effects of conservation agriculture on soil quality and productivity in contrasting agro-ecological...
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Effects of conservation agriculture on soil quality andproductivity in contrasting agro-ecological environments ofZimbabwe
C. Thierfelder & P. C. Wall
Global Conservation Agriculture Program, CIMMYT, P.O. Box MP 163, Mount Pleasant, Harare, Zimbabwe
Abstract
Experimentation by farmers with conservation agriculture (CA) is increasing in southern Africa, but
local longer term data on these new production systems are scarce. This study focuses on CA research
at two contrasting on-farm sites and one on-station long-term trial in Zimbabwe. The on-farm trials
were conducted at Chikato village on a sandy soil at Zimuto Communal Area with low rainfall and at
Hereford farm near Bindura on a clay-rich soil in a high rainfall area. The on-station trial was at
Henderson Research Station near Mazowe where more in-depth soil studies were possible. Results of
CA systems from the on-station site show on average 38 and 65% greater water infiltration on ripline-
seeded (RS) and direct-seeded CA treatments compared with conventionally ploughed control
treatments. Results from on-farm sites show a 123 and 168% greater aggregate stability at Hereford
and 11 and 24% lower dispersion ratio at Chikato on the two CA compared with the conventionally
ploughed control treatments. Soil carbon increased by 46% in the first 20 cm on the sandy soils at
Chikato in RS and by 104% in direct-seeded CA treatments in four cropping seasons from 2004 to
2008, while it stayed at low levels on the conventionally tilled control treatment. Yields on CA plots
were higher on the sandy soils in dry seasons, but lower in very wet seasons because of waterlogging.
Yields on clay soils were less affected by the rainfall season. Crop productivity from CA systems
increased at all sites over time owing to better management although significant differences between
CA and conventional treatments on the three sites were apparent only after several cropping seasons.
Conservation agriculture offers practical solutions to small-scale farmers threatened by future soil
degradation and fertility loss, but its successful use will depend on weed control and adequate
application of fertilizers. The results indicate that there is no immediate increase in maize (Zea mays L.)
yield when changing from a tilled to a CA system, but there is gradual improvement in some soil
quality indicators over time.
Keywords: Sustainable agriculture, water infiltration, residue retention, soil carbon, soil degradation,
no-tillage
Introduction
Conservation agriculture (CA) is based on the three principles
of minimum soil disturbance, surface crop residue retention
and crop rotations (FAO, 2002) and is becoming more
common in southern Africa. Conservation agricultural systems
have been shown to dramatically reduce soil erosion, slow
down physical, chemical and biological soil degradation and
reduce labour and production costs for farmers (Thierfelder
& Wall, 2009; Haggblade & Tembo, 2003; Sorrenson et al.,
1998). Local research data that demonstrate these benefits are
however scarce. The benefits and challenges of CA systems in
other areas have been widely published (Hobbs, 2007; Wall,
2007; Kassam et al., 2009). Nevertheless, the adoption of CA
is regarded as complex involving changes in the ways farmers
prepare and sow land, manage weeds, ensure residue
retention, apply fertilizers and harvest their crops. There is no
one ‘recipe’ for the successful implementation of CA: farmers
need to adapt the principles of CA to their own biophysical
and socio-economic conditions and develop their own ways
for applying these principles (Wall, 2007).
Conventional, tillage-based farming systems without
residue retention have been investigated for decades. SeveralCorrespondence: C. Thierfelder. E-mail: [email protected]
Received January 2011; accepted after revision March 2012
Soil Use and Management doi: 10.1111/j.1475-2743.2012.00406.x
ª 2012 The Authors. Journal compilation ª 2012 British Society of Soil Science 1
SoilUseandManagement
authors conclude that these systems lead to organic matter
decline and are not suitable for tropical environments
(Derpsch et al., 1986; Verhulst et al., 2010; Thierfelder &
Wall, 2010b). Many authors agree that the rapid decline in
soil organic matter is ecologically not sustainable
(Montgomery, 2007; Wall, 2007) and will only lead to various
forms of soil degradation as previously highlighted by
Derpsch et al. (1991) in Brazil, Koch & Stockfisch (2006) in
Germany, and Thierfelder & Wall (2009) in Zambia and
Zimbabwe. The most important challenge for research and
extension systems will, therefore, be the development of more
sustainable and productive cropping systems (Wall, 2007;
Verhulst et al., 2010).
Conservation agriculture has been widely promoted in
southern Africa since the late 1990s. In Zimbabwe, emphasis
was on vulnerable households in an effort to increase food
security. This was by promoting a hand-hoe-based manual
basin planting system, locally labelled as conservation
farming (CF) (Mashingaidze et al., 2006; Mazvimavi et al.,
2008). Other initiatives have focussed on the adaptation and
promotion of animal traction smallholder CA systems.
Widespread promotion of CA without sufficient supporting
scientific data has been criticized by various authors (Bolliger,
2007; Giller et al., 2009). Giller et al. (2009) questioned the
claimed widespread adoption of CA in sub-Saharan Africa
(SSA) and its suitability for most circumstances. However,
they also recognized the existence of ‘niches’ in SSA where
CA would fit. Site-specific research on CA systems offers a
way to understand biophysical and socio-economic challenges
of CA to overcome some of the limitations to agricultural
production in Zimbabwe. The objective of this study was to
compare the effects of CA and conventional systems on
maize productivity and environmental parameters using data
from on-station and on-farm trials.
