left to stand for 2 hours and the
22
hydrometer reading and temperature were noted. A blank sample was analysed in the
same manner using 100 ml of 5% sodium hexametaphosphate (calgon) solution
diluted to 1000 ml in a measuring cylinder. The relative percentages of sand, silt and
clay were calculated as shown in Equations 3.1, 3.2 and 3.3 (Gee and Bauder 1986):
% Sand was calculated as 100 – % (Silt + Clay)………………………… Equation 3.1
Where: % (Silt + Clay) = ((A40 seconds reading – B40 seconds reading) ÷ weight of soil) x 100 A = Sample hydrometer reading + temperature correction* B = Blank hydrometer reading + temperature correction* * Temperature correction: For each degree above 20 °C, 0.3 units were added to the hydrometer reading and 0.3 units subtracted from the hydrometer reading for each degree under 20 °C.
% Silt was calculated as 100 – % Sand – % Clay………………………... Equation 3.2
% Clay was calculated as ((A2 hour reading – B2 hour reading) X 100 weight of soil) …………………... Equation 3.3
3.1.3 Soil pH determination
Soil pH was determined using the CaCl2 method (Thomas, 1996). Fifteen grams (15
g) of previously air dried and ground soil to pass through a 2 mm sieve were weighed
into a 150 ml polythene bottle and 75 ml of 0.01M CaCl2 solution were added. The
resultant mixture was shaken on a reciprocating shaker for 30 minutes. After
removing the suspension from the shaker the bottle was further swirled and a pH
meter electrode was dipped into the supernatant solution. The pH reading was then
taken using a Mettler Toledo SevenEasyTM S20 pH meter once the display was stable.
3.1.4 Determination of exchangeable cations in soils
Exchangeable Ca, Mg, K and Na were determined following extraction from soil by 1
M acidified ammonium acetate (Anderson and Ingram, 1993). Ten (10) grams of air
23
dried soil (<2 mm) were weighed and put into 125 ml plastic jars and 100 ml of
acidified ammonium acetate solution was added. The containers were tightly closed
and put on a reciprocating shaker for 1 hour. The suspension was then filtered through
a Whatman No. 2 filter paper. About 5 ml of the filtrate were extracted into 25 ml
volumetric flasks and 0.5 ml of SrCl2 (21.04 %) were added and made up to mark with
distilled water. The samples were then read on a VarianTM AA-1275 series atomic
absorption spectrophotometer for determining Ca and Mg. Potassium and sodium
were read on a CorningTM 410 flame photometer.
A combined standard was made by taking 10 ml of K (1000 mg L-1) and Ca (1000 mg
L-1) stocks into 100 ml volumetric flask and adding 1 ml of Mg stock (1000 mg L-1)
and making up to the mark by adding distilled water to give 100 mg L-1 K, 100 mg L-1
Ca and 10 mg L-1 Mg. A sodium standard was made by taking 10 ml of Na stock
(1000 mg L-1) into 100 m to give 100 mg L-1 Na. The combined standard was pipetted
into 25 ml volumetric flasks as 1, 2, 3, 5 ml into each volumetric flask. About 2 ml of
SrCl2 (21.04% v/v) was added and made up to the mark with distilled water. Aliquots
of the Na standard were taken into 25 ml volumetric flasks to make 0.15, 0.30, 0.60
and 1.20 mg L-1 Na and 0.5 ml of SrCl2 were added and made up to mark with
distilled water.
The blank was made by adding 0.5 ml SrCl2 to a 25 ml volumetric flask then making up
to mark with distilled water. Calcium and magnesium standards were read on a
VarianTM AA-1275 series atomic absorption spectrophotometer and K and Na
standards were read on a CorningTM 410 flame photometer. Standard curve graphs
were then used to read off concentrations of Ca, Mg, K and Na in samples.
24
3.1.5 Available soil phosphorus determination
Available soil P was measured using the Olsen method (Okalebo et al. 2002).
Phosphorus standards were made by dissolving 0.439 g dry potassium dihydrogen
phosphate in 700 ml of water. To this solution, 25 ml of 3.5M sulphuric acid were
added and de-ionised water was also added to make the solution up to 1 litre.
Aliquots of 1, 2, 3, 4 and 5 ml were each extracted into 100 ml volumetric flasks and
made up to 100 ml using 0.5M sodium bicarbonate to obtain P standards of 1, 2, 3, 4
and 5 mg L-1 P respectively.
Five grams of air dried soil were weighed into 150 ml polythene bottles and 100 ml of
alkaline 0.5M NaHCO3 (pH 8.5) was added. The mixture was shaken for 30 minutes
and then filtered through a Whatman no. 2 filter paper into a 150 ml glass honey jar.
The initial filtrate was discarded until clear and then enough filtrate for 5 ml aliquot
was collected. A 5 ml aliquot of 0.5M NaHCO3 was taken into a 200 ml conical flask
and two drops of p-nitrophenol indicator were added. The solution was then titrated
with 3M H2SO4. At the end-point the colour of the solution changed from yellow to
colourless. The titre was noted and the same amount of 3M H2SO4 was added to all
samples and standards.
A 5 ml aliquot of the filtrate was placed into a conical flask and the titre of 3M H2SO4
added and swirled to release CO2. To the mixture, 20 ml of distilled water and 4 ml of
the ascorbic acid-ammonium molybdate and antimony potassium tartrate
(K2Sb2(C4H2O6)2) reagent were added. The solution was left for 15 minutes for
maximum colour development as a result of a phosphorus/molybade complex formed
25
by addition of acidified ammonium molybdate. The absorbance of the samples and
standards were read using a HitachiTM VIS-UV (100-60) spectrophotometer at 880 nm
wavelength. A standard graph was plotted as absorbance against the concentration of P
in each standard flask. The concentration of P in the sample was then calculated as
shown in Equation 3.4 (Okalebo et al. 2002).
P (mg kg-1) = (a-b) * v * f * 1000………………………………………... Equation 3.4 1000*w
Where: a = the concentration of P in the sample (in mg L-1) b = the concentration of P in the blank (in mg L-1) v = volume of the extracting solution (in ml) f = dilution factor w = weight of sample (in g).
3.1.6 Determination of soil total nitrogen
Total N was determined using the semi-Kjeldhal method (Anderson and Ingram
1993). Soil samples were digested using a solution of selenium (catalyst), lithium
sulphate and concentrated H2SO4. One gram of air-dried soil (< 2 mm) was weighed
into a digestion test tube. A spatula tip of selenium catalyst and 5 ml of concentrated
H2SO4 were added. The tube was placed on Gerhardt KjeldathermTM digestor and the
mixture was digested at 350 °C until the solution was clear. After cooling, the contents
of the digestion tube were transferred into a Kjeldahl flask washing with distilled water
and 10 ml of NaOH were added. Boiling stones were added and the flask was fixed on
the distillation apparatus. In Erlenmeyer receiving flasks graduated to 50 ml, 5 ml of
boric acid indicator were added.
The solution was steam distilled until 50 ml of distillate were collected. The liberated
ammonia collected in the distillate was then titrated with standard H2SO4 (0.07 M) until
26
it turned permanent pink from green. The total N of the sample was calculated as shown
in Equation 3.5 (Anderson and Ingram, 1993);
Total N (%) = (Vsample – Vblank) x conc [H+] (m/L) x 14 (m/g) x 100
Mass of soil (g) x 1000 …………. Equation 3.5
Where: V = volume of titre (ml)
3.1.7 Determination of soil mineral N
A 200 ppm N stock solution for standards was made by dissolving 0.57 g (NH4)2NO3
in one litre of distilled water. A 100 ppm N solution was then made from taking 50 ml
of the 200 ppm N solution into a 100 ml volumetric flask and this was made up to the
mark using distilled water. Standards were then made by pipetting 0, 2, 4, 6, 8, 10,12
mls of the 100 ppm N solution into 100 ml volumetric flasks and making up to the
100 ml mark using distilled water to make 0, 2, 4, 6, 8, 10, 12 ppm N solutions
respectively.
Exactly 10 g of soil were weighed into a 125 ml polyethylene bottle and 50 ml of 1M
KCl solution was added and the suspension was shaken for 30 minutes on an
automatic shaker. The suspension was then filtered through a Whatman No. 42 filter
paper. An aliquot of 5 ml was extracted from the filtrate into a 100 ml distillation
flask and 0.1 g NaOH (1 pellet) and a spatula tip of Devarda`s alloy was added to
reduce NO3 – N to NH4 – N. The mixture was steam distilled with the distillate
collected into an Erlenmeyer flask bearing 5 ml 0.02N HCl. About 35 - 40 ml of the
distillate was collected in about 5 minutes from commencement of condensation and
the receiving flask was lowered for the last minute.
27
The distillate was then transferred to a 50 ml volumetric flask and 1 ml of Nessler`s
reagent was added drop-wise with shaking and the final solution was made up to the
mark with distilled water. After about 10 minutes the solution was read on
spectrophotometer (Spec 20) at 470 mm wavelength. Distilled water was read as the
reference. Aliquots of 5 ml of each working standard were distilled, Nesslerized and
read on spectrophotometer and a graph of absorbance against concentration of N in
each flask was plotted. The value of mineral N in the soil sample was interpolated
from the sample absorbance to X ug N flask -1 on the graph of standards and
calculated as shown in equation 3.6 (Anderson and Ingram 1993);
Mineral N in soil (mg kg-1) = 50 / 10 * X / 5……………………………Equation 3.6
Where: 50 = shaking volume (ml) 10 = mass of sample (g) X = interpolated value 5 = distilled aliquot (ml)
3.2.1 Determination of legume biomass, maize grain and stover yields.
Above-ground biomass of hairy vetch, sunnhemp and cowpea were assessed by
sampling at 45, 60 and 75 days after planting (DAP) using a 1 m2 quadrat placed at
random within a net-plot avoiding one metre borders. The biomass was cut using a
pair of shears just above the soil surface at the immediate soil level (1 cm from the
ground) and separated from weeds. Maize was harvested at physiological maturity
from each plot leaving one metre at all sides of the plot borders. After removing cobs,
all maize stover was cut at ground level in the net plots and the fresh field mass was
recorded. Two maize stalks per plot were randomly collected as plant stover samples
and their fresh mass weighed for moisture determination.
28
The biomass was packaged in 36cm x 20cm x11cm khakhi sample bags and dried at
70o C in a Precision ® mechanical convention oven for at least 48 hours until the
weight was constant. Percentage moisture was subtracted from the field fresh mass of
the maize stover samples to give the stover dry mass. The above ground biomass was
then calculated as shown in Equation 3.7 (Anderson and Ingram 1993);
Biomass (kg ha -1) = Dry weight (g) x 10 000 m2 ………………... Equation 3.7 1000 g x Area harvested (1m2)
From the maize net plot all maize cobs were removed and the fresh field mass of cobs
was recorded. Five cob samples were then randomly collected from each plot. These
were also packaged in khakhi sample bags and weighed for fresh mass and dried in an
oven at 70o C for 48 hours and then reweighed to determine moisture content. Maize
cobs were then shelled and the cobs and grain weighed to determine shelling
percentage. The grain yield was then calculated as shown in equation 3.8 ;
Grain yield (kg ha -1) = Dry weight of cobs (g) x shelling % + 12.5% moisture correction x 10 000 m2 1000 g x Area harvested (1m2) ……... Equation 3.8
3.2.2 Initial plant preparation
Plant samples for laboratory analysis were separated into seed and stover (roots,
aboveground parts and shelled pods). The plant samples were oven-dried at 70o C to a
constant weight and ground to 2mm in a Retsch SK 100 standard rostfrei grinder.
Ground plant samples to be used for chemical analyses were then packaged in labeled
polyethene bottles.
29
3.2.3 Determination of total plant nitrogen
Total N in seed and stover was determined using the Kjeldahl method (Anderson and
Ingram 1993). About 0.20 g of plant material was weighed into a Kjeldahl digestion
flask in a balance room and the digestion flasks were then transferred to a fumehood.
A spatula tip of selenium catalyst was added and 5 ml of concentrated H2SO4. The
flask was placed on a digestion rack until the solution was clear or pale yellow (after
about 45 minutes) and cooled. Methyl red indicator was added (10 ml) together with 3
ml of concentrated sulphuric acid to check on any NaOH contamination. Contents of
the Kjeldahl flask were then transferred into one arm of the distillation apparatus
washing with distilled water and 10 ml of 50 % (w/w) NaOH were added. The arm
was closed with a stopcock lubricated with silicon grease. In the Erlenmeyer receiving
flasks graduated to 50 ml, 5ml of boric acid indicator was added and the solution was
steam distilled until 50 ml of distillate were collected. The green solution was then
titrated with 0.07 M sulphuric acid until it turned permanent pink. The titre was
recorded and Total N (% N) in the sample calculated as shown in Equation 3.9
(Anderson and Ingram 1993);
Total N (%) = (Vsample – Vblank) x conc [H+] (m/L) x 14 (m/g) x 100
0.2 (g) x 1000 …………………… Equation 3.9
Where V = volume of titer (ml)
3.2.4 Estimation of N2 fixation of legumes
N2 fixed by legume species was estimated using the N-difference method. The N-
difference method is based on legume N accumulation when the contribution of soil N
30
to the total legume N is determined. The maize crop with no fertiliser applied was
used as the non-N2-fixing control crop and contribution of symbiotic fixation to the
legume N was computed as shown in Equations 3.10 and 3.11 (Peoples et al. 2002).
%N from N2-fixation = Total legume N − Total maize N uptake * 100
Total legume N ……………………..…… Equation 3.10
Amount of N2 fixed (kg ha-1)
= Legume N uptake – maize N uptake……... Equation 3.11
This method was chosen because it’s simple and requires fewer resources. It is more
reliable for soils with poor capacity to supply available N than soils rich in available
N because when there is low available soil N the legume will fix more atmospheric N2
(Peoples and Giller 1993). Assumptions were that the non fixing reference plant and
the N2 fixing legumes vetch have similar root morphology and have the same
capabilities to exploit soil N. It was also assumed that there were minimal N losses to
the system through denitrification and leaching (Lepo and Ferrenbach 1987).
31
CHAPTER 4
BIOMASS PRODUCTION AND RESIDUAL N CONTRIBUTION
OF SELECTED LEGUMES TO A MAIZE CROP IN A LEGUME-
CEREAL ROTATIONAL SYSTEM
4.1 Introduction
There is a general concurrence by agricultural researchers that growing legumes is
key to soil fertility redress and maintenance in smallholder cereal-based systems
(Franke et al. 2008, Formowitz and Joergensen 2009). Suitability of legumes in low
soil N environments is usually high because of their ability to fix N2 and they can be
grown without nitrogen fertiliser additions (Shah et al. 2003, Lupwayi et al. 2011). A
major benefit of legume rotations is yield increases of a succeeding cereal crop (Chalk
et al. 1993) as a result of several factors. These factors include favourable soil pH
changes (Formowitz and Joergensen 2009) that in turn enhance phosphorus (P)
availability (Alvey et al. 2001), greater soil N availability (Svubure et al. 2010),
arbuscular mycorrhizal infection (Alvey et al. 2003) and reduced pest and weed
occurrence (Van Kessel and Hartley 2000). Legumes are also usually superior in
accessing P and K and this improves availability of these nutrients to subsequent
crops (Fischler et al. 1999, Franke et al. 2008). Rotations also offer the benefit of
spreading the risk of crop failure and provide a break for cereal diseases.
In a study to assess the residual N benefits of soyabean to maize, Kasasa et al. (1999)
found increased maize yields following Magoye and local promiscuous soyabean
varieties. Many studies have noted improvements in yields of cereals grown in
rotation with legumes (Anuar et al. 1995, Ojiem et al. 2007, Lunze and Ngongo 2007,
32
Svubure et al. 2010). Lunze and Ngongo (2007) reported a 33.8% increase in maize
yields in rotation with climbing bean. Ojiem et al. (2007) also reported a 33-47%
increase in maize yield following various legume rotations in Western Kenya.
Nitrogen fixed by a legume crop is dependent upon legume type and variety,
rhizobium strain (introduced and indigenous populations), soil fertility and rainfall
(Lupwayi et al. 2011). In turn the N contribution of legumes to a subsequent cereal
crop will depend upon the legume’s growth habit, yield, N partitioning patterns and
by the efficiency of N2 fixation (effective nodulation) and residue management
(Snapp et al. 1998). Legumes such as cowpea have been shown to contribute about 70
kg N per hectare for the succeeding crop when residues are incorporated into the soil
(Dube 1995). It should also be noted that cereal yield increases after legumes can also
happen without significant legume effects on the levels of total soil N (Bagayoko et
al. 2000). This implies that soil N dynamics as a result of legume inclusion cannot be
singled out as the major contributor to increases in cereal yields.
Challenges of legume inclusion in cereal cropping systems are usually the lack of
quality seed, labour and disease constraints (Ncube et al. 2009). When the legumes
are forage legumes there is a general reluctance of farmers to invest land and inputs
where there are no quick economic returns (Muza 2002, Franke et al. 2008). Such
reasons have led to the rejection of growing of green manures on fallow land by
smallholders farmers (Jeranyama et al. 1998). However where legumes are grown, the
productivity is often low which reduces residual effects on the non-legume rotation
crop (Chikowo et al. 2004a). Tauro et al. (2010) reported that most legumes used for
soil fertility improvement in Zimbabwe fail to produce at least 2 t ha-1 of biomass,
33
resulting in insignificant N contributions to cropping systems. Soil limiting factors to
legume productivity are soil P and N deficiencies when the root system is still
developing (Van Kessel and Hartley (2000) and Tauro et al. (2010)).
Hairy vetch (Vicia villosa Roth) is more tolerant to acidic soil conditions than most
legumes and is described as a legume that can be highly productive in low fertility
soils (Dastikaitė et al. 2009). In subtropical regions hairy vetch produces biomass of
between 3000–5000 kg ha-1, with N content ranging from 100–150 kg ha-1 (Anugroho
et al. 2009b). Benefits of growing hairy vetch, include weed control (Anugroho et al.
2009a), provision of significant amounts of nitrogen (Saha and Grove 1996), as a
mulch to retard soil moisture loss (Sadeghi and Insensee 2001) and to reduce surface
runoff and soil erosion (Czapar et al. 2002). Despite the potential benefits of including
hairy vetch in a maize rotation in tropical systems, there is no published data on its
actual performance under rain fed conditions in the tropical systems of Zimbabwe.
This chapter focuses on the following objectives;
1. To determine the biomass production of hairy vetch and the amount of
nitrogen fixed under different biophysical conditions and compare it with
sunnhemp and cowpea.
2. To evaluate the residual effects of hairy vetch, sunnhemp and cowpea on the
yield of maize in a legume-maize rotational system.
4.2 Materials and Methods
4.2.1 Study sites
Field experiments were set up in Chiota and Wedza communal areas and on station at
Grasslands Research Station (GRS). The study sites are previously described in
34
section 3.1. A composite soil sample of each study site was collected before the
experiments were set up. Each soil sample consisted of a mixture of 5 sub-samples
collected at random in the un-ploughed fields in November 2009. Soil samples were
analysed for pH, mineral N, exchangeable bases, available P and texture.
4.2.2 Experimental design, treatments and management
Two field experiments were set up to determine biomass production of three legume
types and their rotational effects on maize grain yields. The first experiment was laid
out in a randomised complete block design (RCBD) during the 2009/2010 agricultural
season in Chihota, Wedza and at Grasslands Research Station (GRS) and maintained
during the 2010/2011 season. At GRS, the experiment was continued for another
season during the 2011/2012 season. Blocking was done against slope and the
experiment had 6 treatments and 3 replicates in plots measuring 5 m x 4 m. The
treatments were:
i) Hairy vetch, no fertiliser applied
ii) Hairy vetch, 300 kg ha -1 NPK (7 N : 14 P2O5 : 7 K2O)
iii) Cowpea, no fertiliser applied
iv) Cowpea, 300 kg ha -1 NPK (7 N : 14 P2O5 : 7 K2O)
v) Sunnhemp, no fertiliser applied
vi) Sunnhemp, 300 kg ha -1 NPK (7 N : 14 P2O5 : 7 K2O)
Basal fertiliser was applied as compound D (7 N : 14 P2O5 : 7 K2O) at a rate of 300 kg
ha-1 to supply 21 kg N, 42 kg P2O5 and 21 kg K2O. This is the ‘blanket’
recommendation promoted by government extension agents for such soils and it has
also been used by Ngongoni et al. (2007) for forage legumes. Hairy vetch, cowpea
35
(variety CBC 2) and sunnhemp were planted at a spacing of 0.3 m between rows and
0.1 m within rows. Sunnhemp and Cowpea were inoculated with a Rhizobium sp
(Cowpea miscellany group) MAR 411 strain inoculant from the legume inoculant
factory at The Soil Productivity Research Laboratory (SPRL) in Marondera and hairy
vetch was inoculated with Ndure® inoculant imported from Intex Microbials, U.S.A.
The strain MAR 411 is the commercial strain used for cowpea and sunnhemp in
Zimbabwe and it has been shown to be very effective on both crops (unpublished
data). The Ndure® inoculant is used for inoculating hairy vetch in the USA. In the
third season the Rhizobium leguminosarum strain coded as MAR833 from the
Marondera inoculant facility was used to inoculate hairy vetch. This was after the
strain proved to be very effective in nodulating hairy vetch in the greenhouse in 2011.
To avoid cross contamination sterile techniques were used, the seeds, hands and
utensils used were swabbed with 70% alcohol before and after inoculations.