Material and methods
Field sites
Simple multi-season validation trials comparing two CA
options (animal traction ripping and direct seeding) with a
conventionally tilled control plot using a mouldboard plough
were established in Zimbabwe, one in the area around
Chikato village in the Zimuto Communal Area (Chikato),
Masvingo Province (19.85S; 30.88E; altitude 1223 m.a.s.l.,
mean annual rainfall of ca. 620 mm) from 2004 to 2010 and
the other at Hereford Farm near Bindura (Hereford),
Mashonaland Central Province (17.42S; 31.44E; altitude
1054 m.a.s.l., mean annual rainfall ca. 800 mm) from 2005 to
2010. Dominant soils at Chikato are Arenosols developed
from granitic sands of low inherent fertility. Sand and
organic matter contents are around 94–95% and 0.2%,
respectively. Dominant soil types at Hereford are heavy red
clays, commonly described as Chromic Luvisols and Lixisols,
rich in available nutrients and organic matter. Some soil
characteristics of the top 20 cm at the two sites are shown in
Table 1.
A trial comparing several management systems was
established in 2004 at Henderson Research Station (HRS)
near Mazowe (17.57S; 30.98E; altitude: 1136 m.a.s.l., mean
annual rainfall ca. 884 mm). The predominant soils at this
site are Arenosols and Luvisols (Table 1). Results are
available for 2004–2010.
Trial description
Chikato Village. Seven replications of a farmer-managed
validation trial with three treatments were initiated in
Chikato in 2004 with each replication on a different farmer’s
field. By the 2009 ⁄ 2010 cropping season, this had been
expanded to twelve replications in the community. Treatment
characteristics were as follows:
1. The conventional control treatment (CP) with ploughing
at a shallow depth (10–15 cm) using an animal traction
mouldboard plough. Tillage was performed in one single
pass just before seeding, which is common in this area.
Residues were removed after harvest and the remaining
stubble incorporated with the plough.
2. The first CA treatment was a ripline-seeded (RS)
treatment, which was subsoiled in the first season with an
animal drawn subsoiler (Palabana subsoiler, developed by
the Palabana Farm Power and Mechanization Centre,
Zambia) and the crop and fertilizer hand-placed in the
furrow of the subsoiler. In later seasons, a Magoye chisel-
tine opener was used to create riplines (GART, 2002).
Initially, because of the lack of maize stover, thatching
grass was used on both CA options as surface residue.
Thereafter, all residues harvested from the plots in each
year were retained on the field at Hereford and HRS
(Table 2) or stored next to the demonstration plots after
harvest at Chikato and spread on the fields at the onset
of the cropping season to avoid complete grazing. Local
grass species (Hyparrhenia spp.) were used to supplement
the crop residues at Chikato to achieve 2.5–3.0 t ⁄haground cover.
3. The second CA treatment used an animal traction direct
seeder (DS) (Irmaos Fitarelli, Brazil, model no. 12) that
enabled simultaneous seeding and fertilizer addition to the
mulch. As in the first CA treatment, residues were
retained from the previous harvest or supplemented with
Hyparrhenia grass at Chikato to achieve 2.5–3.0 t ⁄haground cover.
All three treatments received equal amounts of basal and
top-dressing fertilizers as well as the same maize varieties in
each year. Although we know that many farmers especially in
lower production areas do not apply the same amount of
fertilizer, the decision was made to have equal rates of
fertilizer and varieties on all plots to exclude this important
2 C. Thierfelder & P. C. Wall
ª 2012 The Authors. Journal compilation ª 2012 British Society of Soil Science, Soil Use and Management
factor and avoid comparing fertilized CA systems with
un-fertilized control plots as has been the case in previous
research and extension projects (Haggblade & Tembo, 2003;
Mazvimavi et al., 2008). We used ‘typical’ but lower than
recommended fertilizer rates in this study. Basal dressing
in Chikato was 165 kg ⁄ha Compound D (11 kg ⁄ha N:
10 kg ⁄ha P: 10 kg ⁄ha K) followed by a top-dressing of
35 kg ⁄ha N (at 4 weeks after crop emergence) in 2004 ⁄ 2005or 69 kg ⁄ha N in succeeding years as ammonium nitrate.
The same maize variety [ZM521 in 2004 ⁄ 05, ZM423 in
2008 ⁄ 09 and 2009 ⁄ 10 (all open-pollinated varieties), SC513
and SC403 (hybrids) in 2005 ⁄ 06 and 2006 ⁄ 07] was sown at
37 000 plants ⁄ha on all plots in a particular year at respective
seeding dates (Table 3). Maize varieties were not kept the
same throughout the study because farmers wanted to make
use of genetic improvements over time and periodically
requested different varieties. A cross-year comparison of
maize performance is, therefore, not possible and not
required because of significant variation in rainfall between
cropping seasons. Maize crops were routinely intercropped
with a spreading type of cowpeas (Vigna unguiculata L.)
following local practice. In most years, cowpea yields were
low. Herbicides were not used, and all weeding was carried
out with hand hoes whenever weeds were 10 cm tall or 10 cm
in circumference (usually 2–3 times per season).