A 1m2 quadrat was used to randomly sample above-ground biomass at 45, 60 and 75
days after planting (DAP). Legumes were cut using a shear at the cotyledonary node
from the soil surface, separated from weeds and put in khakhi paper sample bags. Dry
mass was determined after drying the collected fresh mass in an oven at 70o C for 48
hours. Legume plant samples for total N analyses were collected at 75% flowering
during a time when N content in leaves and stems is highest before translocation of N
to the seed. These were then oven dried at 70o C, ground and analysed for total N by
the micro-Kjeldahl technique as described in section 3.23. Amount of nitrogen fixed
by the legumes was calculated by the N difference method explained in section 3.4.5
using an adjacent unfertilised maize control crop as a reference crop. Maize was
chosen as the reference crop because it was the closest available non N2 fixing crop to
36
the legumes in terms of phenology (development rate), rooting intensity and depth.
Assumptions were that the legumes and maize have the same capabilities to exploit
soil N.
The second experiment was a legume-cereal rotation experiment arranged in a
randomised complete block design (RCBD). It was also set up during the 2009/2010
agricultural season in Chihota, Wedza and at Grasslands Research Station (GRS).
During the 2010/2011 season of maize was planted in all plots, with a spacing of 0.9
m between rows and 0.45 m within rows. The experiment was continued for another
season at GRS during the 2011/2012 season which also had maize planted in all plots.
The treatment plots measured 5 m x 4 m with each treatment replicated three times
and the treatments are shown in Table 4.1
Table 4.1 Treatments (first and second season) for rotation experiments conducted in Wedza, Chihota communal area and at GRS
Treatment number 1st season cropping (2009/2010) 2nd season cropping (2010/2011)
1 Hairy vetch, no fertilizer applied Maize, no fertilizer applied
2 Hairy vetch, 300 kg ha -1 NPK Maize, 300 kg ha -1 NPK
3 Cowpea, no fertilizer applied Maize, no fertilizer applied
4 Cowpea, 300 kg ha -1 NPK Maize, 300 kg ha -1 NPK
5 Sunnhemp, no fertilizer applied Maize, no fertilizer applied
6 Sunnhemp + 300 kg ha-1 NPK Maize, 300 kg ha -1 NPK
7 Maize, 300 kg ha-1 NPK, 200 kg
ha-1 AN
Maize, 300 kg ha-1 NPK, 200 kg
ha-1 AN
8 Maize control, no fertilizer
applied
Maize control, no fertilizer
applied
37
Basal fertiliser was applied on all crops as an NPK compound fertiliser (compound D,
7 N : 14 P2O5 : 7 K2O) at a rate of 300 kg ha -1 and top dressing fertilizer was applied
on maize as ammonium nitrate (AN, 34.5 % N) at a rate of 200 kg ha -1. In the second
season at all sites and the third season at GRS maize was planted on all plots
maintaining the previous season fertilisation rates. Basal fertiliser was applied at
planting and top dressing fertiliser was split applied into two applications (100 kg ha -
1 each). The first application of top dressing fertiliser was between 40 -50 DAP and
the second was between 80 - 90 DAP. Maize grain and stover yields were determined
at physiological maturity after harvesting them from a net plot obtained from
excluding a meter from all plot borders. Cobs and stover in the net plot were counted
and weighed to obtain the field fresh mass of cobs and stover. Thereafter five cobs
and three maize stalks were sampled at random, reweighed and further dried in an
oven at 70o C for 48 hours to determine moisture content. Total grain and stover
yields were then calculated by subtracting percent (%) moisture content for stover
yield and for grain yield the moisture content was adjusted to 12.5% moisture content.
There were prolonged periods without rainfall during both seasons of experimentation
at all three sites during 2009/2010 and 2010/2011 (Figure 4.1) at least 8 days without
precipitation were observed midseason at all sites. The January mid-season drought
was more prolonged at all sites during the 2010/2011 season compared to the
2009/2010 season. During the 2010/11 season Chiota, Wedza and GRS sites
experienced 27, 16 and 21 days without precipitation during the month of February.
Grasslands received more rainfall than all the other sites over the 2 seasons.
38
GRASSLANDS WEDZA CHIHOTA
Figure 4.1 Rainfall distribution at Grasslands Research Station and at Wedza and Chihota communal areas.
1/10/09 29/10/09 26/11/09 24/12/09 21/1/10 18/2/10 18/3/10 15/4/10 13/5/10
Total rainfall for season = 653.6 mm
0
20
40
60
80
100
120
140
1/10/10 29/10/10 26/11/10 24/12/10 21/1/11 18/2/11 18/3/11 15/4/11 13/5/11
Ra
infa
ll (m
m d
ay -1
)
Time (day/month/year)
T otal rainfall for season= 1265.9 mm
1/10/09 29/10/09 26/11/09 24/12/09 21/1/10 18/2/10 18/3/10 15/4/10 13/5/10
Total rainfall for season = 758.5 mm
/10/10 29/10/10 26/11/10 24/12/10 21/1/11 18/2/11 18/3/11 15/4/11 13/5/11
Time (day/month/year)
Total rainfall for season = 836.2 mm
0
20
40
60
80
100
120
140
1/10/09 29/10/09 26/11/09 24/12/09 21/1/10 18/2/10 18/3/10 15/4/10 13/5/10
Ra
infa
ll (m
m d
ay -1
)
Total rainfall for season = 907.7 mm
/10/10 29/10/10 26/11/10 24/12/10 21/1/11 18/2/11 18/3/11 15/4/11 13/5/11
Time (day/month/year)
Total rainfall for season = 880.5 mm
2009
/201
0 20
10/2
011
39
4.2.3 Statistical analysis
A t test was performed on legume above ground biomass to determine whether basal
fertilization was effective on each legume’s biomass accumulation. Data on N uptake,
N2 fixed and maize yields were first tested for normality by plotting normal graphs in
Genstat 8th edition. The data was then analysed for variance (one way ANOVA) in
Genstat 8th edition. Where differences were significant at 95% confidence interval
least significant differences (LSD) were used to separate the different means.
4.3 Results
4.3.1 Soil fertility status of study sites
The soils at the study sites were strongly acidic with pH (CaCl2) ranging from 4.2 to
4.5 (Table 4.2), low available P (5 - 13 mg P2O5 kg -1), low exchangeable Mg (0.38 –
0.97 cmolc kg -1), adequate K (< 0.1 cmolc kg -1) and low clay content (<13%).
Table 4.2 Physical and chemical characteristics of soils at the legume-cereal rotation study sites.
Site Clay
%
Sand
%
Available
P
(mg P2O5
kg -1)
Mineral
N
(mg kg -1)
pH
(CaCl2)
Ca Mg K
(cmolc kg -1)
Wedza 6 88 13 24 4.2 6.95 0.97 0.08
Chihota 9 89 12 28 4.5 5.1 0.85 0.06
GRS* 13 82 5 22 4.5 6.4 0.38 0.09 *GRS – Grasslands Research Station.
4.3.2 Legume biomass production and biological nitrogen fixation (BNF)
Generally for all the legumes, the lowest biomass was at 45 DAP and the highest at 75
DAP at the 3 study sites (Table 4.3). At all sites sunnhemp yielded more above
ground biomass (Table 4.3) followed by cowpea and hairy vetch in that order except
in Chihota during the 2010/2011 season where fertilised cowpea yielded more
40
Table 4.3 Above-ground biomass production of three fertilized and unfertilized legumes at 45, 60 and 75 days after planting across three cropping seasons (2009/10, 2010/11 and 2011/12) in Zimbabwe
Treatment Biomass production t ha-1
Wedza Chihota GRS 2009/2010 2010/2011 2009/2010 2010/2011 2009/2010 2010/2011 2011/2012
45 DAP
60 DAP
75 DAP
45 DAP
60 DAP
75 DAP
45 DAP
60 DAP
75 DAP
45 DAP
60 DAP
75 DAP
45 DAP
60 DAP
75 DAP
45 DAP
60 DAP
75 DAP
75 DAP
Unfertilised hairy vetch
0.06 0.03 0.14 - - - 0.02 0.31 0.15 - - - 0.14 0.22 0.49 0.12 0.30 0.74 2.61
Hairy vetch + NPK
0.09 0.20 0.81 - - - 0.15 0.35 0.53 - - - 0.49 0.63 1.03 0.65 1.53 2.49 5.36
Unfertilised cowpea
1.44 2.37 2.95 0.77 2.17 2.10 0.46 1.12 1.10 0.85 1.32 1.84 0.42 1.08 2.76 1.15 3.35 4.02 5.48
Cowpea + NPK
1.70 2.55 2.95 0.94 2.68 2.44 0.64 1.40 1.59 0.70 2.11 4.61 0.61 1.52 2.62 0.84 1.66 1.73 4.17
Unfertilised sunnhemp
1.85 1.85 2.26 1.47 2.33 3.01 0.56 0.62 1.59 0.57 1.46 1.56 1.06 1.57 3.10 1.13 3.48 5.92 11.43
Sunnhemp + NPK
1.43 4.42 6.19 1.95 6.63 6.54 1.09 1.60 1.60 1.16 2.43 3.55 1.14 2.63 5.51 1.54 4.11 7.73 9.54
LSD (5%) 0.16 0.24 0.29 0.1 0.27 0.34 0.13 0.3 0.37 0.17 0.23 0.46 0.15 0.185 0.20 0.16 0.54 0.41 3.14 DAP – Days After Planting, - failed to establish, NPK – basal compound fertilizer (7 N : 14 P2O5 : 7 K2O) Figures in the same column with the same superscript letter are not significantly different from each other
41
biomass (4.6 t ha -1) than sunnhemp (3.55 t ha -1). Hairy vetch yielded the lowest
biomass and failed to establish completely in the 2010/2011 season in Chihota and
Wedza as a result of the erratic rainfall. Across the sites, there was yellowing of
leaves and unfertilised hairy vetch turned purplish (Plate 4.1) suggesting critical
deficiencies of nitrogen and phosphorus. Root inspection revealed that hairy vetch
was poorly nodulated with very few effective nodules.
In the 2011/2012 season at GRS hairy vetch at 75 DAP yielded biomass (5.4 t ha -1)
that was the same with that of cowpea (5.5 t ha -1) (Table 4.3). When compared across
the three seasons at GRS, basal fertilized hairy vetch’s biomass production at 75 DAP
increased by 142 % and 420 % in the second and third seasons respectively. Basal
fertilization significantly (P < 0.05) improved biomass accumulation of hairy vetch
and sunnhemp at 75 DAP except for sunnhemp in Chiota during the 2009/10 season
(P = 0.941). Basal fertilization of cowpea did not improve its biomass accumulation at
all sites during the 2010/11 season and also in Wedza during the 2010/11 season.
During the 2009/2010 season, basal fertilized sunnhemp had significantly (P<0.05)
higher N uptake of 138, 46 and 165 kg N ha -1 at GRS, Chiota and Wedza respectively
(Figure 4.2). This trend was also the same for N fixed by the legumes and basal
fertilized sunnhemp fixed about 136, 42 and 162 kg N ha -1 at GRS, Chiota and
Wedza respectively. When N uptake of the legumes was compared across the sites,
Chihota had the lowest N uptakes and N2 fixation values which were less than 46 kg
N ha -1 and 43 kg N ha -1 respectively for all the legumes.
42
Plate 4.1 a and b Growth of hairy vetch (HV), sunnhemp (SH) and cowpea (CP) in Marondera at 53 DAP. Unfertilised and uninoculated hairy vetch shows stunted growth and severe purpling of leaves and stem
(a) (b)
HV
HV
SH
CP
SH CP
SH
HV CP
SH
HV
SH
43
Figure 4.2 Nitrogen uptake of legumes grown during the 2009/2010 season at (a) GRS, in (b) Chihota and (c) Wedza. Bars with the same letter within the same graph are not significantly different from each other at 95% confidence interval. NPK – Compound D fertiliser (7 N : 14 P2O5 : 7 K2O)
Figure 4.3 N2 fixed by legumes during the 2009/2010 season at (a) GRS (b) Chihota and (c) Wedza. Bars with the same letter within the same graph are not significantly different from each other at 95% confidence interval. NPK – Compound D fertiliser (7 N : 14 P2O5 : 7 K2O)
0
20
40
60
80
100
120
140
160
180
200
Hairy vetch Cowpea Sunnhemp Hairy vetch +NPK
Cowpea +NPK
Sunnhemp +NPK
Treatment
N2
fixed
(kg
ha
-1)
(a)2 x SEM
a
c c
b
c
d
Hairy vetch Cowpea Sunnhemp Hairy vetch +NPK
Cowpea +NPK
Sunnhemp +NPK
Treatment
(b)2 x SEM
a
c c d
b
c d d
Hairy vetch Cowpea Sunnhemp Hairy vetch +NPK
Cowpea +NPK
Sunnhemp +NPK
Treatment
(c)2 x SEM
a
d
c
b
d
e
0
20
40
60
80
100
120
140
160
180
200
Hairy vetch Cowpea Sunnhemp hairy vetch + NPK
Cowpea + NPK
Sunnhemp + NPK
N u
pta
ke (
kg
ha
-1)
Treatment
(a)
2 x SEM
ab
cc c
d
Hairy vetch Cowpea Sunnhemp hairy vetch + NPK
Cowpea + NPK
Sunnhemp + NPK
Treatment
(b)
2 x SEM
ab
c c c c
Hairy vetch Cowpea Sunnhemp hairy vetch + NPK
Cowpea + NPK
Sunnhemp + NPK
Treatment
(c)
2 x SEM
a
b
c
d d
e
44
Application of NPK fertiliser significantly increased nitrogen uptake and nitrogen
fixation of hairy vetch and sunnhemp at GRS and Wedza (P<0.05). Basal fertilizing
hairy vetch increased N uptake by 136%, 237% and 544% at GRS, Chihota and
Wedza respectively. When sunnhemp received basal fertilizer, N uptake was
improved by 62% and 172% at GRS and Wedza respectively. However, application of
NPK basal fertiliser had no significant effect on nitrogen uptake and nitrogen fixation
of cowpea across the sites. Application of NPK basal fertiliser also had no significant
effect on nitrogen uptake and nitrogen fixation of sunnhemp in Chihota (Figure 4.2
(b) and Figure 4.3 (b)).
4.3.3 Effect of legume rotations on maize grain and stover yield.
4.3.3.1 Maize grain yield
In 2010/2011 maize in Chihota suffered excessive moisture stress because of a long
dry spell and as a result did not yield any grain. Maize grain yields at GRS were less
than those obtained in Wedza with all treatments at GRS having grain yields of less
than 1 t ha -1. In Wedza basal fertilized legume-cereal rotations of hairy vetch, cowpea
and sunnhemp increased maize grain yield by 138%, 72% and 155% respectively
compared to unfertilised maize. A basal fertilized sunnhemp-maize rotation had
significantly higher maize grain yield (2.6 t ha -1) than maize that received basal and
top dressing fertilizer (2.36 t ha -1) (Figure 4.4 (a)). This trend was the same at GRS
during the 2010/11 season where basal fertilized sunnhemp-maize rotation produced
0.96 t ha -1 and fully fertilized maize produced 0.8 t ha -1 of maize grain. A basal
fertilized hairy vetch-maize rotation had grain yields (2.5 and 0.74 t ha -1) that were
not significantly different to maize that received basal and top dressing fertilizer (2.4
and 0.8 t ha -1) at Wedza and GRS respectively.
45
At both sites during the 2010/11 season, the lowest maize grain yields were obtained
from the control and legume-cereal rotation plots without NPK basal fertiliser
application and this ranged between 0.96 – 1.2 t ha -1 in Wedza and 0.07 – 0.3 t ha -1
at GRS (Figures 4.4 (a),(b)). In Wedza, legume-cereal rotations without basal
fertiliser application yielded maize grains that were not significantly different from
the control at 1.03 t ha -1.
Figure 4.4 Maize grain yields after crop rotations during the 2010/2011 season at (a) Wedza and (b) GRS sites. Bars with the same letter in a graph are not significantly different from each other. NPK – Compound D fertiliser (7 N : 14 P2O5 : 7 K2O), AN – ammonium nitrate fertilizer (34.5 % N)
0
0.5
1
1.5
2
2.5
3
Mai
ze G
rain
Yie
ld (t
ha -1
) 2 X S.E.M.
a
cd
b
cd
aa a
(a)
0
0.5
1
1.5
2
2.5
3
Maize afterHairy vetch
Maize afterCowpea
Maize afterSunnhemp
Maize afterHairy vetch
+ NPK
Maize afterCowpea +
NPK
Maize afterSunnhemp
+ NPK
Maize +NPK + AN
Un-fertilisedMaize
Mai
ze G
rain
Yie
ld (t
ha -1
) 2 X S.E.M.
a
d
cbab
e ed
(b)
46
At GRS, during the 2010/11 legume rotations significantly differed in the order
Control (0.07 t ha -1) = Maize after Hairy vetch (0.12 t ha -1) < Maize after Cowpea
(0.17 t ha -1) < Maize after Sunnhemp (0.3 t ha -1) (Figure 4.4(b)). During the
2011/2012 season at GRS, a hairy vetch-maize-maize rotation had the lowest maize
grain yield of 0.14 t ha -1 that was not significantly different from the control (0.32 t
ha -1) (Figure 4.5). However rotations involving cowpea and sunnhemp without
fertiliser application had at least 50% more grain than the unfertilized control and
were not significantly different from a hairy vetch rotation with basal NPK fertilizer
that yielded 0.61 t ha -1.
Figure 4.5 Maize grain yields during the 2011/2012 season after crop rotations at Marondera (GRS). Bars with the same letter are not significantly different from each other at 95% confidence interval. NPK – Compound D (7 N : 14 P2O5 : 7 K2O), AN – ammonium nitrate fertilizer (34.5 % N)
Continuous maize with basal NPK fertiliser and ammonium nitrate yielded the highest
maize grain yield of 1.59 t ha -1 compared to basal fertilized rotations of sunnhemp
and of cowpea that yielded 1.2 and 0.99 t ha -1 respectively.
0
0.5
1
1.5
2
2.5
3
Maize / Maize/ Hairy vetch
Maize / Maize/ Cowpea
Maize / Maize/ Sunnhemp
Maize / Maize/ Hairy vetch +
NPK
Maize / Maize/ Cowpea +
NPK
Maize / Maize/ Sunnhemp +
NPK
Maize + NPK+ AN
Un-fertilisedMaize
Treatment
mai
ze g
rain
yie
ld (t
ha
-1)
2 X S.E.M
aab
f
ed
cbcbc
47
4.3.3.2 Maize stover yields
During the 2010/2011 season; rotations involving sunnhemp and basal fertilisation
yielded significantly higher maize stover yields in Wedza and at Grasslands Research
Station (GRS) (Table 4.4). In Wedza unfertilised both legume-maize rotations of
hairy vetch and cowpea had low maize stover biomass of 0.33 t ha -1 which was not
significantly different to unfertilised maize (0.46 t ha -1). At the same site, when hairy
vetch-maize and sunnhemp-maize rotations were basal fertilized maize stover
biomass was increased by 56% and 104% compared to maize that received both basal
and top dressing fertilizer. However, compared to fully fertilized maize, there was a
reduction in maize stover biomass of a basal fertilized cowpea-maize rotation by 33%.
Table 4.4 Maize stover yields after a legume-cereal rotation during the 2010/2011 and 2011/2012 seasons in Wedza and at Grasslands Research Station (GRS).
Treatment Maize stover yield (t ha-1) Wedza
2010/2011 GRS
2010/2011 GRS
2011/2012 1. Hairy vetch- Maize, no fertilizer 0.33a 2.28a 1.17b
2. Hairy vetch-Maize, + NPK fertilizer 1.83d 2.61a 3.44d
Sunnhemp- Maize, no fertilizer 1.06c 3.11b 1.39b
Sunnhemp-Maize, + NPK fertilizer 2.39e 4.72c 3.50d
Cowpea- Maize, no fertilizer 0.33a 2.94a 3.11d
Cowpea-Maize, + NPK fertilizer 0.78b 3.06a,b 2.33c
Continuous Maize, + NPK fertiliser + Ammonium nitrate 1.17c 4.06c 4.28e
Continuous maize, no fertilizer 0.46a 2.50a 0.72a
LSD 0.25 0.79 0.40 Treatments with the same superscript letter in the same column are not significantly different from each other. GRS – Grasslands Research Station, NPK –compound fertiliser (7 N: 14 P2O5 : 7 K2O), Ammonium nitrate (34.5% N) At GRS during the 2010/11 season, maize that received basal and top dressing
fertilizer and a sunnhemp-maize rotation with basal fertiliser had increased stover
biomass of 62% and 89% respectively compared to unfertilised maize. However,
basal fertilized rotations of hairy vetch and cowpea had maize stover biomass of 2.6
48
and 3.06 t ha -1 that were not significantly different to unfertilised maize (2.5 t ha -1)
(Table 4.4). During the 2011/12 season maize with basal and top dressing fertilizer
had significantly higher stover biomass that was 24%, 84% and 22% more than basal
fertilized rotations of hairy vetch, cowpea and sunnhemp respectively.
4.3.4 Effect of legume crops (mono-cropped and in cereal rotations) on soil
available P and soil N
4.3.4.1 Mono-cropped legumes
Generally across the sites extractable phosphorus was lowest in treatments without
basal fertiliser application (Table 4.5).
Table 4.5 Available soil P and total N after legume mono-cropping in Wedza, Chiota and GRS during the 2010/11 and 2011/12 seasons.