Hereford Farm. Here, the programme started with
replications at four sites in 2005 ⁄ 06, which increased to eight
in 2007 ⁄ 08. All sites were managed with a maize–soyabean
rotation with maize on four replications in a given year and
soyabeans on the other four sites. Treatments were the same
as in Chikato (CP, RS and DS). All crop residues (maize or
soyabeans) were left as mulch on the CA treatments from the
Table 1 Some soil properties of reference profile C, endostagnic dystric Luvisol; Henderson Research Station; Chikato, Zimuto Communal Area,
Mashvingo Province and Herford Farm near Bindura, Mashonaland Central, Zimbabwe
Horizons
Depth
[cm]
Bulk density
[g ⁄ cm3]
Mottling
[vol %] pH [CaCl2]
CEC
[cmol ⁄ kg] BS [%] Corg [%]
Particle size [%]
Sand Silt Clay
Henderson
Ahp 0–28 1.29 – 4.5 3.7 39 0.44 77 16 7
Ah2 )35 1.48 – 4.5 2.0 55 0.22 73 20 7
E )70 1.45 5 4.2 1.6 37 0.06 83 13 4
Bs )105 1.67 20–30 4.5 1.7 44 – 84 14 2
Bt >115 1.73 20–30 4.3 7.0 38 – 66 15 19
Chikato 0–20 1.32 – 4.6 2.52 20 0.26 95 3 2
Hereford 0–20 1.20 – 5.5 20.9 81 1.39 56 20 24
BS, base saturation; CEC, cation exchange capacity; Corg, organic carbon.
Table 2 Biomass yield in kg ⁄ ha at Chikato, Hereford and Henderson Research Station, 2004 ⁄ 05–2009 ⁄ 10
Treatment
Harvest year
2004 ⁄ 05 2005 ⁄ 06 2006 ⁄ 07 2007 ⁄ 08 2008 ⁄ 09 2009 ⁄ 10
Chikato kg dry biomass ⁄ haConventional ploughing 129a 1060b 1492b 1188b 1584b 450b
Ripline seeding 197a 1064b 1870ab 1569a 2553a 795a
Direct seeding 176a 1602a 2366a 1427ab 2874a 778a
Hereford
Conventional ploughing 3750 5821 6127b 5225a 5337a
Ripline seeding 3434 3964 6438a 5175a 5841a
Direct seeding 4567 4967 7549a 5499a 6442a
Henderson
Conventional ploughing 2177b 3071a 4967a 2458a 1752b 1598a
Ripline seeding 2595a 3418a 4764a 2492a 3206a 1906a
Direct seeding 2390ab 3198a 5074a 2451a 2430ab 1694a
Biomass was not analysed at Hereford in 2005 ⁄ 06 and 2006 ⁄ 07 because of the limited number of replications in the sample; means followed by
the same letter in column at each site are not significantly different at P £ 0.05 probability level (LSD-test).
Effects of CA on soil quality in Zimbabwe 3
ª 2012 The Authors. Journal compilation ª 2012 British Society of Soil Science, Soil Use and Management
first year onwards. Maize plant populations at Hereford were
denser than in Chikato (44 444 ⁄ha) because of the higher
rainfall in this area. Soyabeans [Glycine max (L) Merr] were
sown at 333 333 plants ⁄ha. Fertilizer levels for maize were
the same as those used in Chikato Village, but the N top-
dressing to maize was split with 34.5 kg ⁄ha N at both 4 and
7 weeks after crop emergence. The soyabean crop received
only basal dressing (11 kg ⁄ha N, 10 kg ⁄ha P, 10 kg ⁄ha K)
and no top-dressing, but was inoculated with commercial
rhizobium acquired from the Grasslands Research Institute,
Zimbabwe before seeding. Weed control on the CA plots
included an application of Roundup� containing the active
ingredient glyphosate (N-(phosphono-methyl) glycine) at a
rate of 2.5 l ⁄ha immediately after seeding and before crop
emergence, followed by shallow hoe-weeding as necessary
after crop emergence. Weeding with a mouldboard plough or
a mechanical cultivator with seven tines was carried out on
CP. Hybrid maize varieties (SC627 in 2005 ⁄ 06, SC635 in
2006–2008, PAN 67 in 2009 ⁄ 10) were sown at each seeding
date (Table 3) and the soyabean variety was Safari.
Henderson Research Station. The experiment at HRS
consisted of one conventionally tilled control plot (CP)
compared with two CA systems in a randomized complete
block design with initially four replications. A fifth
replication was added in the 2006 ⁄ 2007 crop season. The plot
area was 160 m2. The treatments at HRS were as follows:
1. Shallow ploughing (10–15 cm) using an animal traction
mouldboard plough (CP). The soil was tilled just before
seeding in one single pass. Residues were removed and the
remaining stubble incorporated with the plough.
2. Ripping and hand-seeding (RS) of maize into a 10–15-cm-
deep ripline opened by an animal traction ripper pulled
by a pair of oxen. The RS treatment was subsoiled with
the Palabana subsoiler in the first three cropping seasons
and planted with maize in the subsoiled lines and
thereafter ripped with the Magoye ripper and planted into
the riplines. Residues were retained on the surface.
3. Direct seeding (DS) of maize by an animal traction direct
seeder (2004–2006) (Irmaos Fitarelli, Brazil, model no. 12)
or a manual jabplanter (2006–2010) (Irmaos Fitarelli,
Brazil, model no. 9). Both implements allow direct seed
and fertilizer placement in the soil through the mulch.
Maize hybrid varieties (SC 627 in 2004–2006 and SC 635 in
2006–2008; ZS 261 in 2009 ⁄ 10) were seeded in mid-November
and harvested at the end of April in all years (Table 3). Crops
had the same applications of fertilizers as the basal one and
with the same application rates on farm sites. The N top-
dressing to maize (69 kg ⁄ha N) was split into 34.5 kg ⁄ha at
both 4 and 7 weeks after crop emergence. Weed control was
initially performed with glyphosate and atrazine (2-chloro-4-
ethylamino-6-isopropylamino-s-triazine), but since 2007, only
manual weeding was carried out.
Field measurements
Infiltration. Two different methods of measuring infiltration
were used in the trial at HRS: one using a small rainfall
simulator and the other a simple method using a watering
can. Infiltration using the rainfall simulator (Amezquita
et al., 1999; Thierfelder & Wall, 2009) was measured as the
difference between precipitation and run-off from an
application of ca. 100 mm ⁄h on an area of 32.5 · 40 cm
(0.13 ⁄m2) applied in 60 min. Infiltration was measured 12
times per treatment in the interrow space between two maize
lines when the soil was at or close to field capacity in January
of each year when the maize crop was at, or just before, the
tassling stage. Infiltration measurements were conducted
between the last week of December (4–5 weeks after
ploughing) and the end of January in each year after the first
manual weed control. We did not measure infiltration during
the initial crop establishment to avoid comparing freshly
tilled and no-tilled plots. The final infiltration rate as derived
from Horton’s infiltration model was used to compare
treatments (Kutilek & Nielsen, 1994).
In the ‘time-to-pond’ measurement (Govaerts et al., 2006),
a metal wire ring of 50-cm diameter was placed on the soil
surface between two maize lines and water was added to the
middle of the ring with a watering can. The time taken for
water to flow out of the metal ring was measured and
recorded as the time-to-pond. As a check, the amount of
Table 3 Summary of all seeding and harvest dates from the three research sites from 2004 to 2010
Cropping season
Zimuto Hereford Henderson
Seeding date Harvesting date Seeding date Harvesting date Seeding date Harvesting date
2004 ⁄ 2005 27 ⁄ 12 ⁄ 2004 16 ⁄ 05 ⁄ 2005 10 ⁄ 12 ⁄ 2004 10 ⁄ 05 ⁄ 20052005 ⁄ 2006 29 ⁄ 11 ⁄ 2005 11 ⁄ 05 ⁄ 2006 05 ⁄ 12 ⁄ 2005 25 ⁄ 04 ⁄ 2006 24 ⁄ 11 ⁄ 2005 21 ⁄ 04 ⁄ 20062006 ⁄ 2007 11 ⁄ 12 ⁄ 2006 08 ⁄ 05 ⁄ 2007 15 ⁄ 12 ⁄ 2006 12 ⁄ 05 ⁄ 2007 27 ⁄ 11 ⁄ 2006 20 ⁄ 04 ⁄ 20072007 ⁄ 2008 01 ⁄ 12 ⁄ 2007 01 ⁄ 05 ⁄ 2008 04 ⁄ 12 ⁄ 2007 30 ⁄ 04 ⁄ 2008 29 ⁄ 11 ⁄ 2007 16 ⁄ 04 ⁄ 20082008 ⁄ 2009 24 ⁄ 11 ⁄ 2008 27 ⁄ 04 ⁄ 2009 24 ⁄ 11 ⁄ 2008 13 ⁄ 05 ⁄ 2009 17 ⁄ 11 ⁄ 2008 30 ⁄ 03 ⁄ 20092009 ⁄ 2010 14 ⁄ 11 ⁄ 2009 22 ⁄ 04 ⁄ 2010 07 ⁄ 12 ⁄ 2009 17 ⁄ 05 ⁄ 2010 23 ⁄ 11 ⁄ 2009 08 ⁄ 04 ⁄ 2010
4 C. Thierfelder & P. C. Wall
ª 2012 The Authors. Journal compilation ª 2012 British Society of Soil Science, Soil Use and Management
water left in the watering can was recorded. Six
measurements were taken on each plot of each treatment in
each replication at HRS from the 2007 season on. Time-to-
pond measurements were also carried out at Chikato from
2008 to 2010 and Hereford in 2009 and 2010.
Total carbon, aggregate stability and dispersion test. The soil
was sampled for carbon at three depths at Henderson (0–10,
10–20 and 20–30 cm) in 2004 and 2008. At Chikato and
Hereford, soil samples were collected in October 2004 and
October 2008 and October 2005 and 2008, respectively. The
sample depth at both on-farm sites was 0–20 cm. Total
carbon was measured through a CE Elantech Flash EA1112
dry combustion analyser. Mg ⁄ha carbon was calculated from
the carbon concentration, thicknesses and bulk densities of
the horizons (Ellert & Bettany, 1995):
M element ¼ conc� pb � T� 10000 m2=ha� 0:001 Mg=kg
ð1Þwhere:
M element = element mass per unit area (Mg ⁄ha)conc = element concentration (kg ⁄Mg)
pb = field bulk density (Mg ⁄m)
T = thickness of soil layer (m).
Aggregate stability was measured on surface soil samples
from Hereford in April 2009 (Yoder, 1936; van Bavel, 1949).