Treatment SITE Chihota
(2010/2011) Wedza
(2010/2011) GRS
(2010/2011) GRS
(2011/2012) Available
P (mg P/kg)
Total N (%)
Available P (mg P/kg)
Total N (%)
Available P (mg P/kg)
Total N (%)
Available P (mg P/kg)
Total N (%)
Unfertilised Hairy vetch
4.16a 0.037b 3.99b 0.027a 4.02b 0.059a 3.05bc 0.059a
Hairy vetch + NPK fertilizer
4.94a,b 0.030a 5.84d 0.025a 5.79c 0.055a 5.92d 0.055a
Unfertilised Cowpea
5.12a,b 0.031a 5.88d 0.026a 3.14ab 0.056a 2.58ab 0.056a
Cowpea + NPK fertilizer
9.76c 0.037b 3.14a 0.025a 3.55b 0.057a 3.34c 0.057a
Unfertilised Sunnhemp
4.67a 0.032a 4.10b 0.025a 2.77a 0.056a 2.16a 0.061a
Sunnhemp + NPK fertilizer
6.85b 0.033a 5.06c 0.024a 7.25d 0.057a 6.46e 0.057a
LSD 2.06 0.0038 0.67 ns 0.56 ns 0.48 ns
Figures with the same superscript letters in the same column are not significantly different at 95% confidence interval. ns- not significant (P > 0.05), GRS – Grasslands Research Station
49
During the 2010/11 season in Chihota a basal fertilized cowpea crop had significantly
greater available P (9.76 mg P kg -1) compared to other basal fertilized or unfertilised
legumes. In Wedza basal fertilized hairy vetch and unfertilised cowpea had more soil
P (5.84 and 5.88 mg P kg -1 respectively) than all the other treatments. However at
GRS during the 2010/11 season basal fertilized sunhemp had significantly more
available P (7.25 mg P kg -1 soil), followed by basal fertilized hairy vetch that had
5.79 mg P kg -1 (Table 4.5). During the 2011/12 season at GRS basal fertilized
legumes had at least 9.5% more soil P than unfertilised legumes. In Chihota during
the 2010/11 season, basal fertilized cowpea and unfertilised hairy vetch had
significantly (P < 0.05) higher ( > 12%) total soil nitrogen content compared to other
treatments. However there were no significant differencies in total soil N at the
Wedza and GRS sites.
4.3.4.2 Legume-cereal rotations
During the 2010/2011 season in Chiota plots under maize that received basal and top
dressing fertilizer had 195% more soil P than plots under maize that was unfertilised
(Table 4.6). At the same site, Basal fertilized legume-maize rotations had soil P
increases that ranged 39% - 57% compared to unfertilised maize plots. In Wedza a
basal fertilized cowpea-maize rotation plots had higher soil P (8.46 mg P kg -1) which
was not significantly different to maize plots that received basal and top dressing
fertilizer (8.36 mg P kg -1), basal fertilized rotations of sunnhemp and hairy vetch and
unfertilised rotations of hairy vetch and cowpea. At GRS the trend in soil P was the
same as that observed in Chiota and plots under maize that received basal and top
dressing fertilizer had 72% more soil P that plots that were under unfertilised maize
(Table 4.6). However, during the 2011/12 at GRS plots under basal fertilized
sunnhemp-maize rotation had the highest available P (8.36 mg P kg -1) followed by
50
Table 4.6 Available soil P, mineral N and total N after basal fertilised and unfertilised legume-cereal rotations over two seasons (2010/11 and 2011/12).
Treatment
SITE Chihota 2010/2011 Wedza 2010/2011 GRS 2010/2011 GRS 2011/2012
Avail P
(mg P/kg)
Mineral N (mg N/kg)
Total N (%)
Avail P (mg P/kg)
Mineral N (mg N/kg)
Total N (%)
Avail P
(mg P/kg)
Mineral N (mg N/kg)
Total N (%)
Avail P (mg P/kg)
Mineral N (mg N/kg)
Total N (%)
Unfertilised Hairy
vetch / Maize
3.45a
22.33cd
0.026b
7.27b
10c
0.030b
4.00ab
12.00bcd
0.055b
3.41ab
9.67a
0.054a
Hairy vetch + NPK / Maize + NPK
5.03ab 18.67bc 0.027b 7.32b 11.33c 0.029b 5.45bc 16.67de 0.052ab 4.89c 6.33a 0.052ab
Unfertilised Cowpea / Maize
4.03a 14.33ab 0.026b 8.00b 17.67d 0.029b 4.88bc 9.67abc 0.055b 3.34a 10.33a 0.055b
Cowpea + NPK / Maize + NPK
3.54a 17.00ab 0.023a 8.46b 2.67a 0.028ab 3.82ab 15.67cde 0.053ab 5.70c 9.00a 0.053ab
Unfertilised Sunnhemp / Maize
4.08a 17.00ab 0.024a 3.51a 1.67a 0.025a 3.02a 7.67ab 0.049a 4.20bc 8.67a 0.049a
Sunnhemp + NPK / Maize + NPK
5.67b 25.33d 0.024a 7.40b 10.33c 0.027a 6.43c 18.67e 0.061c 8.36d 11.67a 0.061c
Continuous Maize + NPK + A.N.
10.70c 12.67a 0.031c 8.36b 1.00a 0.028ab 7.03c 10.33bc 0.053ab 3.17ab 9.67a 0.053ab
Maize monocrop without fertiliser (control)
3.62a 23.67d 0.026b 3.08a 6.33b 0.026a 4.09b 4.00a 0.051ab 2.22a 13.00a 0.051ab
LSD 1.63 4.93 0.002 1.05 3.28 0.003 1.04 6.16 0.004 1.33 ns 0.005 a,b,c,d – values with different superscript letters in the same column denote significant difference at P<0.05. ns – not significant, GRS – Grasslands
Research Station, NPK – compound fertilizer (7N : 14 P2O5 : 7 K2O), AN – ammonium nitrate fertilizer (34.5% N)
51
basal fertilized cowpea-maize rotations (5.7 mg P kg -1) and basal fertilized hairy
vetch-maize rotation (4.89 mg P kg -1). The trend in soil mineral N was highly
variable across the different sites (Table 4.6). During the 2010/11 season, plots under
a basal fertilized sunnhemp-maize rotation had significantly more mineral N of 25.33
and 18.67 mg N kg -1 in Chiota and GRS respectively compared to other treatments.
However in Wedza, unfertilised cowpea plots had the most mineral N (17.67 mg N kg
-1). During the 2011/12 season at GRS, treatment effects were not significant (P >
0.05) on soil mineral N. Also total N values for each treatment did not change for the
2010/11 and 2011/12 seasons at GRS. Total soil N at GRS was double that for soils in
Chiota and Wedza.
4.4 Discussion
Although hairy vetch was inoculated it produced considerably less biomass than
cowpea and sunnhemp over two seasons (2009/2010 and 2010/2011). A visual
inspection of plant roots revealed that there was poor nodulation of hairy vetch as
opposed to sunnhemp and cowpea which had a lot of effective nodules with reddish
insides. Reduced or lack of nodulation in hairy vetch resulted in poor N2 fixing
capacity of the crop contributing to the poor biomass accumulation.
While legumes usually have higher P uptake compared to grasses, legumes such as
cowpea are reported to be able to access sparingly soluble P when soils are deficient
in available P (Rusinamhodzi et al. 2012). Hairy vetch is known to have higher P
uptake rates than most legumes and as a result it has been explored as P accumulating
crop from fields were poultry litter is added (Alsup et al. 2002). Due to its high P
requirements, P deficiency in the soils might have impaired hairy vetch’s initial root
development and as a result nodulation was affected. Sunnhemp and cowpea may be
52
more superior in terms of ‘scavenging’ for sparingly soluble soil phosphorus. Basal
fertilised hairy vetch produced more biomass compared to unfertilised hairy vetch
showing that there was a biomass response as a result of NPK basal fertiliser
application.
Generally there was a significant (P<0.05) increase in biomass when the legumes
received basal fertiliser. This agrees with the studies of Zhu et al. (2011) in
subtropical China who found that hairy vetch dry matter yield initially increased as
the phosphorus rate increased and then decreased, with a maximum yield of 3.56 t ha -
1 achieved at a phosphorus rate of 120 kg P ha -1. Emine and Mukerrem (2012) in
Turkey also found that the dry matter yield of a 50:50 mixture of hairy vetch and
barley was highest (11.26 t ha -1) when 13.1 kg P ha -1 was applied and lowest when P
was not applied. Other studies by Singh et al. (2011) showed that higher stover yields
of cowpea were recorded when cowpea was supplied with 26.2 kg P ha -1.
At GRS during the 2011/2012 season, hairy vetch produced twice as much biomass
compared to the previous seasons. In this season, the Ndure® inoculant that was used
during the 2009/10 and 2010/11 seasons had to be changed because nodulation was
poor. This was despite that the inoculant is extensively used for inoculating hairy
vetch in the USA. In this season (2011/12) the Rhizobium leguminosarum strain
MAR833 was used to inoculate hairy vetch after the strain proved to be very effective
in nodulating hairy vetch in the greenhouse in 2011 in experiments described in
chapter 6. The effective nodules observed in the field after inoculating with MAR833
in the 2011/12 season suggest that the bacteria might have been better adapted to local
53
conditions and therefore more effective than the NDURETM inoculant used during the
2009/10 and 2010/11 seasons.
Erratic distribution of rainfall during the 2010/11 seasons might have also affected
production of legume biomass. Lower biomass yields for all legumes were observed
in Chihota and this might have been caused by the fact that this site received less
rainfall than the other sites. In the first season (2009/2010), legumes were planted in
mid January which is considered late in the rainy season and as a result the legumes
had fewer days of precipitation. However, basal fertilised sunnhemp produced the
highest biomass across all sites during the seasons studied showing that sunnhemp
might be better adapted to tropical conditions with low soil fertility and erratic rainfall
than the other legumes.
Due to its quick growth, sunnhemp might have conserved more soil moisture by
providing a shade on the soil surface resulting in less water evaporation from the soil
surface. The conserved soil moisture is likely to have been used for plant growth at
times of prolonged dryness. The cowpea variety used (CBC2) has a vertical growth
habit and as a result it did not form a dense cover on the soil, hairy vetch due to its
initial poor growth also failed to form a dense soil cover. This might have exacerbated
direct moisture losses through evaporation from the soil surface enhancing moisture
stress during the prolonged mid season drought.
Basal fertilised sunnhemp had the highest nitrogen uptake and N2 fixed which was up
to 164.9 and 161.5 kg ha -1 respectively at the 3 study sites because it had greater total
N content and more biomass. Generally there was an increase in N uptake and N2
54
fixation when sunnhemp and hairy vetch received basal fertiliser showing that lack of
fertiliser especially P greatly affects biomass production. This is likely to have
happened by restricting root development which in turn reduces nodulation (Graham
et al. 2003). The situation was different for cowpea because there were no significant
differences in N uptake and N2 fixation for cowpea with and without basal
fertilisation. This suggests that cowpea’s N uptake and N2 fixation was not affected by
basal fertilisation. The abundance of indegeneous rhizobia able to effectively nodulate
cowpea could partly explain this since effective nodulation could still take place at
low soil P levels. Ankomah et al. (1996) reported similar findings working with three
cowpea cultivars in Austria where P application did not have any effect on neither
cowpea root biomass nor N2 fixed. In the current study, cowpea might also have been
able to extract soil P sufficient for N2 fixation even when it was not fertilised with
basal fertiliser. Cowpea is generally considered to be well adapted to low nutrient
soils and low rainfall conditions (Singh et al, 2011). Other soil factors such as the low
pH (4.2 - 4.5) and sandy texture (% sand >82) should have also adversely affected
legume growth. Compared to other sites, legumes planted in Chihota had very low
levels of N uptake (3.5 – 45.7 kg ha -1) and N2 fixation (0.4 – 42.4 kg ha -1) and the
low rainfall might have been a major contributor to this.
Low maize grain yields (0.12 – 1.22 t ha -1) were obtained when unfertilised legumes
were rotated with unfertilised maize and these were not significantly different from
the unfertilised maize control (0.07 – 1.03 t ha -1). This shows that under similar
conditions legume rotations without basal fertilisation will not do much to improve
maize yields of a succeeding crop. In this case legume productivity is compromised
and its potential beneficial effects are reduced. Legume rotations with basal
55
fertilisation had higher maize grain yields (72% < compared to unfertilised maize)
that was equal to or greater than fully fertilised maize. Many studies have noted
similar improvements to cereal yields when grown in rotation with legumes (Anuar et
al. 1995; Ojiem et al. 2007; Svubure et al. 2010). Basal fertilisation caused more
phosphorus to be available during the growth of maize contributing to the increase in
grain yields compared to non-fertilised rotations.
The legumes that were grown with basal fertilisation might have also contributed
more residual N to the succeeding maize crop and led to an increase in grain yields.
The residual N is likely to have been readily available for the next season crop and
was taken up by the maize. The increase in yields however cannot be attributed to
residual N alone but also to other ‘non N’ legume effects such as increased P
availability observed in NPK fertilised sunnhemp rotations in this study. Legumes
may improve the recycling of phosphorus and potassium since they are usually
superior in accessing phosphorus and potassium improving availability of these
nutrients to subsequent crops (Fischler et al. 1999).
Maize yields at GRS during the 2010/2011 season were generally low with the control
yielding 0.07 t ha-1 and the highest yield of 0.96 t ha-1 obtained from a sunnhemp-
maize rotation that received basal fertiliser. The severe nutrient depleted nature of the
soils, erratic rainfall, late planting and damage by the pest maize stock borer
(Busseola fusca) might have contributed to the low yields. Very low maize yields (<1
t ha -1) in Zimbabwe communal farming areas have also been reported by
Mtambanengwe and Mapfumo (2006) in Chikwaka. Chikowo et al. (2004a) also
56
reported similar low maize yields of less than 0.5 t ha -1 in Domboshava even after
Sesbania litter was applied to the soil.
During the 2011/2012 season at GRS maize that received NPK basal fertiliser and
ammonium nitrate yielded the highest grain yield compared to basal fertilized maize
that had been rotated with legumes. When maize was planted for the second time after
legumes a basal fertilized sunnhemp rotation produced maize grain yield that was
25% less than maize that received basal and top dressing fertilizer compared to the
2010/2011 season which had a sunnhemp maize rotation having the highest yields.
This showed that rotational effects were less than when maize immediately follows a
legume crop. Despite this, differences in maize grain yield were still significant in the
order sunnhemp- maize rotation > cowpea- maize rotation > hairy vetch-maize
rotation. Other researchers have also noted significant increases in maize grain yields
after rotations with sunnhemp compared to when land is left fallow (Balkcom and
Reeves, 2005)
The lowest available phosphorus was obtained from treatments without basal
fertilisation suggesting that when legumes are included in cropping systems without
basal fertilisation there can be a depletion in soil available P. When the legumes were
mono-cropped without basal fertilization, sunnhemp depleted more available soil P
than the other legumes but when basal fertilized plots after sunnhemp had increased
soil P. A reason for this could be that when sunnhemp was basal fertilised root
development was improved making the legume more efficient in extracting soil P
from sparingly soluble P pools via increased root access to a greater P pool. The
ability of legumes to extract P from sparingly soluble P pools is attributed to root-
57
induced chemical changes through exudation of organic acids (Pypers et al. 2007).
This extracted soil P might become available when legume biomass decomposes after
leaf fall and legume residues that remain after harvest. Alvey et al. (2001) also found
that P uptake of sorghum was 3-9 times higher after a rotation with cowpea relative to
the continuous sorghum treatment on Arenic Kandiustalf and Haplustalf soils in
Sudan and Burkina Faso respectively.
Maize that received basal and top dressing fertiliser exhibited greater soil available P
across the sites compared to legume/maize rotations that received basal fertiliser. This
could have been because introduced legumes might have depleted more soil P than
maize. This is evident at GRS where unfertilised sunnhemp/maize rotation plots had
significantly lower available P than the unfertilised maize plots hence rotations
without basal fertiliser application are likely to mine soil P resulting in the reduction
of soil P.
Generally fully fertilised maize depleted more soil mineral N implying that the
“blanket” fertiliser recommendations for such soils might not be enough to supply all
the nitrogen needs of a maize crop in such impoverished soils. The provision of P and
N to the maize crop might have made it better at extracting soil nitrate to support its
growth. Basal fertilised sunnhemp- maize rotation had the highest soil mineral N at
two out of three sites during the 2010/2011 season. This could have been as a result of
the decomposition and mineralization of sunnhemp biomass from the first year where
sunnhemp produced the highest biomass. In this scenario sunnhemp might have
contributed more mineral N to the soil than the other legumes by virtue of it having
produced more biomass. This agrees with studies done by Bagayoko et al. (2000) who
58
reported consistently higher mineral N levels of at least 12 % in legume-cereal
rotation plots compared to continuous cereal plots in Niger. In that study millet was
rotated with cowpea. Soil mineral N was not significantly different in the third season
at GRS implying that the effects of legume rotations on soil mineral N are only
significant in the year following the legume.
4.5 Conclusions
Hairy vetch can produce biomass yields of up to 5.3 t ha -1 in Zimbabwe if it is
inoculated with an appropriate Rhizobium leguminosarum strain and fertilised with
basal fertiliser. However, moisture stress during establishment greatly hampers
establishment of hairy vetch and sunnhemp is more appropriate when such conditions
prevail. Cowpea and sunnhemp can produce biomass yields of up to 5.5 t ha -1 and
11.4 t ha -1 respectively. Sunnhemp, cowpea and hairy vetch fixed N2 of up to 161.5,
84.7 and 25 kg N ha -1. In conditions of low soil fertility and erratic rainfall it is more
beneficial to include sunnhemp or cowpea provided that basal fertilizer is also applied
in the rotation for increased maize yields.
Phosphorus fertilization is important in smallholder legume-cereal rotational systems.
When legumes are rotated with maize without any P fertilization in low fertility soils
the residual benefits of legume cropping to the subsequent maize crop will be
insignificant because of poor legume productivity. However, when basal fertiliser is
applied on the legume crop in legume-cereal rotations there is a potential of
increasing grain yields of a subsequent maize crop. Legume rotations without basal
fertilization will lead to more P nutrient mining than continuous maize.
59
CHAPTER 5
MAIZE GRAIN YIELD BENEFIT OF LEGUME RELAY
INTERCROPS IN LOW FERTILITY SOILS IN ZIMBABWE
5.1 Introduction
Intercropping is one of the most common traditional methods of food production by
smallholder farmers in the tropics especially in Southern Africa (Rao et al. 1987;
Sakala et al. 2000; Hardarson and Atkins 2003). Smallholder farmers grow maize,
sorghum and millets sparsely intercropped with beans, groundnuts, cowpeas and/or
pumpkins (Dube 1995; Kumwenda et al. 1995). They do this to avert risk and
maximize utilisation of land and labour by growing legumes mixed with cereals
(Mucheru-Muna et al. 2010). Potential benefits of intercropping with legumes include
cereal yield increase through pest and weed suppression, increased efficiency of soil
resource use and reduction in soil erosion rates (Van Kessel and Hartley 2000;
Jeranyama et al. 2000). Improvement and/or maintenance of soil fertility is however
of mediocre concern to smallholder farmers with returns and labour being most
prioritised (Snapp and Silim 2002). Smallholder farmers also rarely use lime and
apply minimal inorganic fertilisers targeted at the cereal in the intercrop (Muza 2002).
Potential benefits of legume-cereal intercrops in the short term depend on rainfall, soil
type, fertiliser and agronomic management (Waddington et al. 2007). Yield benefits
vary considerably with varying crop varieties that have different plant architecture,
rooting patterns, competitive advantages and potential nitrogen fixing capacity of the
legume (Fujita et al. 1992). As a result, legume species and varieties adapted to low
fertility soils have greater potential for resource-poor farmers (Snapp and Silim 2002).
60
To ensure success in cereal-legume intercropping selection of locally adapted species
with good ability to nodulate and fix N2 becomes crucial (these have high
complementarity and are less competitive in terms of soil resource use) (Ngongoni et
al. 2007). Cereal-legume associations can lead to efficient utilisation of available
resources by having the cereal and legume compliment each other through growth
patterns, aboveground canopy, rooting system, water and nutrient demand (Mucheru-
Muna et al. 2010). The legume chosen must not reduce cereal yield and should benefit
from maize targeted agronomic management for farmer acceptance (Muza 2002).
Intercropping does not take away land devoted to cereals unlike in rotations where
smallholder farmers are reluctant to forgo a season with legumes that do not give an
immediate food benefit because of land and labour challenges, (Snapp et al. 1998;
Jeranyama et al. 1998; Jeranyama et al. 2000). Suitable technologies fitting resource-
poor farmers that are not taxing in terms of labour, land and capital investments can
be explored (Mucheru-Muna et al. 2010). Legume species that would fit well must
therefore have some tolerance to soil acidity in low fertility soils common in most
smallholder areas of Zimbabwe and be able to fix considerable amounts of N2. While
cowpea and sunnhemp have received considerable research in Zimbabwe, information
on the performance of hairy vetch in local intercropping systems is not available. This
chapter therefore focused on the following objectives;
1. To determine the effect of hairy vetch-maize intercropping benefit on maize
yield and compare it with cowpea and sunnhemp.
2. To determine the N and P mineralization patterns of hairy vetch, cowpea and
sunnhemp residues.
61
5.2 Materials and methods
5.2.1 Study sites
Field relay-intercropping experiments were conducted in Wedza, Chihota and at
Grasslands Research Station as described in Sections 3.1.
5.2.2 Determination of the effect of legume inter-crop N benefit on maize yield
A Randomised complete block design (RCBD) experiment was set up during the
2009/2010 agricultural season one each in Chihota, Wedza and at Grasslands
Research Station (GRS) and repeated during the 2010/2011 season. The experiment
was continued on the same plots for another season at GRS during the 2011/2012
season. Blocking was done against slope and the experiment had ten (10) treatments
and three (3) replicates. The treatment plots measured 7.2 m x 6 m and the treatments
were:
i) Maize + Hairy vetch + NPK fertiliser (300 kg ha-1)
ii) Maize + Cowpea + NPK fertiliser (300 kg ha-1)
iii) Maize + Sunnhemp + NPK fertiliser (300 kg ha-1)
iv) Unfertilised Maize + Hairy vetch
v) Unfertilised Maize + Cowpea
vi) Unfertilised Maize + Sunnhemp
vii) Maize + NPK fertiliser (300 kg ha-1)
viii) Maize + NPK fertiliser (300 kg ha-1) + Amonium nitrate (200 kg ha-1)
ix) Unfertilised Maize
x) Maize + Ammonium nitrate (200 kg ha-1)
Basal fertiliser was applied at a rate of 300 kg ha-1 Compound D fertiliser (7 N: 14
P2O5: 7 K2O) and top dressing fertiliser was applied at a rate of 200 kg ha-1
62
ammonium nitrate (34.5 % N). The top dressing fertiliser was applied twice (split
applied) between 30-40 days after planting (DAP) and the second application was
during 60-70 DAP. Maize was planted with a spacing of 0.9 m between rows and 0.45
m within rows. Sunnhemp, hairy vetch and cowpea were relay intercropped in the
maize plots at approximately 4 weeks after planting maize. Two rows of legumes
were planted between two rows of maize, the rows of legumes were placed 30 cm
apart and the within row spacing was 10 cm.
In Chihota and Wedza the land was tilled using an ox drawn plough and a tractor was
used to till the land each year at Grasslands Research Station (GRS). Prolonged
periods without rainfall during the 2009/2010 and 2010/2011 seasons at all sites
resulted in the delay of legume relay intercrop planting. Legumes were planted ideally
when rainfall water had penetrated to a depth of 20 cm to ensure survival of rhizobia
applied in inoculants. Weeding was done manually using hoes whenever necessary.
At the end of each season maize grain and stover were harvested at physiological
maturity from net plots measuring 5.2 m x 4 m formed by excluding a meter at each
plot border. Field fresh mass of stover and maize cobs was measured using a field
hand-held scale. Four maize cob samples and three maize stalks were taken at random
from each plot and packaged in khakhi paper bags. The samples were then dried in an
oven at 70o C for 48 hours to determine moisture content at harvest. Maize cobs were
then shelled and shelling percentage was calculated as explained in section 3.2.1. Soil
samples were collected after harvest and analysed for N and P, using standard
procedures described in section 3.1.5 -7.
63
5.2.3 Determination of mineralization of sunnhemp, cowpea and hairy vetch
residues
A leaching tube experiment was setup using soil that was collected from Grasslands
Research Station from the plough layer (0 to 200 mm depth). The soil was air-dried
with stubble and roots removed by hand and sieved to pass through a 2 mm sieve.
Hairy vetch, sunnhemp, cowpea and maize residues were collected from the same
area where they had been grown with basal fertilization (section 4.2.2) before they
had senesced (at 60 DAP). They were dried and ground to 0.250 mm. Glass tubes of
130 mm length with a diameter of 40 mm and a thickness of 1 mm were used as
lysimeters. The lower end was closed with a small tube inserted in the centre for
drainage. At the lower end a gauze and glass wool were placed to prevent soil and
residue particles from washing out. Rates equivalent to 3 t ha-1 of residues were used
to reflect typical minimal field crop residue application rates. A mixture of 75 g soil
and 0.11 g of residues was placed in each tube. For the treatments with maize residues
and legume residues combined 0.056 g of each was mixed with the soil. Each
treatment mixture was thoroughly mixed in a glass jar before transferring to the tube.
The treatment mixture was placed in the tubes to fill about half the volume of the
columns and 10g of acid washed sand added at the top of the soil-residue mixer to
avoid disturbance of particles from the sample on pouring the leaching solution.
Aluminium foil was used to loosely cover the tops of each tube to minimise moisture
loss.
The experiment was arranged as a completely randomised design (CRD) with 4
replicates and 7 treatments:
i) Soil + hairy vetch residues
64
ii) Soil + cowpea residues
iii) Soil + sunnhemp residues
iv) Soil + hairy vetch + maize residues
v) Soil + cowpea + maize residues
vi) Soil + sunnhemp + maize residues
vii) Un-amended soil (Control)
Leaching tubes were randomised on racks and the experiment was incubated for 56
days. The tubes were periodically leached with 100 ml of leaching solution (0.01 M
CaCl2) in 50 ml aliquots at 0, 3, 7, 14, 28, 42 and 56 days. After each leaching the
moisture of the soil-sand mixture in the tube was brought back to approximately 70%
of the water holding capacity (WHC) and kept at room temperature. Collected
leachates were then analysed for mineral-N (NH4-N and NO3-N) and P (P2O5). The P
content of the leachates was determined by the colorimetric, molybdenum-blue
method of Murphy and Riley (1962). Mineral-N (NH4-N and NO3-N) in leachates was
determined colorimetrically according to Anderson and Ingram (1993).
Net N and P release was calculated by subtracting N or P released from the un-
amended soil controls from the N or P released from the residue-amended treatment.
Cumulative nutrient release was calculated by adding net nutrient release on each
leaching sampling day. On day 1, treatments were leached with 0.01 M CaC12 while
dry and then incubated moist. After each leaching event the tubes were subjected to
vacuum suction to bring the water content of each tube to approximately 75 % WHC
by extracting 25% of the moisture content of the soil at field capacity.
65
5.2.3.1 Determination of nitrate N in leachates
Colorimetric determination of nitrate was done according to Anderson and Ingram
(1993). Exactly 0.5 ml of leachate was micro-pippeted into a 40 ml test tube. To the
test tube 1.0 ml of 5% salicyclic acid solution was added and the test tube was shaken
on a vortex mixer and left to stand for 30 minutes. After 30 minutes 10.0 ml of 4 M
NaOH solution was added, mixed and left to stand for an hour. The sample
absorbance was then read at 410 nm on a UV-VIS Spectrometer. The above procedure
was done for standards prepared from KNO3 and a graph of absorbance against
standard concentration was plotted. A blank was also run using the extractant. Nitrate-
N was then calculated as shown in Equation 5.1 (Anderson and Ingram, 1993);
NO3- - N (µg g soil -1) = (C * V)…………………………………………. Equation 5.1
W
Where: C = corrected concentration (µg ml-1) V = extract volume (ml) W = weight of sample (g)
5.2.3.2 Determination of ammonium N in leachates
Reagent N1 was made by dissolving 34 g sodium salicylate, 25 g sodium citrate and
25 g sodium tartrate in 750 ml distilled water. Then 0.12 sodium nitroprusside was
added until dissolved and the solution was made up to the 1 L mark with distilled
water. Reagent N2 was made by dissolving 30 g sodium hydroxide in 750 ml water
and 10 ml of 5% sodium hypochlorite were added and made up to the 1L mark with
distilled water. Both reagents N1 and N2 were made 24 hours before use. Standards
were made by first drying 7 g (NH4)2 SO4 at 105o C for two hours and then cooling it
in a dessicator. About 4.714 g of the dry (NH4)2 SO4 were then dissolved in distilled
water in a 1L volumetric flask and distilled water was added up to the 1L mark to
make a 100 µg ml -1 NH4+ - N stock solution. Exactly 50 ml of the resultant solution
66
was then pipetted into a 500 ml volumetric flask and distilled water added up to the
500 ml mark to make a 100 µg ml -1 N solution. About 0, 5, 10, 15, 20 and 25 ml of
the 100 µg ml -1 N solution were each pipetted into 100 ml volumetric flasks and
leaching solution (0.01 M CaCl2) added up to the 100 ml mark to make standards
containing 0, 5, 10, 15, 20 and 25 µg ml NH4+ - N. From the standards and leachates,
0.1 ml was pipetted into 40 ml test tubes and 5 ml of reagent N1 was added, mixed
and left to stand for 15 minutes. After 15 minutes, 5 ml of reagent N2 was added,
mixed and left to stand for one hour. The sample absorbance, blank and standards
were then read on a UV-VIS spectrometer at 655 nm. Ammonium N was then
calculated as shown in Equation 5.2 (Anderson and Ingram, 1993);
NH4+ – N (µg g -1) = C x V …………………………………... Equation 5.2
W
Where C = corrected concentration (µg ml -1) V = extract volume (ml) W = weight sample (g) 5.2.3.3 Determination of Total N mineralized Total N mineralized was obtained from adding values of nitrate N and ammonium N
obtained from sections 5.2.3.1 and 5.2.3.2 respectively. Results were converted to mg
kg -1 from µg g -1 by using a convertion factor of 1.
5.2.3.4 Determination of P in leachates
Ascorbic acid solution was made by dissolving 1 g ascorbic acid in distilled water in a
100 ml volumetric flask. A molybdate reagent was made by weighing 4.3 g
ammonium molybdate into a 1L volumetric flask filled with 400 ml of distilled water.
Another solution was made by dissolving 0.4 g antimony sodium tartrate in a 500 ml
beaker containing 400 ml of distilled water and added to the moybdate solution in the
67
1L volumetric flask. To the resultant mixture, 54 ml of concentrated H2SO4 was
carefully added with stirring, allowed to cool then distilled water was added to the 1L
mark.
Phosphorus standards were made by dissolving 4.394 g of desiccated KH2PO4 in a 1L
volumetric flask and distilled water was added up to the 1L mark to make a 1000 µg
ml -1 P stock solution. About 10 ml of the 1000 µg ml -1 P solution were pipetted into
a 500 ml volumetric flask and the leaching solution (0.01 M CaC12) was added up to
the 500 ml mark to make a 20 µg ml -1 P solution. About 0, 5, 10, 15, 20 and 25 ml of
the 20 µg ml -1 P solution were pipetted into labeled 100 ml volumetric flasks and
0.01 M CaC12 was added to the 100 ml mark to make 0, 1, 2, 3, 4 and 5 µ g ml -1 P
standards.
Exactly 1 ml of each leachate and standard were pipetted into a 40 ml test tube, 4 ml
of ascorbic acid solution and 3 ml of molybdate solution were added and mixed well.
The solution was left to stand for one hour for the blue colour to develop. After full
colour development the samples were read on a UV-VIS spectrometer at 880 nm
wavelength. Sample P concentrations were then read from a standard graph plotted
for P concentration against absorbance (Anderson and Ingram 1993). The
concentration of P in the sample was then calculated as shown in Equation 5.3
(Okalebo et al. 2002).
P (mg kg-1) = (a-b) * v * f * 1000………………………………………... Equation 3.4 1000*w
Where: a = the concentration of P in the sample (in mg L-1) b = the concentration of P in the blank (in mg L-1) v = volume of the extracting solution (in ml) f = dilution factor w = weight of sample (in g).
68
5.3.4 Statistical analysis
Data on maize grain and stover yields and soil N and P was first checked for
normality by plotting normal graphs in Genstat 8th edition. An analysis of variance
(ANOVA) was then run on maize grain and stover yield, soil N and P data in Genstat
8th edition. Where differences in means were significant at 95% confidence interval,
least significant differences (LSD) were used to seperate the means.
5.4 Results
5.4.1 Rainfall
The rainfall pattern is previously described in section 4.2.2 and Figure 4.1.
5.4.2 Intercropped maize grain and stover yields
During the 2009/10 season in Wedza, maize that received basal and top dressing
fertilizer produced higher maize grain (3.14 t ha -1) compared to other treatments
except a basal fertilized maize-sunnhemp intercrop (2.6 t ha -1). At the same site, basal
fertilized intercrops of hairy vetch and cowpea produced grain yields that were 24%
and 21% less than maize that received basal and top dressing fertilizer respectively.
Unfertilised intercrops of hairy vetch, cowpea and sunnhemp produced grain yields
(1.52 – 1.96 t ha -1) that were not significantly different to unfertilised maize (1.81 t
ha -1). The trend observed in Wedza was the same as that observed in Chiota where
maize that received basal and top dressing fertilizer produced higher maize grain yield
(1.45 t ha -1) compared to all the other treatments. However, this was not significantly
different to basal fertilized intercrops of sunnhemp and cowpea that produced 1.36
and 1.25 t ha -1 respectively. Unfertilised intercrops of all legumes had maize grain
yields (0.29 – 0.34 t ha -1) that were not significantly different to unfertilised maize
(0.26 t ha -1). At GRS during the 2009/10 season, basal fertilized legume intercrops
69
were not significantly different (2.36 – 2.44 t ha -1) to maize that received basal and
top dressing fertilizer (2.04 t ha -1).
The maize crop in Chihota suffered excessive moisture stress during the 2010/2011
mid-season drought and as a result the crop failed to produce any grain for this
season. However in Wedza during the 2010/11 season, basal fertilized
sunnhemp/maize intercrop produced higher maize grain (2.27 t ha -1) than other
treatments except maize that received basal and top dressing fertilizer (1.85 t ha -1)
which was not significantly different to the basal fertilized sunnhemp/maize intercrop.
Basal fertilized maize-legume intercrops of hairy vetch and cowpea had maize grain
yields that were 39% and 52% less than a basal fertilized maize/sunnhemp intercrop.
All unfertilised maize/legume intercrops produced maize grain yields (0.57 – 0.92 t ha
-1) that were not significantly different to unfertilised maize (0.83 t ha -1).
The trend in maize grain yields at GRS during the 2010/11 season was the same as
that observed in Wedza during the same period despite the fact that grain yields at
GRS were very low (> 0.7 t ha -1). During the 2011/12 season at GRS maize that
received basal and top dressing fertilizer had maize grain yields that were at least 25%
higher than all the basal fertilized maize/legume intercrops. Unfertilised
maize/legume intercrops produced maize grain yields that were not significantly
different to the unfertilised maize (0.48 t ha -1). Generally across all sites, unfertilised
maize and unfertilised maize/legume intercrops yielded the lowest maize grain yields.
In Wedza during the 2009/10 season, a basal fertilized maize-sunnhemp intercrop
produced the highest maize stover (1.98 t ha -1) followed by maize that received basal
70
and top dressing fertilizer (1.39 t ha -1). Unfertilised maize/legume intercrops had
maize stover that was at least 76% higher than unfertilised maize. In Chiota during the
2009/10 season, unfertilised maize had maize stover yields (2.07 t ha -1) that was not
significantly different to all the other treatments. However at GRS, basal fertilized
maize/legume intercrops had significantly (P < 0.05) more stover biomass (251 -
283%) than unfertilised maize (0.8 t ha -1) and was not significantly different to maize
that received basal and top dressing fertilizer (2.8 t ha -1).
During the 2010/11 season, a basal fertilized hairy vetch intercrop produced more
maize stover biomass than all the other treatments in Wedza and at GRS (1.98 and
3.06 t ha -1 respectively). However in Chiota, maize that received basal and top
dressing fertilizer and a basal fertilized sunnhemp/maize intercrop had maize stover
yields that were 28% higher than unfertilised maize. During the 2011/12 season at
GRS, maize that received basal and top dressing fertilizer produced 159% more maize
stover than unfertilised maize but was not significantly different to basal fertilized
maize, unfertilised hairy vetch/maize intercrop and basal fertilized maize intercrops of
sunnhemp and cowpea.
5.4.3 Intercropped legume biomass yields
During the 2009/10 season at Wedza, basal fertilized sunnhemp produced biomass of
4.56 t ha -1 followed by unfertilised sunnhemp (2.01 t ha -1), basal fertilized cowpea
(1.9 t ha -1) and hairy vetch produced 0.95 t ha -1 (Table 5.2). The trend was the same
for the 2010/11 season at Wedza. However, in Chihota for the 2009/10 and 2010/11
seasons biomass was in the order basal fertilized sunnhemp > basal fertilized cowpea
> unfertilised sunnhemp > unfertilised cowpea > basal fertilized hairy vetch.
71
Table 5.1 Maize grain and stover yields at Wedza, Chihota and Grasslands Research Station (GRS) between the 2009/10 and 2011/2012 seasons.
a,b,c,d – letters with the same superscript letter are not significantly different (P > 0.05) from each other, AN – ammonium nitrate fertilizer (34.5
% N), NPK – compound D fertilizer (7 N: 14 P2O5: 7 K2O), nd - not determined
Treatment
2009/2010 2010/2011 2011/2012
Wedza Chihota GRS Wedza Chihota GRS GRS
Grain
yield
(tha-1)
Stover
yield
(tha-1)
Grain
yield
(tha-1)
Stover
yield
(tha-1)
Grain
yield
(tha-1)
Stover
yield
(tha-1)
Grain
yield
(tha-1)
Stover
yield
(tha-1)
Grain
yield
(tha-1)
Stover
yield
(tha-1)
Grain
yield
(tha-1)
Stover
yield
(tha-1)
Grain
yield
(tha-1)
Stover
yield
(tha-1)
Maize / Hairy vetch, no fertilizer
1.96a 1.06bc 0.33a 2.04abc 1.52b 1.36b 0.92ab 1.06bc nd 0.45abc 0.14ab 1.36b 0.73bc 1.66cd
Maize / Cowpea, no fertilizer
1.52a 0.95bc 0.29a 1.90ab 0.61a 1.41b 0.70ab 0.93bc nd 0.30a 0.26abcd 1.75b 0.38a 0.99ab
Maize / Sunnhemp, no fertilizer
1.64a 0.90b 0.34a 2.06abc 0.87a 1.32a 0.57a 0.90b nd 0.49bc 0.23abc 1.32b 0.37a 1.01ab
Maize / Hairy vetch + NPK fertilizer
2.40c 1.05bc 0.95b 2.20bc 2.36c 3.06d 1.39c 1.98d nd 0.50bc 0.43cde 3.06d 0.35a 1.27bc
Maize / Cowpea + NPK fertilizer
2.47c 0.99bc 1.25bc 1.69a 2.35c 2.81cd 1.09bc 0.99b nd 0.40ab 0.34bcd 2.81cd 0.48ab 1.70cd
Maize / Sunnhemp + NPK fertilizer
2.60cd 1.98e 1.36c 2.33bc 2.44c 2.93cd 2.27d 0.95b nd 0.60c 0.64e 2.93d 1.04cd 1.73d
Maize + NPK fertilizer 2.44c 1.23c 1.01b 1.98abc 1.62b 2.39c 0.97abc 1.23bc nd 0.48bc 0.14ab 2.39c 0.98cd 1.67cd
Maize + AN 2.22bc 1.15bcd 0.33a 2.37c 1.61b 1.39b 0.59a 0.51a nd 0.42ab 0.22abc 1.39b 1.22de 1.52cd
Maize + NPK + AN 3.14d 1.39d 1.45c 2.15bc 2.04c 2.80cd 1.85d 1.39c nd 0.61c 0.48de 2.80cd 1.40e 1.94d
Maize, no fertilizer 1.81ab 0.51a 0.26a 2.07abc 0.95a 0.80a 0.83ab 1.15bc nd 0.47ab 0.07a 0.80a 0.48ab 0.75a
LSD 0.56 0.318 0.320 0.446 0.40 0.535 0.45 0.33 nd 0.17 0.23 0.49 0.31 0.45
72
A sunnhemp-maize intercrop that received NPK basal fertiliser yielded at least 18%
higher biomass than unfertilised sunhemp and other legumes at all sites over the two
seasons. The sunnhemp intercrop grew to the height of maize (Plate 5.1) while
cowpea and hairy vetch remained inconspicuously below the foliage of the maize
intercrop. Legume intercrops that received basal fertiliser produced significantly (P <
0.05) more biomass than those that did not.
Table 5.2 Biomass production of legumes relay intercropped into maize in Wedza, Chiota and at Grasslands Research Station during the 2009/2010 and 2010/2011 seasons
Legume in intercrop Biomass production t ha -1 Wedza Chihota GRS
2009/2010 2010/2011 2009/2010 2010/2011 2009/2010 2010/2011 Unfertilised hairy vetch/ maize 0.26 0.29 0.12 0.26 0.33 0.56
Hairy vetch / maize + NPK 0.95 0.62 0.50 0.56 0.70 1.64
Unfertilised cowpea 1.0 1.71 0.76 0.74 0.74 1.30
Cowpea / maize + NPK 1.90 1.96 1.08 2.95 1.11 2.18
Unfertilised sunnhemp / maize 2.01 2.75 0.82 1.17 1.73 3.48
Sunnhemp / maize + NPK 4.56 3.24 1.57 3.75 3.75 4.29
LSD 0.32 0.25 0.29 0.22 0.40 0.45 GRS – Grasslands Research Station, NPK – compound fertilizer (7N : 14 P2O5 : 7K2O) 5.4.4 Soil available phosphorus and nitrogen after legume intercrops.
There were significant differences in available phosphorus at all locations after
various legume relay intercrops except at GRS in the 2009/2010 and 2010/2011
seasons (Table 5.3). During the 2009/10 season in Wedza all basal fertilized and
unfertilised maize-legume intercrop plots had at least 43% more available soil P than
unfertilised maize except unfertilised maize/sunnhemp intercrop that was not
significantly different to the control. During the same season in Chihota, all basal
fertilized and unfertilised maize-legume intercrop plots had at least 47% more
73
Plate 5.1 Biomass production of sunnhemp when intercropped with maize in Wedza communal area during the 2010/11 season. The intercrop: (a) received NPK fertiliser (b) was unfertilised
(a) (b)
74
available soil P compared to unfertilised maize. Differences in available soil P at GRS
were not significant (P > 0.05) during the 2009/10 season. During the 2010/11 season
in Wedza a basal fertilized maize/sunnhemp intercrop had the highest available soil P
(7.29 mg P kg -1) followed by basal fertilized maize/hairy vetch intercrop (5.93 mg P
kg -1). An unfertilised maize/sunnhemp intercrop (2.9 mg P kg -1) was not
significantly different to unfertilised maize (3 mg P kg -1). During the 2010/11 season
in Chihota, basal fertilized maize/sunnhemp intercrop plots had the highest available
soil P (6.72 mg P kg -1) followed by a basal fertilized maize/cowpea intercrop (5.35
mg P kg -1). All unfertilised maize/legume intercrops were not significantly different
(3.73 – 4.84 mg P kg -1) to unfertilised maize (3.9 mg P kg -1). Treatment effects on
available soil P were not significant (P > 0.05) at GRS during the 2010/11 season.
However during the 2011/12 season a basal fertilized maize/sunnhemp intercrop had
the highest available soil P (5.6 mg P kg -1) followed by a basal fertilized
maize/cowpea intercrop (5.28 mg P kg -1).