A sample of the soil was placed on a 2-mm sieve and soaked
for 10 min in water in the laboratory. After soaking, the
samples were agitated in water for 10 min at 48 strokes per
min with strokes of 35 mm. Aggregates that remained on the
2-mm sieve were dried at 105 �C, weighed, and the
percentage of water stable aggregates was then calculated.
Because of the sandy soil texture in Chikato, no aggregate
stability test was conducted but was replaced with the
dispersion test described by Middleton (1930). A soil sample
was wetted with distilled water, diluted to one litre and an
aliquot of the soil–water suspension taken after 1 h. This
aliquot was dried and weighed and the percentage of
dispersion calculated.
Harvest procedure and rainwater use efficiency. The crop
was harvested at physiological maturity. Cobs and above-
ground biomass were collected from 10 samples of 9 m2
selected at random from each treatment in Chikato and
Hereford and from eight samples of 4.5 m2 at HRS. Samples
were weighed in the field and subsamples taken for the
determination of grain moisture content. A sample of 20
cobs per treatment was shelled to calculate the shelling
percentage (ratio of grain to total cob weight) and grain
yield calculated on a per hectare basis at 12.5% moisture
content. Grain yield in kg divided by the total amount of
rainfall (mm) gave the rainwater use efficiency for each
treatment, site and year.
Statistical analysis
Statistical analyses were carried out using STATISTIX for
personal computers (Statistix, 2008). Infiltration, time-to-
pond, total carbon, aggregate stability, dispersion ratio, yield
data and rainfall use efficiency were tested for normality and
subjected to an analysis of variance (anovas) using completely
randomized block design. Where the F-test was significant, a
least significant difference (LSD) test was used at P £ 0.05, if
not stated otherwise, to separate the means.
Results
Infiltration and time-to-pond
Final infiltration rate at HRS was significantly higher on CA
plots than in the control plot (CP) from 2006 to 2007
onwards (Table 4). In most years, the direct-seeded (DS)
treatment had the highest final infiltration rates, but only in
2008 ⁄ 09 was it significantly higher than the RS treatment.
Infiltration rate averaged over both CA treatments exceeded
the control plot by 40% in cropping season 2006 ⁄ 07, 91% in
2007 ⁄ 08, 180% in 2008 ⁄ 09 and 56% in 2009 ⁄ 10. Infiltrationmeasured by the rainfall simulator and time-to-pond
measured on the same day at HRS in March 2009 (Figure 1)
were highly correlated (r2 = 0.67).
Time-to-pond was significantly greater (P £ 0.01) in the
CA treatments than in the conventionally ploughed control
plot at both Chikato and Hereford in all seasons (Table 5).
At Chikato, the increase in time-to-pond of both CA
treatments compared with the control ranged between 69 and
90% in 2008–2010. The increase was slightly lower at
Hereford Farm and ranged between 61% in cropping season
2008 ⁄ 09 and 45% in 2009 ⁄ 10. Measurements at HRS
followed the same trend with 47–151% higher time-to-pond
on the two CA treatments compared to CP in the 3 yr.
Total carbon, dispersion test and aggregate stability
There were no significant differences in SOC between
treatments at the start of the trials at all sites, but significant
differences were discovered in 2008 but at the P £ 0.10 level
only (Tables 6 and 7). High variability on plots and between
sites did not allow for higher significance levels. Significantly
higher SOC was recorded on direct-seeded treatments in
Chikato and Hereford (Table 6). At Chikato, CP had only
6% higher carbon (from the initial site mean of 6.5 Mg ⁄ha in
2004 to 6.9 Mg ⁄ha in 2008), whereas both CA treatments
gained substantial amounts of carbon over time, 46% in RS
and 104% in DS. At Hereford, SOC was highest on DS
(43.3 Mg ⁄ha) in 2008 and lowest on CP (37.5 Mg ⁄ha).Although the results from the three depth layers at HRS
suggest some stratification of organic matter in the first
horizons on CA plots, the differences within individual depth
Effects of CA on soil quality in Zimbabwe 5
ª 2012 The Authors. Journal compilation ª 2012 British Society of Soil Science, Soil Use and Management
layers were not significant. For the combined 0–30 cm
horizon, the greatest SOC was from the RS treatment at
HRS (22.4 Mg ⁄ha) compared with the conventional control
treatment (18.7 Mg ⁄ha). Gains were lower on the direct-
seeded treatment at HRS (46%) and Hereford (38%)
(Tables 6 and 7).
The dispersion ratio test at Chikato indicated the highest
dispersion of soil in CP (53%) and the lowest on DS (40%)
(Table 8). Similarly, at Hereford, the lowest aggregate
stability (in %) was on CP (19%), with far greater stability
under both DS (52%) and RS (43%) treatments (Table 8).
Yield results
Chikato. The distribution of rainfall was different in the dry
years. In 2004 ⁄ 05, nearly all rainfall fell in three major event
periods, whereas in 2006 ⁄ 07, although the mean rainfall was
Table 4 Effect of two conservation agriculture and one conventional
system on final infiltration rate as measured with simulated rainfall
of ca.100 mm ⁄ h for 60 min. Henderson Research Station, 2006–2010
Treatment
Final infiltration rate (mm ⁄ h)
2006 2007 2008 2009 2010
Conventional
ploughing
31.6 51.4b 26.3b 25.3c 40.7b
Ripline seeding 36.6 69.7a 50.5a 63.3b 60.0a
Direct seeding 47.2 74.8a 50.3a 78.2a 67.1a
P NS 0.05 0.05 0.01 0.05
LSD 16.9 8.4 13.3 16.6
Means followed by the same letter in column are not significantly
different at the respective P-level (LSD-test).