During the 2009/10 season treatment effects on total soil N were not significant (P >
0.05) at the three study sites (Table 5.3). During the 2010/11 season, there were also
no significant differences in total N in Wedza. Although significant differences
existed during the 2010/11 in Chihota and GRS, at both sites there were no significant
differences between unfertilised maize and basal fertilized and unfertilised
maize/legume intercrops (Table 5.3). During the 2011/12 season at GRS treatment
effects on total soil N were not significant.
The trend in soil mineral N was highly variable with varying sites and seasons (Table
5.4). In Chihota during the 2010/2011 season, soil mineral N was not significantly
75
Table 5.3 Soil available P and total N after maize/legume relay intercrops in Wedza, Chihota and at Grasslands Research Station (GRS) during the 2009/10 to 2011/12 seasons.
MZ – maize, HV – hairy vetch, CP – cowpea, SH – sunnhemp, NPK – compound D basal fertilizer (7 N: 14 P2O5: 7 K2O), AN – ammonium nitrate fertilizer (34.5% N)
Treatment
2009/2010 2010/2011 2011/2012
Wedza Chihota GRS Wedza Chihota GRS GRS
Avail
P (mg
P/kg)
Total
N %
Avail
P (mg
P/kg)
Total
N %
Avail
P ( mg
kg -1)
Total
N %
Avail
P (mg
P/kg)
Total
N %
Avail
P (mg
P/kg)
Total
N %
Avail
P (mg
P/kg)
Total
N %
Avail P
(mg
P/kg)
Total
N %
MZ + HV 10.17cd 0.035 11.65ab 0.030 9.25 0.056 12.8cd 0.030 8.55a 0.027ab 8.53 0.059ab 7.96ab 0.054
MZ + CP 9.67cd 0.034 10.34ab 0.026 10.75 0.050 10.18bc 0.030 11.10ab 0.029ab 10.43 0.049a 10.26abcd 0.054
MZ + SH 5.73a 0.037 13.31bc 0.025 8.97 0.057 6.64a 0.029 8.94a 0.026ab 9.77 0.058ab 7.74ab 0.059
MZ + HV + NPK 9.97cd 0.035 11.58ab 0.032 8.96 0.052 13.6de 0.032 11.49ab 0.028ab 9.53 0.052a 10.78bcd 0.061
MZ + CP + NPK 12.27d 0.034 12.31b 0.027 10.84 0.054 10.38e 0.030 12.27b 0.028ab 11.87 0.054ab 12.11cd 0.054
MZ + SH + NPK 9.13abc 0.034 17.90c 0.029 9.54 0.054 16.72e 0.030 15.41c 0.028ab 11.70 0.055ab 12.85d 0.054
MZ + NPK 11.9d 0.035 12.27b 0.032 10.30 0.053 11.1cd 0.029 12.08b 0.033b 11.03 0.053a 9.22abc 0.057
MZ + AN 8.17abc 0.031 15.19bc 0.033 10.37 0.057 7.88ab 0.028 10.51ab 0.031ab 10.57 0.061b 7.30a 0.057
MZ + NPK + AN 8.83abcd 0.033 12.27b 0.028 10.01 0.053 11.76cd 0.033 10.67ab 0.030ab 10.63 0.055ab 10.27abcd 0.055
MZ 6.4ab 0.038 7.00a 0.030 9.66 0.056 6.88a 0.030 8.94a 0.025a 10.33 0.057ab 9.89abcd 0.057
LSD 3.59 ns 4.974 ns Ns ns 3.17 ns 3.095 0.007 ns 0.007 3.12 ns
76
Table 5.4 Soil mineral N after a maize-legume intercrop in Wedza, Chihota and at Grasslands Research Station (GRS).
Treatment Site (Soil Mineral N (mg N/kg))
Wedza Chihota Grasslands Research Station
(2009/2010) (2010/2011) (2009/2010) (2010/2011) (2009/2010) (2010/2011) (2011/2012)
MZ + HV 10.33bc 9.33c 20.67bc 5.00 10.00bc 11.00bc 7.67bc
MZ + CP 4.00a 11.00c 11.67a 5.67 12.00cd 17.33cd 14.67d
MZ + SH 12.00bcd 21.33d 11.33a 4.00 6.33ab 14.33bcd 7.67bc
MZ + HV + NPK 15.67d 1.67a 21.67c 3.00 2.33a 2.67a 11.00cd
MZ + CP + NPK 12.33cd 11.67c 13.33ab 3.33 3.67a 1.33a 3.33a
MZ + SH + NPK 5.67a 3.33ab 18.67abc 3.00 13.33cd 19.00d 4.00ab
MZ + NPK 7.67ab 11.67c 20.00bc 6.00 5.67ab 9.67b 3.00a
MZ + AN 13.67cd 10.67c 18.00abc 3.67 16.00d 14.00bcd 14.33d
MZ + NPK + AN 13.67cd 5.33b 35.00d 3.67 2.33a 13.33bcd 7.00abc
MZ 13.00cd 2.00a 20.67bc 3.00 5.67ab 14.67bcd 14.33d
LSD 4.58 3.21 7.51 ns 4.43 6.62 4.32
MZ – maize, HV – hairy vetch, CP – cowpea, SH – sunnhemp, NPK – compound D basal fertilizer (7 N: 14 P2O5: 7 K2O), AN – ammonium nitrate fertilizer (34.5% N)
different with varying treatments. At GRS during the 2010/2011 season the basal
fertilised maize-sunnhemp treatment had the highest soil mineral N (19 mg N kg -1)
while in the 2011/2012 season it had one of the lowest soil mineral N (4 mg N kg -1).
5.4.5 Third season intercropped maize grain N accumulation at GRS
There was a significant difference (P < 0.05) in maize grain N during the 2011/2012
season at GRS and higher grain N content was observed in the maize + AN (14.63 mg
g -1), maize + NPK fertiliser + AN (14 mg g -1) and basal fertilised maize/sunnhemp
intercrop (13.93 mg g -1) (Figure 5.1). The lowest grain N contents were obtained
77
from unfertilised intercrops, unfertilised maize and basal fertilized maize (10.93 –
12.23 mg g -1).
Figure 5.1 N content of maize seed at GRS in the 2011/2012 season. Bars with the same letters are not significantly different from each other.
There was a significant (P<0.05) difference in maize grain N uptake with various
treatments. Fully fertilised maize yielded significantly the highest grain N uptake
(19.48 kg N ha -1) followed by top dressed maize (18.04 kg N ha -1) and basal
fertilised maize/sunnhemp (14.56 kg N ha -1) (Figure 5.2). Very low grain N uptake
was observed for unfertilised maize/cowpea and maize/sunnhemp intercrops and basal
fertilised maize/hairy vetch intercrop which yielded 4.41, 4.13 and 4.34 kg N ha -1
respectively.
0
2
4
6
8
10
12
14
16
18
Maize +HV
Maize +CP
Maize +SH
Maize +HV + D
Maize +CP + D
Maize +SH + D
Maize Maize +AN
Maize +D + AN
Maize +D
Treatment
Gra
in N
mg
g -1
2 X SEM
aabab ab b
cd
ab
d d
ab
78
Figure 5.2 Grain N uptake of maize at GRS in the 2011/2012 season. Bars with the same letters are not significantly different from each other.
5.4.6 N and P mineralization of legume residues
There was net mineral N from the soil residue mixtures at the start of the incubation
and treatment effects were significant (P < 0.05) on each leaching day. Sole legume
residues and their mixtures with maize did not exhibit a net immobilisation phase;
however the (1:1) legume-maize mixtures of hairy vetch and sunnhemp exhibited a
decrease in net N mineralization on day 3 from 3.23 and 3.52 mg kg -1 to 2.86 and
1.38 mg kg -1 respectively (Figure 5.3). Sole legume residues showed a positive trend
in net N mineralisation from day 0 while net N mineralization of legume-maize
mixtures only started to increase from day 7. At each leaching interval from day 3,
sole legumes had at least 18% more mineralised N than mixtures of maize and legume
residues. However, net N mineralized by the maize/cowpea residues was not
significantly different to those of sole legumes on day 3 and 14. Sole hairy vetch
residues yielded the highest amount of mineral N (45.5 mg kg-1 soil) with the
0
5
10
15
20
25
Maize +HV
Maize +CP
Maize +SH
Maize +HV + D
Maize +CP + D
Maize +SH + D
Maize Maize +AN
Maize +D + AN
Maize +D
Treatment
Gra
in N
upt
ake
kg h
a -1
2 X SEM
bc
a a aab
de
ab
eff
cd
79
sunnhemp and maize (1:1) mixture yielding the lowest (28.76 mg kg-1 soil) after the
56 day incubation period.
Figure 5.3 Net N mineralization patterns of cowpea, sunnhemp and hairy vetch residues when incubated solely and in combination with maize residues. Error bars represent LSDs at 95% confidence interval.
Figure 5.4 Net P mineralization patterns of cowpea, sunnhemp and hairy vetch residues when incubated solely and in combination with maize residues. Error bars represent Least Significant Differences (LSD) at 95% confidence interval.
0123456789
10111213
0 5 10 15 20 25 30 35 40 45 50 55 60
Net
N m
iner
alis
ed (m
g kg
-1)
Time (days)
Hairy vetch
Cowpea
Sunnhemp
Hairy vetch + maize
Cowpea + maize
Sunnhemp + maize
-2
-1
0
1
2
3
4
5
6
7
8
9
10
11
0 5 10 15 20 25 30 35 40 45 50 55 60
Net
P m
iner
alis
ed (m
g kg
-1)
Time (days)
Hairy vetch
Cowpea
Sunnhemp
Hairy vetch + maize
Cowpea + maize
Sunnhemp + maize
80
On day 3 there was net P immobilisation for all the other residue treatments except
sole hairy vetch (Figure 5.4). From day 3 net mineralised P was not significantly
different for all the treatments up to day 56.
5.5 Discussion
Maize grain yields at all sites during the 2010/2011 season were lower compared to
the 2009/2010 season despite the 2010/2011 season receiving more rainfall. This was
most likely due to the fact that rainfall distribution during the 2010/2011 season was
more erratic. Basal fertilised maize/legume intercrops yielded the same maize grain
yield as mono-cropped maize that received NPK fertiliser and ammonium nitrate at
most sites in the first two seasons showing that legume intercropping can improve
maize grain yields from the first season of cropping to almost match the N needs of a
companion maize crop. However, Kamanga et al. (2003) working in a drier region of
Zimbabwe (natural region IV) reported no maize grain yield when sunnhemp was
intercropped with maize in the Zimuto communal area in Zimbabwe. A likely reason
for this could be that sunnhemp is less suited to drier regions of Zimbabwe.
Low yields obtained from the unfertilised maize control confirm that due to the
inherent low fertility nature of soils, maize grain yields are compromised without
external fertiliser additions. Maize-legume intercrops that were not basal fertilised had
the same maize grain yields compared to the unfertilised maize control. This shows
that maize grain yield benefits of relay intercropping are reduced when intercrops are
not basal fertilised. Intercropping increases soil nutrient and water demand per unit
area and can limit not only maize growth but also legume growth and N2 fixation if
not basal fertilized (Fujita et al. 1992). Legume biomass yields in intercrops were
81
generally low (0.26 – 4.56 t ha -1) primarily because the legume plant populations in
the intercrops were lower compared to sole legumes.
Hairy vetch and cowpea suffered more from shading due to their low stature
compared to sunnhemp that outgrew the maize in the intercrop. These observations
are in agreement with Jeranyama et al. (1998) who reported declining biomass yield
of cowpea in a maize intercrop when N was applied. In the authors’ study, sunnhemp
was not affected because it was still able to intercept light when maize had grown
vigourously due to external N additions.
During the 2010/2011 season at sites where there were significant changes in soil P,
the maize-sunnhemp intercrop that received NPK basal fertiliser had significantly
higher soil P at all sites (Table 5.3). A probable explanation of this could be that
sunnhemp’s P uptake was higher because labile P was provided in the basal fertiliser
and decomposition of leaf litter falling during the growing season mineralized P.
Increased P uptake could also be partly explained by the ability of sunnhemp to access
a larger rhizosphere by developing roots in deeper soil layers. In a study by Miyazawa
and Murakami (2010) sunnhemp distributed roots deeper than sorghum and sunflower
and stimulated deeper root development of sorghum when intercropped.
Soil total N was generally not significantly different even when the treatments were
maintained for a third season at GRS. Soil total N is known to change gradually with
various fertility management options. The trends in soil mineral N levels were
difficult to establish at the three sites because not only was the trend different at each
site but also at GRS it was different each season. The response of soil mineral N
82
levels to maize/legume intercropping is highly influenced by rainfall, soil
characteristics and overall biomass production of the intercropping system. The
interaction of such variables is likely to have influenced trends at each site. Chikowo
et al. (2004b) reported mineral N flushes at the beginning of the season and
diminishing mineral N three weeks after planting up to when mineral N had been
depleted such that no significant differences existed in differently managed plots.
At GRS, unlike the other sites where each year from July cattle free range the fields,
consuming what is left of planted crops. Legume and maize residues were left in the
field and only ploughed under at the onset of the next growing season. The high maize
grain N content observed for the basal fertilised maize/sunnhemp intercrop may be
partly explained by that sunnhemp was effective in providing available N to a third
season maize-sunnhemp intercrop. The increase in maize grain N content of the basal
fertilised maize-sunnhemp treatment compared to other basal fertilised intercrops is
likely due to the release of N from decomposition and mineralization of sunnhemp
and maize residues (Mtambanengwe and Mapfumo, 2006). This agrees with the
studies by Jeranyama et al. (2000) in Zimbabwe who found that maize grain N uptake
was slightly greater with a sunnhemp intercrop than without. However, higher maize
grain N uptake was observed for maize that received basal and top dressing fertilizer
which was not significantly different from maize that received top dressing fertilizer
only and basal fertilised maize/sunnhemp intercrop was third.
Approximately 44 mg N kg -1 was added to the leaching tubes as sole legumes which
translate to biomass additions of about 3 t ha-1 in the field. The 3 tons are a minimal
average of what communal farmers are likely to get in their fields. Nitrogen
83
mineralization of sole and mixed residues reached slightly less than 50% of applied N
by day 14. It is evident from the results that when legume residues are mixed with
maize residues there will be a reduction in the net N mineralization rate especially in
the first 3 days; however net N mineralization then steadily increased afterwards.
Nitrogen immobilization by microorganisms due to an increased C:N ratio when
legume residues are mixed with cereal residues that have low N could partly explain
this. Sakala et al. (2000) also found that when senesced pigeon pea leaves were mixed
with maize residues there was a strong interaction resulting in a general N limitation.
However in the study by Sakala et al. (2000) maize stover was 3 times more than
pigeon pea leaves.
Mineralization of plant residues in soil is not only dependant on quality and quantity
of the plant material but also decomposer organisms and environmental conditions
(Tian et al. 1997). The leaching tube technique employed in this study provides near
optimum moisture and temperature conditions for mineralization which is not the case
in the field. Leaching tube incubations do not factor in the effect of soil organisms
that enhance the biodegradation and humification of organic residues. However the N
mineralization kinetics information from such a study is important in obtaining choice
of legumes through comparisons in cropping systems (Ranzluebbers et al. 1994;
Thippayarugs et al. 2008).
From this study it was shown that hairy vetch mineralises N faster than cowpea and
sunnhemp. Net P immobilisation observed for all other treatments at day 3 and
subsequent minimal increases in net P mineralization indicate that at 3 t ha -1 legume
residues studied will not be effective in providing P to crops in a sandy granitic soil.
84
Reasons for low net P mineralization could be that the soil used was already low in P
and was an acidic soil with high P fixation capability. Early studies by Sibanda and Le
Mare (1984) showed that P fixation in Harare 5E.2, Marondera 7G.2 and Chipinge
7E.2 soils was more than 200 µg P g-1 soil. From the lysimetry experiment it is
evident that falling legume leaf litter if incorporated into soil for instance during
weeding, might quickly decompose and mineralise N to provide N to a companion
maize crop. The quantities of N that can be provided to a maize companion crop
through such a way still need to be further quantified.
5.6 Conclusions
The study showed that legume relay intercropping without basal fertilization will not
improve maize grain yields. However when basal fertilized, all the legumes tested can
improve maize grain yield to match maize that received basal and top dressing
fertilizer. Generally across the sites and over the three seasons, basal-fertilised
sunnhemp intercrop was superior to other legumes in improving maize grain yields.
In this regard sunnhemp is a better crop to include in a relay intercropping system
compared to hairy vetch and cowpea. Sunnhemp intercropping with NPK fertiliser
also was more effective than cowpea and hairy vetch in improving soil available P
and maize grain N uptake.
Differences in net N mineralization between the sole legumes were not significant
showing that all the legumes studied can supply N to a companion maize crop through
N mineralization of leaf litter. However at field application rates of 3 t ha -1 the
legume residues studied mineralise very little P. A further study that evaluates the
85
effect of different quantities of legume residues on P mineralization in P impoverished
acidic soils is recommended.
86
CHAPTER 6*
EFFECTIVENESS OF Rhizobium leguminosarum bv. Viceae STRAINS IN
NODULATING HAIRY VETCH ( Vicia villosa Roth) IN THE SANDY SOILS
OF ZIMBABWE
6.2 Introduction
Legume productivity is limited by prevailing soil conditions such as nutrients
deficiency, high soil acidity and low indigenous rhizobia population, including poor
infectivity and effectiveness (Wollum 1998, Cheming’wa and Vessey 2006). The
benefits derived from the use of legume crops in soil fertility improvement are
reduced because of these soil conditions. Soils in most Zimbabwean smallholder
farming areas are of poor quality making it difficult for farmers to practice sustainable
crop production. Low pH (< 5) is an important constraint in these soils (Dhliwayo et
al. 1998) and as shown by earlier research, liming is necessary to increase the
availability of molybdate and phosphate (Tanner and Grant 1977, Tanner 1982) that
are highly essential for effective biological nitrogen (N2) fixation.
A realistic option to improve soil fertility is the use of legumes that are more tolerant
to acidic soil conditions, such as hairy vetch (Vicia villosa Roth) (Dastikaitė et al.
2009) than most common legume crops, such as soyabean and groundnut. Hairy vetch
is often described as one of the best legumes in its ability to be productive in low soil
fertility or acid soils, with some varieties showing considerable germination strength
*This chapter has been published as: Tumbure A, Wuta M & Mapanda F (2013) Preliminary evaluation of the effectiveness of Rhizobium leguminosarum bv. viceae strains in nodulating hairy vetch (Vicia villosa) in the sandy soils of Zimbabwe, South African Journal of Plant and Soil, 30:4, 233-239, DOI: 10.1080/02571862.2013.868536
87
at pH as low as 3.3 (Dastikaitė et al. 2009). It is a temperate legume that is suggested
as a preferred legume cover crop to supply N in maize production systems (Jost 1998,
Czapar et al. 2002). In a six-year study on cotton in tropical Australia, Rochester and
Peoples (2005) found that hairy vetch added 33% more N to the soil than faba beans
and increased the crop yield potential by up to 13% relative to the non-legume
system. In the Americas, Parr et al (2011) estimated hairy vetch cover crop biomass N
to fall between 100 and 230 kg N ha-1 and N2 fixed by hairy vetch under field
conditions was about 110 kg N ha–1 yr –1 (Burton 1984). Lanyasunya et al. (2007)
demonstrated that in a tropical climate (Kenya) irrigated hairy vetch can yield dry
matter of up to 9.5 t ha–1 and sole hairy vetch produced dry mass yields of up to 6.14 t
ha–1 (Shobeiri et al. 2010) and 4.47 t ha–1 (Anugroho and Kitou 2011), for Iran and
Japan respectively.
In smallholder areas of Zimbabwe hairy vetch shows great potential for use in soil
fertility improvement, reduction of soil erosion and also as a good protein source for
dairy cattle. Hairy vetch protein content ranges from 12 to 20 % (2.5-3.5 % N)
depending on age and makes good hay, silage, pasture and green manure (Shobeiri et
al. 2010). Studies by Lanyasunya et al. (2007) in Kenya showed that V. villosa has
adequate mineral content for dairy and beef cattle except for Cu and Mg. The same
authors attributed the Mg and Cu deficiency to the high pH (7.4) in that soil. Other
benefits of hairy vetch include reducing weed infestation by forming a dense
groundcover in fields (Baldwin and Creamer 1999) and facilitating the availability of
potassium to other shallower-rooted crops (Undersander et al. 1990). Lybecker et al.
(1988) noted that hairy vetch cover crops can reduce weed biomass by up to 96%.
88
In order to realize high biomass yields hence soil fertility benefits from growing hairy
vetch, it must be properly nodulated by an efficient Rhizobium strain for N2 fixation.
Hairy vetch nodulates with Rhizobium leguminosarum strains that are in the pea
(Pisum spp) cross-inoculation group (Somasegaran and Hoben 1994). However, since
indigenous rhizobia populations are usually of varied populations and effectiveness,
use of high quality rhizobial inoculants is ideal in order to maximize N contributions
to farming systems (McInnes et al. 2004). Indigenous rhizobia can be abundant but
un-infective and/or inefficient in fixing nitrogen from the atmosphere (Chemining’wa
and Vessey 2006), hence the need to select effective strains.
Rhizobial strain selection is founded on the principle that certain strains are better
suited for N2 fixation in particular environments than others and highly effective
strains can be obtained through a selection program (Hungria et al. 2000). Screening
of Rhizobium leguminosarum bv. vicea strains is therefore essential in selecting the
best strain/s that effectively nodulate and fix N2 under Zimbabwean conditions. While
research in tropical conditions has centered on hairy vetch’s mineral content for dairy
and beef cattle (Lanyasunya et al. 2007), literature on the amount of N2 fixed by hairy
vetch in tropical conditions is limited.
The aim of this study was to compare the efficacy of Rhizobium leguminosarum bv.
viceae strains found at the local strain bank at Grasslands Research Station (GRS) in
Marondera, Zimbabwe, on nodulation and N2 fixation of hairy vetch on sandy soils.
An evaluation of these strains will allow recommendations for selected strains to be
used for hairy vetch inoculant production.
89
6.3 Materials and methods
6.3.1 Study site and rainfall
The study was conducted in Zimbabwe at Grasslands Research Station (GRS). The
site is previously described in section 3.1. Rainfall distribution was erratic within each
month and 7 continuous days without precipitation were common in each month. At
the time the field trial was planted on 4 January 2012, 47% of the total season’s
precipitation had already fallen (Figure 1). Effectively, hairy vetch in the field trial
received 743 mm of rainfall. There was a week without precipitation from the time
that hairy vetch was planted in the field and from 18 February up to March 2012 there
were 18 continuous days without any precipitation.
Figure 6.1 Daily rainfall distribution and the times of crop planting and harvesting during 2011/2012 cropping season (1 October 2011 to 5 May 2012) at the experimental site at Grasslands Research Station in Marondera.
6.3.2 Culture preparation
Bacteria strains used in this research were MAR 833 from Sydney, Australia (391/SU,
1993), MAR 346 from Pretoria, South Africa (1993), and MAR 1504 from nifTAL,
Hawaii (638/TAL, 1993). These strains were selected because they have been shown
0
20
40
60
80
100
120
140
1/10/11 31/10/11 30/11/11 30/12/11 29/01/12 28/02/12 29/03/12 28/04/12
Time (day/month/year)
Rai
nfal
l (m
m d
ay -1
)
Planting Harvest
90
to be highly effective on pea (Pisum sativum) and were originally isolated from
tropical climates and might be better adapted to local conditions (Marufu et al. 1995)
The laminar flow cabinet was sanitized by spraying 75% alcohol and wiping surfaces
with clean cotton wool dipped in 75% alcohol before commencement of microbiology
work. Rhizobia were transferred in a laminar flow cabinet from agar slant bottles
containing yeast extract mannitol (YEM) using inoculation loops sterilised by running
through a Bunsen flame into medical flat bottles containing YEM broth. The starter
culture in the medical flat bottles was then incubated at 28 °C for six days in a
Precision scientific ® 815 laboratory incubator (Somasegaran and Hoben 1994).
YEM broth consisted of 10 ml of 5 % K2HPO4 in distilled water (Solution A), 10 ml
of 1 % FeCl3.6H2O in distilled water (Solution B), 1 ml of 2 % MgSO4.7H2O and 1 %
NaCl both dissolved in distilled water (Solution B), 10 g of mannitol and 0.18 g of
CaCO3 and 100 ml of yeast water. The solutions were added to a 1 L volumetric flask
and made up to the 1 L mark by adding distilled water and stirred using a mechanical
stirrer for one hour. For YEM agar an additional 15 g of Oxoid agar was added to the
solution.
Rhizobia cell counts were done by counting colonies on a colony counter, bacteria
was grown on a YEM agar plate supplemented with Congo red after serial dilutions
(up to 10 -7) in sterile water (Somasegaran and Hoben, 1994). Inoculants were used as
liquid cultures with at least 109 rhizobial cells ml -1.
91
6.3.3 Greenhouse experiment
A greenhouse experiment was run using natural (unsterilised) field soil from June
2011 up to 80 days after planting (DAP). Soil for pots was collected from a depth of
15 cm at GRS and sieved through a 5 mm mesh screen to remove stones and twigs.
Half of the soils were limed with dolomitic limestone containing approximately 78%
CaCO3, 16% MgCO3 and 6% impurities at a rate of 800 mg kg -1 soil and allowed to
equilibrate for 14 days to bring the pH to 6.5. Soil pH was measured using 15g of soil
in 75ml of 0.01 M CaCl2 solution. Each pot received 4 kg of soil and 21 mg kg–1 soil
of phosphorus (P). The experiment was arranged as a completely randomized design
in a factorial arrangement with treatments replicated four (4) times. Three (3) strains
were tested with a negative control in limed and acidic soil to give eight (8)
treatments: MAR 833 strain, MAR 346 strain, MAR 1504 strain, No inoculation,
MAR 833 strain + lime, MAR 346 strain + lime, MAR 1504 strain + lime and no
inoculation + lime.
Ten seeds of hairy vetch seed were planted in each pot and the various bacterial
strains were applied as liquid inoculants (1 ml seed -1) to their respective pots at
planting. At two weeks after planting hairy vetch plants were thinned to five plants
per pot and a further application of rhizobium culture (1 ml plant -1) was added at two
weeks after germinating. Each plant received an excess of 109 viable rhizobial cells in
each inoculation. Pots receiving no inoculant had the equivalent volume applied as
distilled water. During the initial phase of seed germination and seedling
establishment the soil moisture was maintained at field capacity using distilled water.
Thereafter, samples showing vigorously growing plants were weighed to determine
the volume of water needed to replace the water lost.
92
Plants were harvested at 80 days after planting (DAP). The number of nodules and
nodule mass per plant were recorded. A visual assessment of whether a nodule was
active was done by checking if the nodules had the pink, red, or brown coloration
typical of actively N-fixing nodules. Shoot and root dry mass were recorded after
drying the plants at 60°C to a constant mass. Total N in plant samples was determined
by the micro-Kjeldahl technique (Anderson and Ingram 1993) by digesting 0.2 g of
sample with concentrated H2SO4 and a catalytic mixture in a Gerhadt Kjeldatherm®,
distillation of digest in 50% NaOH using a Gehardt® N distiller apparatus and
titration of the distillate in a boric acid mixture with 0.07M H2SO4. The method is
previously described in section 3.2.3. Nodules were not included in root N analysis.
6.3.4 Field experiment
A field experiment was set up to assess nodulation and N2 fixation of the various R.
leguminosarum strains under field conditions. A randomised complete block design
field experiment with 4 treatments and 4 replicates was setup during the 2011/2012
rainy season at Grasslands Research Station. The treatments were: MAR 833 strain,
MAR 346 strain, MAR 1504 strain and No inoculation (control). Hairy vetch was
planted in 5 m x 5 m plots in the 2011/2012 cropping season at 0.1 m inter- and 0.3 m
intra-row spacing. Basal fertiliser was broadcasted within each plot at planting as
Single Super Phosphate (SSP) at a rate of 200 kg ha–1 (19% P205). The un-inoculated
hairy vetch treatment was used as a reference crop since it failed to nodulate.
Rhizobia inoculant was applied as slurry to the hairy vetch seeds at planting. The
plots were kept weed free throughout the experiment period by hoeing weeds
whenever necessary.
93
An average of 5 plants were sampled for assessment of nodulation at 80 days after
planting and nodule counts and mass were recorded for each plot. Above ground
biomass was harvested at 120 days after planting and dried to a constant mass in an
oven at 60°C. Plant samples were ground and total N was analysed using the micro-
Kjeldahl technique (Anderson and Ingram 1993). Nitrogen fixation was calculated by
the N difference method (Peoples et al. 2002) by comparing the amount of N
accumulated by the N2-fixing hairy vetch with a neighbouring non-nodulated hairy
vetch crop representing the amount of soil mineral N available for plant growth
during the growing season as shown in equation 6.1 (Peoples et al. 2002);
N2 fixed (kg ha-1) = N yield in fixing species - N yield in non-fixing species
………………………………………………………………………… Equation 6.1
This method is more reliable for soils with poor capacity to supply available N than
soils rich in available N because when there is low available soil N the legume will fix
more atmospheric N2 (Peoples and Giller 1993). Assumptions were that the
uninoculated hairy vetch and inoculated hairy vetch have similar root morphology and
have the same capabilities to exploit soil N. It was also assumed that there were
minimal N losses to the system through denitrification and leaching (Lepo and
Ferrenbach 1987).
6.3.5 Data analysis
A multiple analysis of variance (ANOVA) was performed on greenhouse data (nodule
weight and total N content) to test the effects of strain type and soil pH on nodulation
and N2 fixation by hairy vetch. A one way ANOVA was performed to compare
nodule mass and counts, % N in shoots and roots, dry mass yield and N2 fixed data for
the different treatments using Genstat statistical software 8th edition. Least Significant
94
Differences (LSDs) were calculated for each attribute and this was used to separate
significantly different means at 95% confidence interval.
6.4 Results
6.4.2 Hairy vetch nodulation and biomass production in the greenhouse
In the greenhouse, hairy vetch nodulation was not significantly affected by liming (P
= 0.63) and strain differences were not significant in affecting nodule mass per unit
root mass (Table 6.1). However, there was significant interaction (P <0.01) between
rhizobia strain and lime on above ground-biomass of hairy vetch. Treatments
receiving inoculant had significantly higher (P<0.001) above-ground biomass
compared to the un-inoculated controls in both limed and unlimed soil except for the
MAR 346 treatment in limed soil that was not significantly different from the controls
(Table 6.1).
Table 6.1 Nodule weight per unit root mass and biomass production of hairy vetch from the greenhouse pots in acidic and limed soil at 80 days after planting at the Grassland Research Station.
a,b,c,d – values with the same letter in the same column are not significantly different from each other (at 0.05 probability level)
Treatment Nodule mass root mass-1
Shoot Dry mass g pot–1
Root dry mass g pot–1
Unlimed soil ( pH 4.7)
MAR 833 0.199b 10.34c 3.74
MAR 1504 0.151b 9.96c 4.49 MAR 346 0.155b 6.69b 3.73 Uninoculated (control) 0a 4.09a 3.53 Limed soil ( pH 6.5)
MAR 833 0.151b 9.31c,b 4.38 MAR 1504 0.167b 9.95c 3.85 MAR 346 0.151b 7.20b,a 3.66 Uninoculated (control) 0a 5.32a 3.40 L.S.D. 0.053 2.196 Ns
95
The effect of liming on above-ground biomass was not significant (P= 0.949). A
visual inspection of hairy vetch pots revealed that both the limed and un-limed pots
that were not inoculated had stunted growth and yellowing and purpling of leaves
(Plate 6.1).
There was no significant interaction of rhizobia strain and lime on root biomass and
hairy vetch root biomass was neither affected by rhizobia strain nor liming (Table
6.1). Strain MAR1504 yielded the highest shoot dry mass in both limed and unlimed
soil and shoot dry mass was in the order MAR 1504 = MAR 833> MAR 346> un-
inoculated. In un-limed soil MAR833 had the highest shoot biomass. Strains MAR
1504 and MAR 833 had significantly higher shoot dry mass than MAR 346 in limed
and unlimed soil.
Inoculation in acidic soil achieved above-ground dry mass increases of at least 140%
for strains MAR 1504 and MAR 833 and 63% for strain MAR 346 compared with the
control. In limed soil increase in shoot dry mass was 87%, 56% and 31% for strains
MAR 1504, MAR 833 and MAR 346 respectively.
6.4.3 N content of hairy vetch in the greenhouse
When hairy vetch was grown in the greenhouse, there was significant interaction (P<
0.001) between rhizobia strain and lime for N content in shoots. However, the main
effects of rhizobia strain and lime were also significant in affecting N content in
shoots. Inoculation with rhizobia significantly (P<0.05) increased hairy vetch’s
above-ground and root N concentration in acidic and limed soils (Figure 6.2).
Compared to the un-inoculated treatment, inoculation increased N concentration by at
96
Plate 6.1 Un-inoculated hairy vetch in the greenhouse with yellow and purple colouration suggesting critical deficiencies of nitrogen and phosphorus.
Un-inoculated
MAR 833
MAR 833 + Lime
Limed and Un-inoculated
MAR 1504
MAR 346
MAR 1504 + Lime
97
Figure 6.2 Total N content of hairy vetch (a) above ground parts and (b) belowground parts (roots) grown on acidic soil (pH 4.7) and limed soil (pH 6.5) in the greenhouse at 80 DAP. Error bars denote Standard error of mean (S.E.M.)
least 119% in limed and un-limed soil except for the limed + MAR 346 treatment that
had only 45% more N concentration (Figures 6.2 (a) and (b)).In hairy vetch shoots
liming significantly reduced N content except for the un-inoculated treatment. Liming
had no significant effect (P>0.05) on total N in hairy vetch roots.
6.4.4 Hairy vetch nodulation and biomass production in the field
When sampled from the field at 80 DAP, strains MAR 1504 and MAR 833 had
significantly (P<0.05) greater nodule biomass per plant and total plant mass than
0
5
10
15
20
25
30
35
40
45A
bo
vegr
oun
d N
(mg
g -1
) (a)2 x SEM
a a
b
d
ba
c
d
0
5
10
15
20
25
30
35
MAR833 MAR1504 MAR346 control
N in
roo
ts (m
g g
-1)
Rhizobium leguminosarum strain
(b)
b
a
c
2 x SEMa
a ba
bc
Legend Unlimed soil (pH 4.7) Limed soil (pH 6.5)
98
MAR 346 and the control treatment (Table 6.2). Inoculation increased plant biomass
(root and shoot) for all treatments compared to the un-inoculated control. Inoculation
with rhizobia significantly (P < 0.05) increased above-ground biomass production of
field grown hairy vetch compared to the control at 120 DAP. Above-ground biomass
accumulation was greatest in plants inoculated with MAR 833 (3.11 t ha–1) and MAR
1504 (2.53 t ha–1) followed by MAR 346 (1.86 t ha-1) and the un-inoculated yielded
the lowest biomass (0.37 t ha–1) (Figure 6.3).
Table 6.2 Nodule counts per plant, nodule weight per plant and aboveground biomass of hairy vetch in the field at Grasslands Research Station (GRS) in Marondera at 80 days after planting
Treatment Nodule count
plant–1 Nodule dry mass g plant–1
Shoot and root dry mass g plant–1
MAR 833 24c 0.1758c 9.52c
MAR 346 22c 0.1292b 3.79b MAR 1504 15b 0.1575c 8.86c
No Rhizobium 0a 0a 0.77a LSD 3.579 0.02937 0.836
a,b,c – values with the same letter in the same column are not significantly different from each other (at 0.05 probability level)
Figure 6.3 Above-ground dry mass of hairy vetch grown in the field at 120 DAP. Error bars denote Standard error of mean (S.E.M.)
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
MAR 833 MAR 1504 MAR 346 uninoculated
Ab
oveg
roun
d bi
omas
s t h
a -1
Rhizobium leguminosarum strain
2 x SEM
aa
b
c
99
6.4.5 N content and N2 fixation of hairy vetch in the field
Inoculation significantly increased N concentration of field grown hairy vetch shoots
by 35% (P<0.05) for MAR 833 and MAR 346 and by 41% for MAR 1504 as
compared to the control but strain differences were not significant (Figure 6.4).
Values of N2 fixed by hairy vetch calculated by employing the use of an un-nodulated
hairy vetch reference crop revealed that strains MAR 833 and MAR 1504 fixed the
greatest amount of N2 which reached 66.5 kg N ha–1 and 56.6 kg N ha–1 respectively
(Figure 6.5). Strain MAR 346 fixed significantly less N (P<0.05) than MAR 833 and
MAR 1504.
Figure 6.4 Total N concentration of hairy vetch (above-ground parts) grown in the field at GRS and harvested at 120 D.A.P. Error bars denote Standard error of mean (S.E.M.)
0
5
10
15
20
25
30
35
40
45
MAR 833 MAR 1504 MAR 346 uninoculated
Tota
l N (
mg
g-1
)
Rhizobium leguminosarum strain
2 x SEMaa a
b
100
Figure 6.5 N2 fixed by hairy vetch when grown in an acidic sandy soil at GRS, Marondera. Error bars denote Standard error of mean (S.E.M.)
6.5 Discussion
The absence of nodulation in the un-inoculated treatments in both greenhouse pots
and in the field plots suggested that indigenous Rhizobium leguminosarum bacteria
able to nodulate hairy vetch were absent in the soil. Rhizobium leguminosarum bv.
viceae nodulates temperate legumes such as peas (Pisum spp), vetches (Vicia and
Lathyrus spp) and lentils (Lens esculenta) (Somasegaran and Hoben 1994) and as a
result they might be absent because such crops have not been grown on these soils
before. Most smallholder farming areas in Zimbabwe have no known history of
temperate legume cropping except in home fields and vlei areas. Home fields are
fields close to the homestead which are usually more fertile compared to away fields
because of differential long-term management favouring fields close to the homestead
(Tittonel et al. 2007). Vleis are wetlands in which communal farmers are usually
involved in all year round cropping growing mostly vegetables, maize and rice
(Svotwa et al. 2008). Even on sites where temperate legumes have been grown before
-10
0
10
20
30
40
50
60
70
80
90
100
MAR 833 MAR 1504 MAR 346 uninoculated
N fi
xed
kg
ha
-1
Rhizobium leguminosarum strain
2 x SEM
a
a
b
c
101
and the bacteria introduced, low rhizobial counts may result from a prolonged cereal
cropping history that does not include the host legumes (Mothapo et al. 2013). In the
absence of a host, survival of Rhizobium leguminosarum strains in soil is greatly
reduced depending on their saphrophytic competence (Peoples et al. 2009).
Rhizobia saprophytic survival is the ability of rhizobia bacteria to utilize various
substrates from decaying organic matter in soil. It depends on availability of C and N
sources (organic matter content), soil moisture regimes, soil aggregation, temperature
and pH (Hungria and Vargas 2000). The sandy granite derived soils in the
smallholder farming areas of Zimbabwe offer challenges to rhizobia bacteria survival
as a result of their low organic matter content, low clay content and low pH (acidic)
(Zengeni et al. 2006). The need to inoculate such soils with a commercial strain
becomes essential as shown by Chatel et al. (1968) who found that R. leguminosarum
bv. trifolii did not persist well in hot or dry sandy soils of Australia. The failure to
nodulate of un-inoculated hairy vetch implies that an introduced commercial strain
may not have to compete for nodulation with other indigenous strains that might be
inferior in fixing N2. In such a scenario the inoculation response becomes significant
and the benefits of inoculation are apparent.
Liming did not significantly affect biomass production of hairy vetch inoculated with
the same strains suggesting that the symbiosis and/or the host (hairy vetch) and/or
micro-symbionts performed equally well in both acidic and near neutral soil
conditions. Hairy vetch is known to be more tolerant to acid soil conditions than most
legume crops as shown by the studies of Dastikaite et al. (2009) who observed that of
the two hairy vetch genotypes he studied, above-ground productivity was good in
102
substrates with a pH range of 4.5-6.5. Mothapo et al. (2013) also observed that
nodulation success of hairy vetch was not significantly influenced by pH between two
fields having different pH of 5.7 and 6.5. The pH tolerance range of the Rhizobium
leguminosarum strains used in this study was not known and the strains might have
been better adapted to acidic conditions.
Inoculation improved biomass production, N concentration and N2 fixation of hairy
vetch in the greenhouse and in the field showing that the strains used in this study
formed actively N2-fixing nodules. Strains MAR 833 and MAR 1504 fixed more N2
per ha and as a result stimulated more biomass production than MAR 346. Apart from
fixing N2, rhizobia bacteria have been shown to have other benefits such as C sink
stimulation of photosynthesis. Plants associated with rhizobia have higher rates of
photosynthesis and delayed leaf senescence which increases biomass production of
the legumes (Kaschuk 2009). By this mechanism, strains MAR 833 and MAR 1504
might be more demanding (in terms of C sink strength) to the host than MAR 346
leading to a greater photosynthetic stimulation of the host.
Nitrogen fixation figures obtained for hairy vetch from the field experiment (37.9 –
66.5 kg N ha–1) were much less than those reported in temperate areas of about 110 -
230 kg N ha–1 yr–1 (Burton 1984, Parr et al 2011) and 225 kg N ha–1 yr–1 in Australia
(Zablotowicz et al. 2011). This could be as a result of edaphic limitations to N2
fixation such as periodic soil moisture stress and high soil temperatures (Mpepereki
1992, Zengeni et al. 2006). Hairy vetch fixed up to 66.5 kg N ha–1 in above-ground
parts that can potentially be used by cereal crops in acidic sandy soils, however the
amount of N2 fixed in the roots might also be substantial since hairy vetch is deep
103
rooted. Results from greenhouse grown hairy vetch showed that differences in %N
and biomass between hairy vetch shoots and roots were in the range 0.18 - 9.02 mg g–
1 and 0.56 – 6.1 g g–1 respectively. The range of hairy vetch N content in above
ground parts and roots in inoculated treatments (21.23 – 32.95 mg g–1) was in the
range of that found by Jo et al. (1980) in subtropical Japan. This suggests that the
potential N contributions from hairy vetch below ground parts should not be ignored
since N partitioning in hairy vetch can achieve substantial N amounts in below ground
parts.
The highest hairy vetch above-ground biomass produced from the field trial (3.11 t
ha–1) was much less than that reported by Lanyasunya et al. (2007) who reported dry
mass yields of up to 9.5 t ha–1 in tropical Kenya. The difference could be attributed to
the fact that in the study by Lanyasunya et al. (2007), hairy vetch was irrigated and
the soil in which hairy vetch was grown was a more fertile clayey soil. In contrast in
this study a yield of 3.11 t ha–1 was obtained in infertile sandy soils with 743 mm of
erratic precipitation. The erratic distribution of the rainfall might have affected
biomass production. At 43 DAP there was a mid-season drought lasting 18 days
without any precipitation, but hairy vetch did not wither but continued to grow
indicating its resilience when established. Such a quality is advantageous where
biomass production is not critically hampered. Other factors such as seeding rate, time
of planting and harvesting and whether hairy vetch is grown as an intercrop or sole
crop will also affect its biomass production.