Time to pond data (s)3 4 5 6
Fin
al in
filtr
atio
n ra
te (
mm
/h)
0
20
40
60
80
100
Y = 22.3x – 43r2 = 0.67P < 0.001
Figure 1 Correlation between time-to-pond and final infiltration rate
measured by rainfall simulation, Henderson Research Station,
March 2009.
Table 5 Influence of conservation agriculture and conventional crop-
ping systems on the time-to-pond (s) in Chikato, Zimuto Communal
Area and Hereford Farm, Mashonaland Central, Zimbabwe, 2008–
2010
Time-to-pond (s)
2007 ⁄ 08 2008 ⁄ 09 2009 ⁄ 2010
Chikato
Conventional ploughing 6.6b 3.1b 3.2b
Ripline seeding 11.5a 5.5a 6.2a
Direct seeding 10.8a 5.4a 6.0a
P 1 1 1
LSD 2.7 0.8 0.9
Hereford
Conventional ploughing N ⁄A 4.9b 4.9b
Ripline seeding N ⁄A 7.7a 7.0a
Direct seeding N ⁄A 8.1a 7.2a
P 1 1
LSD 1.4 0.9
Henderson Research Station
Conventional ploughing 3.8b 3.5b 3.6b
Ripline seeding 9.4a 4.9a 5.3a
Direct seeding 10.1a 5.5a 5.3a
P 0.01 0.01 0.01
LSD 3.2 3.4 0.8
Means followed by the same letter in column are not significantly
different at the respective P-level (LSD-test).
Table 6 Changes in total soil carbon (Mg ⁄ ha) in 2004 and 2008
(Chikato) and in 2005 and 2008 (Hereford) in two conservation
agriculture and one conventional treatment
Depth
(cm)
Total
carbon
(Mg ⁄ ha)
Total
carbon
(Mg ⁄ ha)
Change compared
to the initial
site mean (%)
Chikato 2004 2008
Conventional
ploughing
0–20 8.3a 6.9b 6
Ripline seeding 0–20 5.4a 9.5ab 46
Direct seeding 0–20 5.8a 13.3a 104
Mean 6.5 9.9
LSD 5.2 4.9
Hereford 2005 2008
Conventional
ploughing
0–20 30.6a 37.5b 19
Ripline seeding 0–20 30.6a 38.2ab 21
Direct seeding 0–20 33.1a 43.3a 38
Mean 31.4 39.7
LSD 4.7 5.7
Means followed by the same letter in column are not significantly
different at P £ 0.10 probability (LSD-test); samples were all taken in
October of each respective year before the cropping season. Samples
were corrected for bulk density and calculated to Mg ⁄ ha.
6 C. Thierfelder & P. C. Wall
ª 2012 The Authors. Journal compilation ª 2012 British Society of Soil Science, Soil Use and Management
almost the same, it was more evenly distributed and hence
average crop yields were higher (Table 9). Mean yields were
affected by severe dry spells in 2004 ⁄ 05 when only three
replications yielded any grain (Figure 2). In all other seasons,
one or both of the CA treatments yielded significantly more
than CP. There was only one season (2006 ⁄ 07) with a
significant difference between the two CA treatments when
DS yielded significantly more than RS. On average over all
seasons, DS yielded 56% more and RS 35% more than CP.
Hereford. Average yields were much higher at Hereford
because of the more fertile soil and the higher, better
distributed and more reliable rainfall. In 2007 ⁄ 08, the year
with the highest rainfall, DS yielded significantly more than
the other two treatments and 23% more than CP (Figure 3).
In 2008 ⁄ 09, there were no differences in yield between
treatments, whereas again in 2009 ⁄ 10, both CA treatments
tended to yield more than CP. However, there was
considerable variability and differences were not significant.
Henderson Research Station. Treatment yields at HRS were
not significantly different in the first four cropping seasons
(Figure 4). However, in 2008 ⁄ 09 and 2009 ⁄ 10, RS yielded
significantly more than CP, with DS intermediate and not
significantly different from the other treatments. On average over
all seasons, DS yielded 9%more and RS 20%more than CP.
Rainwater use efficiency
Rainfall use efficiency in Chikato (Table 9) was very low in
2004 ⁄ 05 and 2009 ⁄ 10 because of a long mid-season drought.
The other two comparably dry seasons, 2006 ⁄ 07 and
2008 ⁄ 09, had a better distribution of rainfall and much
higher rainfall use efficiency, while the very wet conditions of
2005 ⁄ 06 and 2007 ⁄ 08 had negative effects on crop yield and,
therefore, on rainfall use efficiency. Nevertheless, CP had a
significantly lower rainfall use efficiency than the CA
treatments from 2005 to 2006 onwards.
Rainfall use efficiency was generally higher at Hereford
than at the other sites because of the heavier soil, and rainfall
was more evenly distributed that resulted in higher crop
yields. Although CP yielded the lowest from 2007 to 2008
onwards, it was only significantly lower than DS in the very
wet 2007 ⁄ 08 cropping season.