Further studies are needed to evaluate the performance of the strains tested in this
study with more strains isolated elsewhere over a longer period in varying
104
Zimbabwean agro-ecological regions. It is also crucial to determine the ability of the
strains found to be most effective in increasing the amount of N2 fixed to survive and
maintain high cell counts on baggasse media used for inoculant production in
Zimbabwe. The performance of different hairy vetch cultivars at varying planting
times, seeding rates and as intercrops is also worthwhile looking at. Currently there is
no market for hairy vetch seed which might hamper better utilisation of the crop for
soil fertility management. A challenge also exists when a farmer grows the crop to
harvest seed because hairy vetch pods mature at different stages while mature pods
shatter easily which might require multiple seed harvesting (Clark 2007).
6.6 Conclusions
Indigenous rhizobia that effectively nodulate hairy vetch are absent or undetectable in
the sandy soils of Zimbabwe that do not have a history of pea, vetch and lentil
cropping. Thus inoculation with Rhizobium leguminosarum strains is essential to
increase biomass production and N2 fixation of hairy vetch when grown in
Zimbabwean acidic sandy soils. Soil acidity is not a major problem when growing
hairy vetch in local acidic soils but further studies are needed to determine the range
of pH tolerance in the soils. As the preliminary results show, the strains MAR 833 and
MAR1504 available at the Grasslands Rhizobium Culture Collection were the most
effective in increasing the amount of N2 fixed per hactare and are recommended for
further multi-location field testing in farmer’s fields for a longer period. This will
allow recommendations to be made for strains to use in commercial production of
inoculants for hairy vetch.
105
CHAPTER 7
OVERALL DISCUSSION, CONCLUSION AND RECOMMENDATIONS
7.1 Introduction
Despite potential benefits of legume cropping options available to farmers, the options
must be able to fit well into current maize cropping systems without giving much
resource strain to farmers. In this study hairy vetch, sunnhemp and cowpea were
evaluated for their suitability in rotations and as intercrops with maize. Previous
studies show that when smallholder farmers grow legumes they hardly apply mineral
fertilisers as nutrient resources are often only dedicated to the maize crop (Hardason
and Atkins 2003; Nhamo et al. 2003). This study evaluated the performance of the
legumes when unfertilised or fertilised with basal fertiliser.
7.2 Biomass production of sole legumes
The study sites had low available P levels (P < 6.54 mg kg -1) and response of legumes
to P fertilization was significant for sunnhemp and hairy vetch. Inorganic basal
fertilisation improved legume biomass production, nitrogen fixation and N uptake.
This shows that in smallholder areas with similar soils, it is crucial to apply basal
fertilisation in-order to maximise benefits of growing legumes (Zhu et al. 2011).
Sunnhemp produced the highest biomass of up to 11.4 t ha-1 and fixed up to 161.5 kg
N ha-1, cowpea reached 5.48 t ha-1 and fixed up to 84.72 kg N ha-1 while hairy vetch
produced biomass of up to 5.36 t ha-1 and fixed up to 66.5 kg N ha-1 that can be
potentially available to a next season’s maize crop.
Initially hairy vetch showed poor nodulation and its biomass production was the
lowest of the three legumes tested during the first two seasons at all sites. However, in
106
the third season at GRS when hairy vetch was inoculated with a rhizobium strain
MAR 833 biomass production reached up to 5.36 t ha-1. It is apparent that when hairy
vetch is grown in similar local soils the need to provide a commercial inoculant strain
becomes important.
There were challenges observed in weeding of hairy vetch in the field in its early
growth stages due to its slow initial growth. Hairy vetch’s initial growth is slow
compared to sunnhemp and cowpea and as a result it is more prone to smothering by
weeds in its early stages making the need for early weeding obvious. At times of
erratic rainfall the quick growth of cowpea and sunnhemp might be advantageous in
providing a shade on the soil surface leading to reduction in direct soil water
evaporation losses.
While for comparison’s sake legume biomass was sampled at 75 DAP, it was evident
that hairy vetch was still vegetatively growing although it was also flowering at the
same time. This implies that it might not have reached its maximum biomass at the
time that cowpea and sunnhemp had reached their peak. Hairy vetch was observed to
remain green till the beginning of winter each year in late May to beginning of June
(Plate 7.1). This quality might make it more suitable for providing fresh cattle feed at
a time when most crops would have senesced. Proper separation of hairy vetch pods
and stover would be important to avoid cattle poisoning through seed ingestion
(Kamo et al. 2003).
Plate 7.1 Hairy vetch still green and flowering in June 2011physical maturity and dried up.
flowering in June 2011 (at 170 DAP) in Marondera after cowpea, sunnhemp and maize have r
107
Marondera after cowpea, sunnhemp and maize have reached
108
7.3 Benefits of legume rotations and legume relay intercrops to a maize crop
Maize yields from legume-cereal rotations without any inorganic basal fertiliser
additions were not significantly different from a continuous unfertilised maize
monocrop. As a result of the severe soil fertility limitations especially P deficiency in
most smallholder cropping areas, legume-cereal rotations without some external P
applications will not improve cereal yields. However, as shown in this study when
legumes in rotation are basal fertilised they significantly improve maize grain yields
by at least 37% compared to unfertilised maize. It was shown from this study that
basal fertiliser applied at 300 kg ha -1 (7N : 14 P2O5 : 7 K2O) was important for both
the legume and cereal in a rotation and top dressing fertiliser could be foregone if the
previous legume managed to produce at least 2 t ha -1 of biomass. When the rotation
experiment was continued for a third season the residual effects of legume rotations
were still significant though diminished. It becomes apparent from this study that the
residual effects of growing legumes can span into the next two cereal cropping
seasons (sections 4.3.3, 4.4, 5.5).
The trend observed for legume rotations was similar to that of relay intercrops with
basal fertilised maize-legume intercrops yielding the same maize grain as maize that
was basal and top dressed. While sunnhemp performed better than hairy vetch and
cowpea, its seed is not readily available on the local market. A few local companies
are beginning to commercially produce the seed but quantities produced are still
minimal. Hairy vetch seed is the least available locally.
109
7.4 Recommendations
Rotations and intercrops involving legumes in maize-based cropping systems are
encouranged. These should be basal fertilized inorder to realize significant maize
yield benefits. The commercialization of forage seed production in Zimbabwe is also
recommended to enable farmer access and to improve the quality of seed of legumes
such as sunnhemp and hairy vetch. Smallholder farmers should be taught through
extension services, on the role and benefits of legume inclusion in maize based
cropping systems.
7.5 Areas for further research
It is crucial to look at;
• Optimising biomass production of hairy vetch through exploring best planting
times and agronomic management options.
• Soil moisture conservation under legume intercrops and ideal relay
intercropping times for improved cereal yields.
• Mechanisms of plant nutrient “scavenging” in intercropping and rotational
systems
• Multi-location and multi-season Rhizobium leguminosarum strains
effectiveness testing for identifying a strain for commercial inoculant
production.
• Investigating legume residue transformations in soils through incorporating
13C enriched legume material into soil and performing solid state 13C nuclear
magnetic resonance (NMR) spectroscopy studies.
110
REFERENCES
Alsup CM, Kahn BA and Payton ME. (2002) Using Hairy Vetch to Manage Soil Phosphorus Accumulation from Poultry Litter Applications in a Warm-season Vegetable Rotation, 37(3), 490–495.
Alvey S, Bagayoko M, Neumann G. and Buerkert A. (2001) Cereal / legume rotations affect chemical properties and biological activities in two West African soils. Plant and Soil 231: 45–54
Alvey S, Yang CH, Buerkert A. and Crowley DE. (2003). Cereal / legume rotation effects on rhizosphere bacterial community structure in West African soils, Biology and Fertility of Soils Journal 37: 73–82.
Anderson IP, Brinn PJ, Moyo M and Nyamwanza B. (1993) Physical resource inventory of the communal lands of Zimbabwe – an overview. Natural Resources Institute bulletin 60. Chatham. UK
Anderson JM and Ingram JSI. (1993) Tropical Soil Biology and Fertility: A
Handbook of Methods (2nd edn). CAB International Wallingford 221 pp.
Ankomah AB, Zapata F, Danso, SKA and Hardarson G. (1996). Yield, nodulation and N2 fixation by cowpea cultivars at different phosphorus levels. Biology and fertility of soils. 22: 10–15.
Anuar A.R, Shamsuddin Z.H and Ehsan S.D (1995) Contribution of legume-N by nodulated groundnut for growth of maize on an acid soil. Soil Biology and Biochemistry Journal 27: 595 - 601
Anugroho F and Kitou M. (2011) Effect of live hairy vetch and its incorporation on weed growth in a subtropical region. Weed Biology and Management Journal 11:1– 6
Anugroho F, Kitou M, Nagumo F and Kinjo K. (2009a) Effect of the sowing date on the growth of hairy vetch (Vicia villosa ) as a cover crop influenced the weed biomass and soil chemical properties in a subtropical region. Weed Biology and Management 9: 129–136
Anugroho F, Kitou M, Nagumo F, Kinjo K and Tokashiki Y. (2009b). Growth, nitrogen fixation and nutrient uptake of hairy vetch as a cover crop in a subtropical region. Weed Biology and Management 9: 63–71
Bagayoko M, Buerkert A, Lung G, Bationo A, Römheld V, Ier R, Bamako B.P (2000) Cereal / legume rotation effects on cereal growth in Sudano-Sahelian West Africa : soil mineral nitrogen, mycorrhizae and nematodes. Plant and Soil 218: 103–116
Baldwin KR and Creamer NG. (1999) Cover crops for organic production systems. Raleigh: North Carolina Cooperative Extension Service.
111
Balkcom KS and Reeves DW (2005) Sunnhemp Utilized as a Legume Cover Crop for
Corn Production. Agronomy Journal. American Society of Agronomy 97:26–31
Burton J. (1984) Legume inoculants and their use – a pocket manual. Nitrogen Fixation for Tropical Agricultural Legumes (NifTAL) Project and FAO.
Carr PM, Martin GB, Caton JS, Poland WW (1998). Forage and Nitrogen Yield of Barley–pea and Oat–pea Intercrops. Agronomy Journal. 90: 79-84.
Chalk P.M, Smith C.J, Hamilton S.D. and Hopmans P. (1993) Characterization of the N benefit of a grain legume (Lupinus angustifolius L.) to a cereal (Hordeum vulgate L.) by an in situ 15N isotope dilution technique. Biology and Fertility of Soils 15:39-44
Chatel DL, Greenwood RM and Parker CA. (1968) Transactions of the International Congress of Soil Science 2:65 - 66
Cheming’wa GN and Vessey JK. (2006) The abundance and efficacy of Rhizobium
leguminosarum bv. viciae in cultivated soils of the eastern Canadian prairie. Soil Biology and Biochemistry 38: 294–302
Chikowo R, Mapfumo P, Nyamugafata P and Giller K.E. (2004a) Maize productivity and mineral N dynamics following different soil fertility management practices on a depleted sandy soil in Zimbabwe. Agriculture, Ecosystems and Environment 102: 119–131
Chikowo R, Mapfumo P, Nyamugafata P and Giller K.E. (2004b) Mineral N dynamics, leaching and nitrous oxide losses under maize following two-year improved fallows on a sandy loam soil in Zimbabwe, Plant and Soil 259: 315–330
Chikowo R, Mapfumo P, Nyamugafata P and Giller KE (2004c). Woody legume fallow productivity, biological N2-fixation and residual benefits to two successive maize crops in Zimbabwe, Plant and Soil 262: 303–315.
Chivenge P, Mwale M and Murwira H. (2003) Biomass accumulation of green
manures and grain legumes in soils of different characteristics in Zambia and Zimbabwe. In S.R. Waddington, ed. Grain legumes and green manures for soil fertility in Southern Africa: taking stock of progress. Proc. conference held 8–11 October 2002, Leopard Rock Hotel, Vumba, Zimbabwe. pp. 129-133
Chuma E, Mombeshora B.G, Murwira H.K. and Chikuvire J. (2000) The dynamics of
soil fertility management in communal areas of Zimbabwe. In: Hilhorst, T. and Muchena, F.M. (Eds). Nutrients on the move-Soil fertility dynamics in African Farming systems. International Institute for Environment and Development, London. pp. 45-63
Clark A (ed) (2007) Managing cover crops profitably. 3rd ed. National SARE
Outreach Handbook Series Book 9. National Agricultural Laboratory, Beltsville, MD.
112
Cobo J, Dercon G, Monje C, Mahembe P, Gotosa T, Nyamangara J, Delve R. J. and
Cadisch G. (2009), Cropping strategies, soil fertility investment and land management practices by smallholder farmers in communal and resettlement areas in Zimbabwe. Land degradation and Development Journal. DOI: 10.1002/ldr.927.
Czapar GF, Simmons WF, Bullock DG. (2002) Delayed control of a hairy vetch
(Vicia villosa Roth) cover crop in irrigated corn production. Crop Protection Journal 21:507–510
Dastikaitė A, Sliesaravičius A and Maršalkienė N. (2009) Sensibility of two hairy
vetch (Vicia villosa Roth) genotypes to soil acidity. Agronomy Research Journal 7:233–238
Dhliwayo DKC, Sithole T and Nemasasi H. (1998) Soil Acidity – is it a problem in
maize based production systems of the smallholder areas of Zimbabwe? In: Waddington SR, Murwira HK, Kumwenda JDT, Hikwa D, Tagwira F (eds) Soil Fertility Research for Maize based farming systems in Malawi and Zimbabwe. Proceedings of the Soil Fert Net Results and Planning workshop held from 7 July 1997 at Africa University, Mutare Zimbabwe. Soil Fert Net and CIMMYT- Zimbabwe. Harare, pp 217-221
Dube (1995) Selecting cowpea varieties for intercropping with maize under marginal
dryland conditions; Zimbabwe Journal of Agricultural Research 33: 167-174
Emine BC and Mukerrem MT (2012) Effects of the nitrogen and phosphorus fertilization on the yield and quality of the hairy vetch (Vicia villosa Roth.) and barley (Hordeum vulgare L) mixture. African Journal of Biotechnology 11: 7208-7211
FAO (2006) Fertiliser use by crop in Zimbabwe. Food and Agricultural Organisation of The United Nations (FAO). Rome. Italy
Fischler M, Wortmann C and Feil B (1999) Crotalaria (C. ochroleuca) as a green manure in maize–bean cropping systems in Uganda. Field Crops Research 61: 97–107
Formowitz B and Joergensen R.G. (2009) Impact of legume versus cereal root residues on biological properties of West African soils. Plant Soil (2009) 325:145–156
Foti R, Rusike J and Dimes J (2003) Risk diversification opportunities through legumes in smallholder farming systems in the semi-arid areas of Zimbabwe. In S.R. Waddington (ed) Grain legumes and green manures for soil fertility in Southern Africa: taking stock of progress. Proc. conference held 8–11 October 2002, Leopard Rock Hotel, Vumba, Zimbabwe. pp. 79-85
Franke AC, Laberge G, Oyewole BD. And Schulz S. (2008) A comparison between legume technologies and fallow, and their effects on maize and soil traits, in two
113
distinct environments of the West African savannah. Nutrient Cycling in Agroecosystems. 10.1007/s10705-008-9174-2
Fujita K, Ofosu-budu K.G. and Ogata, S. (1992). Biological nitrogen fixation in mixed legume-cereal cropping systems. Plant and Soil 141: 155–175
Gee GW and Bauder JW (1986) Particle size analysis. In Klute A (ed) Methods of Soil Analysis. Part 1. 2nd edition. Madison: W1:ASA and SSSA. Pp 383-411
Gentile R, Vanlauwe B, van Kessel C and Six J. (2009) Managing N availability and losses by combining fertiliser-N with different quality residues in Kenya. Agriculture, Ecosystems and Environment 131: 308–314
Graham PH, Rosas JC, Jensen CE De, Peralta E, Tlusty B, Acosta-gallegos J and Pereira PAA (2003) Addressing edaphic constraints to bean production : the Bean / Cowpea CRSP project in perspective. Field crops research 82: 179–192. doi:10.1016/S0378-4290(03)00037-6
Grant P.M. 1970. Restoration of production of depleted sands. Rhodesia Agricultural Journal 67: 134–137
Hardarson and Atkins (2003) Optimising biological N2 fixation by legumes in farming systems. In Hardarson and Broughton (eds) Maximising the use of biological nitrogen fixation in agriculture. Report of an FAO/IAEA Technical Expert meeting held in Rome, 13-15 March 2001.
Henson PR. and Schoth HA. (1955) Hairy vetch culture and uses. United States
Department of Agriculture. Farmers’ bulletin No. 1740. Holm L, Doll J, Holm E, Pancho J, Herberger J (1997) World Weeds. Natural
Histories and Distribution. New York, USA: John Wiley and Sons, Inc Hungria M and Vargas MAT. (2000) Environmental factors affecting N2 fixation in
grain legumes in the tropics, with an emphasis on Brazil. Field Crops Research 65: 151-164
Hungria M, Andrade DS, Chueire LMO, Probanza A, Guttierrez-Manero FJ, Megias
M. (2000) Isolation and characterization of new efficient and competitive bean (Phaseolus vulgaris L.) rhizobia from Brazil. Soil Biology and Biochemistry 32: 1515-1528
Jeranyama P, Hesterman O.B, Waddington S.R and Harwood R.R. (1998) Relay-Intercropping of Sunnhemp and Cowpea into a Smallholder Maize System in Zimbabwe. Agronomy Journal 92: 239–244
Jeranyama P, Hesterman OB, Waddington SR, and Harwood RR (2000) Relay-Intercropping of Sunnhemp and Cowpea into a Smallholder Maize System in Zimbabwe. Agronomy Journal. American Society of Agronomy 92:239–244
114
Jo J, Yoshida S and Kayama R. (1980) Growth and nitrogen fixation of some leguminous forages grown under acidic soil conditions. Japan Grasslands Science Journal 25:326 – 334
Jokela W.E and Randall G.W (1992) Fate of fertilizer as affected by time and rate of
application on corn. Soil Science Society of America Journal 61: 1695–1703 Jost J. (1998) Hairy vetch. Sustainable Agriculture Management Guide MG1C.1.
Whiting: Kansas Rural Center.
Kamanga BCG, Shamudzarira Z and Vaughan C. (2003) On-Farm Legume experimentation to improve soil fertility in the Zimuto communal area, Zimbabwe: Farmer Perceptions and Feedback. Risk Management Working Paper Series 03/02. Harare, Zimbabwe: CIMMYT
Kamo T, Hiradate S and Fujii Y. (2003) First isolation of natural cyanamide as a possible allelochemical from hairy vetch Vicia villosa. Journal of Chemical Ecology, 29(2): 275–83.
Kasasa P, Mpepereki S, Musiyiwa K, Makonese F and Giller KE. (1999) Residual nitrogen benefits of promiscuous soya- beans to maize under field conditions. African Crop Science Journal 7: 375– 382
Kaschuk G. (2009) Sink stimulation of leaf photosynthesis by the carbon costs of rhizobial and arbuscular mycorrhizal fungal symbioses. PhD Dissertation, Wageningen University, The Netherlands ISBN: 978-90-8585-392-3
Kendir H. (1999) Determination of some yield components of wintwer vetch species
(Vicia spp) grown in Ankara conditions. Tarim Bilimleri Dergisi 5(2): 85 - 91 Kumwenda J.D.T, Waddington S.R, Snapp S.S and Jones R.B. (1995) Soil Fertility
Management In The Smallholder Maize-Based Cropping Systems Of Africa. A paper for a Workshop on “The Emerging Maize Revolution in Africa: The Role of Technology, Institutions and Policy”. Michigan State University, USA, 9-12 July 1995 (pp. 9–12).
Lanyasunya T.P, Wang H.R, Kariuku D.M, Kuria Chek L, Mukisira A. (2007) Effect
of maturity on the mineral content of Vicia villosa Roth. Tropical and Subtropical Agroecosystems Journal 7: 53-58
Lepo and Ferrenbach. (1987) Measurement of Nitrogen by direct means. In Elkan
G.H. (ed) Symbiotic Nitrogen Fixation Technology. Marcel Dekker Inc. New York
Liebman M. and Davis A.S. (1999) Integration of soil, crop and weed management in low-external-input farming systems. Weed Research Journal 40: 27–47
Lunze L. and Ngongo M. (2007) Potential Nitrogen Contribution of Climbing Bean to Subsequent Maize Crop in Rotation in South Kivu Province of Democratic Republic of Congo. In A. Bationo, B. Waswa, JM. Okeyo, F. Maina and J.
115
Kihara (eds) Innovations as Key to the Green Revolution in Africa: Exploring the Scientific Facts. Springer Science+Business Media B.V. 677 - 682
Lupwayi N.Z, Kennedy A.C. and Chirwa R.M. (2011) Grain legume impacts on soil biological processes in sub-Saharan Africa. African Journal of Plant Science 5: 1-7
Lybecker D.W, Schweizer E.E. and King R.P. (1988) Economic intercrops and cover crops in a corn polyculture system. Agronomy Journal 79: 792–798
Mafongoya P.L, Bationo A, Kihara J and Waswa B.S. (2006) Appropriate technologies to replenish soil fertility in southern Africa. Nutrient Cycling in Agroecosystems 76: 137–151
Mahieu S, Germon F, Aveline A, Hauggaard-nielsen H, Ambus P. and Jensen E.S. (2009) The influence of water stress on biomass and N accumulation , N partitioning between above and below ground parts and on N rhizodeposition during reproductive growth of pea (Pisum sativum L.). Soil Biology and Biochemistry 41: 380–387
Mapfumo P, Mtambanengwe F, Giller K.E, Mpepereki S. (2005) Tapping indigenous herbaceous legumes for soil fertility management by resource-poor farmers in Zimbabwe. Agriculture, Ecosystems and Environment 109: 221–233
Mapfumo P. (2011) Comparative analysis of the current and potential role of legumes in integrated soil fertility management in Southern Africa. In Bationo A, Waswa B, Okeyo JM and Mokwunye U (eds) Fighting poverty in sub-Saharan Africa: The multiple roles of legumes in integrated soil fertility management. Springer science+Business Media B.V. 175 - 200
Mapiye C, Mwale M. and Mupangwa J.F. (2007) Utilisation of ley legumes as livestock feed in Zimbabwe. Tropical Grasslands 41: 84–91
Marufu L, Karanja N and Ryder M. (1995) Legume inoculant production and use in East and Southern Africa. Soil Biology and Biochemistry 27: 735–738
Mavedzenge BZ, Mahenehene J, Murimbarimba F, Scoones I and Wolmer W. (2006)
Changes in the livestock sector in Zimbabwe following land reform: the case of Masvingo province. IDS: Brighton 125pp
McInnes A, Thies JE, Abbott LK, Howieson JG. (2004) Structure and diversity
among rhizobial strains, populations and communities–a review. Soil Biology and Biochemistry 36: 1295–1308
Miller PR, Gravs WL, Williams WA, and Madson BA. (1989) Cover crops for California agriculture. Leaflet 21471. Univ. of California, Div. of Agric. and Natural Resources, Oakland. 24 pp.
116
Miyazawa K. and Murakami T. (2010). Intercropping green manure crops: effects on rooting patterns. Plant Soil 331:231–239
Mothapo N.V, Grossman J.M, Sooksa-nguan T, Maul J, Bräuer S.L and Shi W. (2013) Cropping history affects nodulation and symbiotic efficiency of distinct hairy vetch (Vicia villosa Roth.) genotypes with resident soil rhizobia. Biology and Fertility of Soils doi10.1007/s00374-013-0781-y
Mpepereki S. (1992) Identification of cowpea rhizobia adapted to high temperatures
[Abstract]. African Association for Biological Nitrogen Fixation (AABNF) fifth conference (14-19 September 1992) abstracts. Rabat, Morocco
Mtambanengwe F. and Mapfumo P. (2008) Combating food insecurity on sandy soils
in Zimbabwe: The legume dilemma. In Dakora et al. (eds) Biological Nitrogen fixation: towards poverty alleviation through sustainable agriculture. Springer Science + Business Media B.V. 29 – 30
Mtambanengwe F. and Mapfumo P. (2003) Integrating organic resource quality and farmer management practices to sustain soil productivity in Zimbabwe. In S.R. Waddington, ed. Grain legumes and green manures for soil fertility in Southern Africa: taking stock of progress. Proc. conference held 8–11 October 2002, Leopard Rock Hotel, Vumba, Zimbabwe. pp. 57-63
Mtambanengwe F. and Mapfumo P. (2006) Effects of organic resource quality on soil profile N dynamics and maize yields on sandy soils in Zimbabwe. Plant and Soil 281:173–191
Mucheru-Muna M, Pypers P, Mugendi D, Kung’u J, Mugwe J, Merckx R and Vanlauwe B. (2010). A staggered maize–legume intercrop arrangement robustly increases crop yields and economic returns in the highlands of Central Kenya. Field Crops Research 115: 132–139
Mudzonga E and Chigwada T. (2009) Agriculture: Future scenarios for Southern Africa; A case study of Zimbabwe’s food security. International Institute for Sustainable Development (IISD). Winnipeg, Canada. 14pp
Mugwira L.M, Murwira H.K. (1997) Use of cattle manure to improve soil fertility in Zimbabwe: Past, Current Research and Future Needs. Network Research Results Working Paper No.2. Department of Research and Specialist Services, Chemistry and Soil Research Institute, Harare, Zimbabwe.
Mupangwa W, Nemasasi H, Muchadeyi R. and Manyawu JG (2003) Residual effects of forage legumes on subsequent maize yields and soil fertility in the smallholder farming sector of Zimbabwe. In: Waddington S.R. (ed.) Grain legumes and green manures for soil fertility in Southern Africa: Taking stock of progress. Proceedings of a conference held 8 -11 October, 2002 at the Leopard Rock Hotel, Vumba, Zimbabwe. Soil Fert Net and CIMMYT-Zimbabwe, Harare, Zimbabwe. pp. 119-127.
117
Murphy J and Riley JP (1962) A modified single solution method for the determination of phosphate in natural waters. Analytica Chimica Acta 27:31–36
Murray M.I. (1997) A survey of completed IDRC projects in Southern Africa,
commercialisation case study -agri-industry. Johannesburg. South Africa. Muza L. (2002) Green manuring in Zimbabwe from 1900 to 2002. In Waddington S (ed)
Grain legumes and green manures for soil fertility in Southern Africa: Taking Stock of Progress. Proceedings of a Conference held 8-11 October 2002 at the Leopard Rock Hotel, Vumba, Zimbabwe. Soil Fert Net and CIMMYT-Zimbabwe, Harare, Zimbabwe, pp 103-108.
Ncube B, Dimes JP, van Wijk MT, Twomlow SJ. and Giller KE. (2009) Productivity and residual benefits of grain legumes to sorghum under semi-arid conditions in south-western Zimbabwe: Unravelling the effects of water and nitrogen using a simulation model. Field Crops Research 110: 173–184
Nezomba H, Tauro TP, Mtambanengwe F. and Mapfumo P. (2007) Biomass productivity of N 2 -fixing indigenous legumes on sandy soils under smallholder rain-fed conditions of Zimbabwe. African Crop Science Conference Proceedings Vol. 8. pp. 1505-1512
Ngongoni NT, Mwale M, Mapiye C. and Moyo MT. (2007) Evaluation of cereal-legume intercropped forages for smallholder dairy production in Zimbabwe. Livestock Research for Rural Development 19: 1–11
Nhamo N, Mupangwa W, Gatsi T. and Chikazunga D. (2003) The role of cowpea (Vigna unguiculata) and other grain legumes in the management of soil fertility in the smallholder. In: Waddington, S.R. (Ed.). Grain legumes and green manures for soil fertility in Southern Africa: Taking stock of progress. Proceedings of a conference held 8 -11 October, 2002 at the Leopard Rock Hotel, Vumba, Zimbabwe. Soil Fert Net and CIMMYT-Zimbabwe, Harare, Zimbabwe. pp. 119-127.
Nyamadzawo G, Nyamangara J, Nyamugafata P, Muzulu A. (2008), Soil microbial biomass and mineralization of aggregate protected carbon in fallow-maize systems under conventional and no-tillage in Central Zimbabwe. Soil and Tillage Research 102: 151–157
Nyamapfene K. (1991) Soils of Zimbabwe. Nehanda Publishers. Harare Ojiem JO, Vanlauwe B, de Ridder N, Giller KE (2007) Niche-based assessment of
contributions of legumes to the nitrogen economy of Western Kenya smallholder farms. Plant and Soil 292: 119–135
Okalebo J.R, Ganthua K.W. and Woomer P.L. (2002). Laboratory methods of soil and
plant analysis: A working manual. 2nd Edition. TSBF-CIAT and SACRED Africa, Nairobi.
Ong C. (1994) Alley cropping – ecological pie in the sky? Agroforestry Today 6: 8-10
118
Parr M, Grossman JM, Reberg-Horton SC, Brinton C, and Crozier C. (2011) Nitrogen
Delivery from Legume Cover Crops in No-Till Organic Corn Production. Agronomy Journal 103:1578–1590
Peoples MB and Giller KE. (1993) Techniques for quantifying nitrogen fixation. In
Anderson JM and Ingram JSI. Tropical Soil Biology and Fertility: A Handbook of Methods (2nd edn). CAB International Wallingford 221 pp.
Peoples MB, Boddey RM and Herridge DF. (2002) Quantification of Nitrogen
Fixation In: Leigh GJ (ed) Nitrogen fixation at the millennium. Elsevier Science B.V.
Peoples MB, Brockwell , Herridge DF, Rochester , Alves B1R, Urquiaga S, Boddey
RM, Dakora FD, Bhattarai S, Maskey SL, Sampet C, Rerkasem B, Khans DF, Hauggaard-Nielsen H, and Jensen BS. (2009) The contributions of nitrogen-fixing crop legumes to the productivity of agricultural systems. Symbiosis Journal 48:1-17
Perin A, Henrique R, Santos S, Urquiaga SS, Cecon R, Guilherme J and Guerra M. (2006). Sunnhemp and millet as green manure for tropical maize production. Scientia Agricola (Piracicaba, Brazil), 63: 453-459
Pypers P, Huybrighs M, Diels J, Abaidoo R, Smolders E and Merckx R. (2007) Does the enhanced P acquisition by maize following legumes in a rotation result from improved soil P availability? Soil Biology and Biochemistry 39: 2555–2566
Ranzluebbers K, Weaver RW, Juo SR and Franzluebbers AJ. (1994) Carbon and nitrogen mineralization from cowpea plants part decomposing in moist and in repeatedly dried and wetted soil. Soil Biology and Biochemistry 26:1379-1387
Rao MR, Rego TJ and Willey RW. (1987) Response of cereals to nitrogen in sole cropping and intercropping with different legumes. Plant and Soil 101: 167-177
Reid MG. (1977) Early Agriculture of Matabeleland and Mashonaland. Rhodesia Agricultural Journal 74: 97-102
Rochester I. and Peoples M. (2005) Growing vetches (Vicia villosa Roth) in irrigated
cotton systems: inputs of fixed N, N fertiliser savings and cotton productivity. Plant and Soil Science Journal 271: 251–264
Rowe E. and Giller KE. 2003 Legumes for soil fertility in southern Africa: needs,
potential and realities. In: Waddington, S.R. (Ed.), Grain Legumes and Green Manures for Soil Fertility in Southern Africa: Taking Stock of Progress. Soil Fert Net-CIMMYT, Harare, Zimbabwe. pp. 15–19.
Rusinamhodzi L, Corbeels M, Nyamangara J and Giller KE. (2012). Maize – grain legume intercropping is an attractive option for ecological intensification that reduces climatic risk for smallholder farmers in central Mozambique. Field Crops Research 136: 12–22. doi:10.1016/j.fcr.2012.07.014
119
Rusinamhodzi L, Murwira HK and Nyamangara J. (2006) Cotton–cowpea intercropping and its N2 fixation capacity improves yield of a subsequent maize crop under Zimbabwean rain-fed conditions. Plant and Soil 287: 327–336
Sadeghi AM and Insensee AR. (2001) Impact of hairy vetch cover crop on herbicide transport under field and laboratory conditions. Chemosphere 44:109-118
Saha HM and Grove JH. (1996) No-tilling Corn into Hairy Vetch : Fertilizer Nitrogen Substitution Without Penalty Due to Delayed Planting. Soil Science News and Views 17: 3–5
Sakala WD, Cadisch G. and Giller KE. (2000) Interactions between residues of maize and pigeonpea and mineral N fertilizers during decomposition and N mineralization. Soil Biology and Biochemistry 32: 679–688
Sanchez PA. (2002) Soil fertility and hunger in Africa. Science 295: 2019-2020 Sanginga N. (2003) Role of biological nitrogen fixation in legume based cropping
systems; a case study of West Africa farming systems. In Hardarson G. and Broughton W.J. (Eds), Maximising the use of biological nitrogen fixation in Agriculture. F.A.O. and Kluwer academic publishers. pp.25 -39
Saunder DH. and Grant PM. (1962) Rate of mineralization of organic matter in cultivated Rhodesian soils. Transactions of the International Society of Soil Science conference in New Zealand. Wright and Carman ltd, Wellington A6. 235-239
Shah Z, Shah SH, Peoples MB, Schwenke GD and Herridge DF. (2003) Crop residue and fertiliser N effects on nitrogen fixation and yields of legume – cereal rotations and soil organic fertility. Field Crops Research 83: 1–11
Shobeiri SS, Habibi D, Kashani A, Paknejad F, Jafary H, Al-Ahmadi M, Tookalloo R and Lamei J. (2010) Evaluation of hairy vetch (Vicia villosa Roth) in pure and mixed cropping with barley (Hordeum vulgare L.) to determine the best combination of legume and cereal for forage production. American Journal of Agricultural and Biological Sciences 5: 169-176
Shumba EM, Dhliwayo HH, Mukoko OZ. (1990) The potential of maize-cowpea
intercropping in low rainfall areas of Zimbabwe. Zimbabwe Journal of Agricultural Research 28: 33-38
Sibanda HM and Le Mare PH. (1984) Phosphate adsorption as a function of soil
mineralogy and soil organic matter in some Zimbabwean soils. The Zimbabwe Journal of Agricultural Research 22: 157-162
Singh A, Baoule AL, Ahmed HG, Dikko AU, Aliyu U, Sokoto MB and Alhassan J. (2011) Influence of phosphorus on the performance of cowpea (Vigna unguiculata (L) Walp.) varieties in the Sudan savanna of Nigeria. Agricultural Science 2:313–317
120
Smaling, EMA. (1993). An Agro-Ecological Framework of Integrated Nutrient Management with Special Reference to Kenya, Wageningen Agricultural University, Wageningen.
Snapp SS and Silim SN. (2002) Farmer preferences and legume intensification for low nutrient environments. Plant and Soil 245: 181–192
Snapp SS, Mafongoya PL and Waddington S. (1998) Organic matter technologies for integrated nutrient management in smallholder cropping systems of southern Africa. Agriculture, Ecosystems and Environment 71: 185-200
Somasegaran P. and Hoben HJ. (1994) Handbook of Rhizobia: Methods in Legume-Rhizobium Technology. Springer- Verlag, New York
Steiner AK. (2002) Conservation Tillage – Gateway to Food Security and Sustainable Rural Development: Crop Residue Management and Cover Crops: Types and Sources of Soil Cover. African Tillage conservation network, information series no. 3. 1–4.
Svotwa E, Manyanhaire O. and Makombe P. (2008) Sustainable gardening on wetlands in the communal lands of Zimbabwe. Electronic Journal of Environmental, Agricultural and Food Chemistry. ISSN: 1579-4377
Svubure O, Mpepereki S. and Makonese F. (2010) Sustainability of maize-based cropping systems in rural areas of Zimbabwe : an assessment of the residual soil fertility effects of grain legumes on maize (Zea mays L.) under field conditions. International Journal of Engineering, Science and Technology 2:141-148
Tanner PD and Grant PM. (1977) Response of maize (Zea Mays L.) to lime and molybdenum on acid red and yellow-brown clays and clay loams. Rhodesian Journal of Agricultural Research 15:143- 149
Tanner PD. (1982) Effect of incorporation depth of lime and phosphate uptake by
soyabeans. Zimbabwean Journal of Agricultural Research 20:129-138 Tanner PD. and Mugwira L. (1984), Effectiveness of communal area manures as
sources of nutrients for young maize plants. Zimbabwe Agricultural Journal 81: 31-36
Tanner PD. (1978) Relations of sorption of molybdate and phosphate by clays and
clay loams to soil pH and other chemical factors. Rhodesian Journal of agricultural research. 16: 31 – 41
Tauro TP, Nezomba H, Mtambanengwe F and Mapfumo P. (2010) Population dynamics of mixed indigenous legume fallows and influence on subsequent maize following mineral P application in smallholder farming systems of Zimbabwe. Nutrient Cycling in Agroecosystems 88:91–101
Teasdale JR. and Daughtry CS. (1993) Weed suppression by live and desiccated hairy vetch (Vicia villosa).Weed Science 41: 207– 212
121
Thippayarugs S, Toomsan B, Vityakon P, Limpinuntana V, Patanothai A and Cadisch G. (2008) Interactions in decomposition and N mineralization between tropical legume residue components. Agroforest Systems 72:137–148
Thomas GW. (1996) Soil pH and soil acidity. In Sparks DL, Page AL, Helmke PA, Loeppert RH, Soltanpour PN, Tabatabai MA, Johnson CT and Summer ME (eds). Methods of soil analysis, part 3, Chemical methods. Soil science society of America and American society of Agronomy. Madison, Wisconsin, USA. pp. 475-490
Tian G, Brussaard L, Kang BT and Swift MJ (1997), Soil Fauna-mediated
decomposition of plant residues under constrained environmental and residue quality conditions. In Cadisch and Giller (eds), Driven by nature plant litter quality and decomposition. CABI publishing International. pp.125 – 134
Tittonel P, Zingore S, van Wijk MT, Corbeels M and Giller KE. (2007) Nutrient use
efficiencies and crop responses to N, P and manure applications in Zimbabwean soils: exploring management strategies across soil fertility gradients. Field Crops Research 100:348-368
Toomsan B, McDonagh FJ, Limpinuntana V, and Giller KE. (1995) Nitrogen fixation
by groundnut and soybean and residual nitrogen benefits to rice in farmers’ fields in Northeast Thailand. Plant and Soil 175: 45 – 56
Undersander DJ, Ehlke NJ, Kaminski AR, Doll JD, Kelling KA. (1990) Hairy vetch.
Alternative Field Crops Manual. University of Wisconsin-Madison and University of Minnesota. Available at http:// www.hort.purdue.edu/newcrop/afcm/vetch.html [accessed 5 February 2012].
Van Kessel C. and Hartley C. (2000) Agricultural management of grain legumes : has
it led to an increase in nitrogen fixation? Field Crops Research 65: 165-181
Vance CP, Graham PH. and Allan DL. (2000) Biological nitrogen fixation: phosphorus - a critical future need? In Pedrosa F.O. (ed.) Nitrogen Fixation: from molecules to crop productivity, pp. 509–514.
Vanlauwe B. and Giller KE. (2006) Popular myths around soil fertility management in sub-Saharan Africa. Agriculture, Ecosystems and Environment 116: 34–46
Waddington SR, Karingwindi J. and Chifamba J. (1998) Productivity and profitability of maize-groundnut rotations when compared to continuous maize under smallholder management in Zimbabwe. In Waddington S, Murwira H.K, Kumwenda J.D.T, Hikwa D and Tagwira F., Soil fertility Research for maize-based farming systems in Malawi and Zimbabwe. Proceedings of the Soil-Fert-Net Results and planning workshop. Soil Fert Net and CIMMYT – Zimbabwe.
Waddington SR, Karigwindi JM, Chifamba J. (2007) The sustainability of a ground- nut plus maize rotation over 12 years on smallholder farms in the sub-humid zone of Zimbabwe. African Journal of Agricultural Research. 2: 342–348
122
Waddington, Sakala WD. and Mekuria M. (2004) Progress in lifting soil fertility in Southern Africa. "New directions for a diverse planet". Proceedings of the 4th International Crop Science Congress, 26 Sep – 1 Oct 2004, Brisbane, Australia. Published on CDROM.
Wollum AG. (1998) Constraints and opportunities for biological nitrogen fixation in a
changing world. In Mpepereki S and Makonese F (1998) Harnessing Biological nitrogen fixation in African Agriculture. Proceedings of The Sixth International Conference of the African Association for biological Nitrogen Fixation. University of Zimbabwe Publications.
Woodend J. (1995) Biotechnology and sustainable crop production in Zimbabwe.
Technical Paper No. 109, produced as part of the research programme on Sustainable Development: Environment, Resource Use, Technology and Trade, December 1995.
Wuta M. and Nyamugafata P. (2012) Management of cattle and goat manure in
Wedza smallholder farming area, Zimbabwe. African Journal of Agricultural Research, Vol. 7(26), pp. 3853-3859.
Zablotowicz RM, Reddy KN, Krutz LJ, Gordon RE, Jackson RE and Price LD.
(2011) Can leguminous cover crops partially replace nitrogen fertilization in Mississippi delta cotton production?. International Journal of Agronomy doi:10.1155/2011/135097
Zahran HH. (1999) Rhizobium-legume symbiosis and nitrogen fixation under severe conditions and in an arid climate. Microbiology and Molecular Biology Reviews 63: 968–989.
Zahran, HH. (2001). Rhizobia from wild legumes: diversity, taxonomy, ecology, nitrogen fixation and biotechnology. Journal of biotechnology 91: 143–53
Zengeni R, Mpepereki S. and Giller KE. (2006) Manure and soil properties affect survival and persistence of soyabean nodulating rhizobia in smallholder soils of Zimbabwe. Applied Soil Ecology 32:232–242
Zhu X, Liu R and Zhang Y. (2011). Interactions of a hairy vetch-corn rotation and P
fertilizer on the NPK balance in an upland red soil of the Yunnan plateau. 10 (45), 9040–9050. doi:10.5897/AJB11.902
Zingore S, González-Estrada E, Delve R.J, Herrero M, Dimes J.P. and Giller KE.
(2009) An integrated evaluation of strategies for enhancing productivity and profitability of resource-constrained smallholder farms in Zimbabwe. Agricultural Systems 101: 57–68
123
APPENDIX
LIST OF PUBLICATIONS AND CONFERENCE PROCEEDINGS FRO M
THIS STUDY
1. Tumbure A, Wuta M. and Mapanda F. (2013) Preliminary evaluation of the effectiveness of Rhizobium leguminosarum bv. viceae strains in nodulating hairy vetch (Vicia villosa) in the sandy soils of Zimbabwe. South African Journal of Plant and Soil. 30:4, 233-239, DOI: 10.1080/02571862.2013.868536
2. Tumbure A, Wuta M. and Mapanda F. (2012) Effectiveness of Rhizobium leguminosarum strains nodulating hairy vetch, an introduced forage legume in the sandy soils of Zimbabwe. In Tusiime G, Majaliwa Mwanjololo J.G, Nampala P and Adipala E. (Eds.), Proceedings of the Third RUFORUM Biennial Regional Conference on Partnerships and Networking for Strengthening Agricultural Innovation and Higher Education in Africa, held 24 – 28 September 2012, Entebbe, Uganda. RUFORUM. Working Document Series No. 7.
3. Tumbure A, Wuta M. and Mapanda F. (2010) An evaluation of the effect of hairy vetch (Vicia villosa Roth) on soil fertility of sandy soils in central Zimbabwe. Proceedings of the Second RUFORUM Biennial Regional Conference held 20 - 24 September 2010, Entebbe, Uganda.