At HRS, the rainfall use efficiency was markedly lower in
the extremely wet and unevenly distributed 2007 ⁄ 2008 rainfall
season (1060 mm), which led to severe waterlogging. On the
other hand, very high rainfall use efficiency was attained in
the comparably dry 2006 ⁄ 2007 rainfall season (534 mm).
There were significant differences between treatments only in
Table 7 Changes in total soil carbon (Mg ⁄ ha) content in 2004 and 2008 (Henderson) in two conservation agriculture and one conventional
treatment
Total
Carbon
0–10 cm
depth
(Mg ⁄ ha)
Change to
initial
site mean
(%)
Total
Carbon
10–20 cm
depth
(Mg ⁄ ha)
Change to
initial
site mean
(%)
Total
Carbon
20–30 cm
depth
(Mg ⁄ ha)
Change to
initial
site mean
(%)
Total
Carbon
0–30 cm
depth
(Mg ⁄ ha)
Change to
initial
site mean
(%)
Henderson 2004 2008 2004 2008 2004 2008 2004 2008
Conventional ploughing 6.5a 7.0a 6 4.1a 6.7a 40 3.7a 4.6 31 14.4a 18.7a 26
Ripline seeding 6.2a 9.2a 39 4.6a 7.6a 72 3.1a 5.2 48 13.9a 22.4a 50
Direct seeding 7.1a 9.0a 36 5.6a 7.3a 52 3.7a 5.3 51 16.5a 21.9b 47
Mean 6.6 8.4 4.8 7.2 3.5 5.0 14.9 20.9
LSD 1.8 2.4 1.9 2.8 1.4 1.9 3.3 3.1
Means followed by the same letter in column are not significantly different at P £ 0.10 probability (LSD-test); samples were all taken in October
of each respective year before the cropping season. Samples were corrected for bulk density and calculated to Mg ⁄ ha.
Table 8 Effect of conventional and conservation agricultural systems
on dispersion ratio (Chikato) and aggregate stability (Hereford),
Zimbabwe, 2009
Dispersion test (%)
Chikato
Conventional ploughing 52.5b
Ripline seeding 46.7a
Direct seeding 39.7a
Mean 46.3
LSD 6.7
Aggregate stability (%)
Hereford
Conventional ploughing 19.2b
Ripline seeding 42.9a
Direct seeding 51.5a
Mean 37.9
LSD 15.6
Means followed by the same letter in column are not significantly
different at P £ 0.01 probability (LSD-test).
Effects of CA on soil quality in Zimbabwe 7
ª 2012 The Authors. Journal compilation ª 2012 British Society of Soil Science, Soil Use and Management
the two last cropping seasons when the rainfall use efficiency
of CP was significantly lower than that of RS.
Discussion
Higher infiltration rates were found with the CA treatments
in most years and across sites compared with conventionally
ploughed treatments. This result accords with those from
previous research results (Derpsch et al., 1986; Roth et al.,
1988; Thierfelder & Wall, 2009). Bescansa et al. (2006)
attribute these improvements not only to residue retention
and associated improvements in biological activity but also to
changes in pore size distribution in un-tilled compared with
tilled soils. Differences between years in infiltration at HRS
are mainly due to differences in seasonal rainfall distribution.
In January 2009, the soils at HRS had two to three times
higher infiltration on CA compared to the conventional
control plot (Table 4), water which could potentially be
available in seasonal dry spells (Thierfelder & Wall, 2010b).
However, as water data are not presented in this study, it is
difficult to draw firm conclusions from higher infiltration
rates on different treatments with reference to available soil
moisture for plant growth.
Too much water infiltration can result in a lowering in
crop performance when there is an impediment to drainage.
At Chikato, granitic bedrock at 50–90 cm, and in HRS, a
subsoil layer with higher clay content, both common for
granitic sandy soils in Zimbabwe (Nyamaphene, 1991),
contributed to increased waterlogging in very wet years
(Thierfelder & Wall, 2009). Yield results at Hereford, where
the soils have a heavier texture, are free draining and have
better aggregation because of higher clay and organic matter
contents, were not negatively affected by excess rainfall.
Differences in SOC between tillage treatments were found
at all sites after 3–4 cropping seasons but were highest at
Chikato. At this site, both CA treatments had higher SOC
(46 and 104%) after four seasons of CA compared to the
initial site mean. In the conventional control, it increased by
only 6% over the same period compared to the initial site
mean. The amount of SOC can be maintained if tillage is not
continued and residues are applied in sufficient quantity;
however, most of the organic carbon will accumulate on the
soil surface if it is not mixed into the soil by tillage (Baker
et al., 2007; Luo et al., 2010). If tillage is continued, there will
be better soil aeration resulting in greater microbial and
enzyme activity that leads to accelerated SOC decay
(Magdoff & Weil, 2004; Alvear et al., 2005; Janzen, 2006).
Crop residues are the main contributors to slightly higher
SOC values. However, in the drier areas of Zimbabwe,
residues are in short supply because of strong competition for
livestock needs. Farmers feed the residues to their cattle
which lead to lower SOC in those areas. Nevertheless,
livestock plays an important role in smallholder farmers’
livelihood, (e.g. as income, as traction and energy source, as
Table 9 Rainfall use efficiency in kg of grain ⁄mm of rainfall at Chikato, Hereford and Henderson Research Station, 2004 ⁄ 05–2009 ⁄ 10
Treatment
Harvest year
2004 ⁄ 05 2005 ⁄ 06 2006 ⁄ 07 2007 ⁄ 08 2008 ⁄ 09 2009 ⁄ 10
Chikato
Rainfall 393 mm 870 mm 412 mm 1377 mm 382 mm 561 mm
kg grain ⁄mm of rain
Conventional ploughing 0.3a 1.3b 3.8b 0.8b 3.9b 0.8b
Ripline seeding 0.4a 1.3b 4.7ab 1.1a 6.5a 1.3a
Direct seeding 0.4a 1.9a 5.9a 1.0ab 7.1a 1.3a
Hereford
Rainfall 557 mm 664 mm 1033 mm 938 mm 815 mm
kg grain ⁄mm of rain
Conventional ploughing 6.3 8.2 5.5b 5.2a 6.1a
Ripline seeding 5.8 5.6 5.7b 5.2a 6.7a
Direct seeding 7.6 7.0 6.8a 5.4a 7.4a
Henderson
Rainfall 772 mm 1096 mm 534 mm 1060 mm 734 mm 617 mm
kg grain ⁄mm of rain
Conventional ploughing 3.3a 3.0a 8.2a 1.1a 2.4b 1.9b
Ripline seeding 3.3a 3.0a 8.1a 1.4a 4.8a 3.5a
Direct seeding 3.1a 2.2a 9.8a 1.1a 3.8ab 2.7ab
Rainfall use efficiency was not statistically analysed at Hereford in 2005 ⁄ 06 and 2006 ⁄ 07 because of the limited number of replications in the
sample; means followed by the same letter in column are not significantly different at P £ 0.05 probability level (LSD-test). Rainfall use
efficiency is calculated by dividing the average yield in each treatment (in kg) by the annual rainfall at each site (in mm).
8 C. Thierfelder & P. C. Wall
ª 2012 The Authors. Journal compilation ª 2012 British Society of Soil Science, Soil Use and Management
saving and insurance in times of drought, as a source of
manure and as wealth status in the community) (Thierfelder
& Wall, 2011). The difference in SOC between the CA and
conventional treatments was not greater on the sandy soils
because the applied residue amounts of 2.5–3 t ⁄ha of maize
stalks on CA plots were not sufficient to add much more to
the existing soil carbon pool (Mapfumo et al., 2007).
Greater amounts of OC are stored in the clay-rich soils of
Hereford so that mobilization and depletion will take much
longer than at Chikato. Differences between treatments
were significant only at P £ 0.10 after three cropping
seasons but will probably become more significant in the
Harvest year2005 2006 2007 2008 2009 2010
Mai
ze g
rain
yie
ld (
kg/h
a)
0
500
1000
1500
2000
2500
3000
3500
4000Conventional ploughing, maize (CP)Ripline seeding, maize (RI)Direct seeding, maize (DS)
c
a
a
NS
a a
b
b
b b
a b
ab
a
aab
b
Figure 2 Effect of conservation agriculture and conventionally ploughed cropping systems on maize yield at Chikato, Zimuto Communal Area,
2005–2010. NS, not significant; mean bars labelled by the same letter in each year are not significantly different; vertical error bar represents
SEDs in each year.
Harvest year2006 2007 2008 2009 2010
Mai
ze g
rain
yie
ld (
kg/h
a)
0
1000
2000
3000
4000
5000
6000
7000
8000
9000Conventional ploughing, maize (CP)Ripline seeding, maize (RI)Direct seeding, maize (DS)
a
N/ANS
bb
N/A
NS
Figure 3 Effect of conservation agriculture and conventionally ploughed cropping systems on maize yield at Hereford Farm, Bindura, 2006–
2010. NS, not significant; N ⁄A, not analysed because of limited numbers of replications under maize in this particular season; mean bars
labelled by the same letter in each year are not significantly different; vertical error bar represents SEDs in each year.
Effects of CA on soil quality in Zimbabwe 9
ª 2012 The Authors. Journal compilation ª 2012 British Society of Soil Science, Soil Use and Management
longer term if surface residue retention continues at the
same intensity.
Results from Zingore et al. (2005) and Chivenge et al.
(2007) show that the SOC fraction on sandy soils is much
more vulnerable than that on loamy and clay soils as
aggregates on the clay-rich soils protect the organic matter
from being decomposed, which reduces nutrient leaching
(Feller & Beare, 1997). It is, therefore, not surprising that the
SOC was maintained at higher level at Hereford and HRS
even with CP. On the sandy soils in Chikato, there is less
protection of organic matter through aggregation, fungal
hyphae and bacterial exudates and, therefore, faster
decomposition and decline in SOC on treatments without
additional organic matter input (Six et al., 2002, 2004). Other
improvements in soil quality, such as improved aggregate
stability in CA treatments (Hereford) and decreased soil
dispersion (Chikato), add to the longer term benefits of CA.
Higher aggregate stability, for example, reduces soil erosion,
surface crusting and water run-off, which benefits the system
(Thierfelder et al., 2005).
Significant increases in maize productivity cannot be
expected immediately, especially when moisture is not a
major limiting factor. If all plots in the comparison had
received the same amount of fertilizer and had the same
maize variety, it would have taken some time before benefits
were evident. At Chikato, the driest and most degraded site
in this study, the benefits occurred faster than on the more
fertile sites of HRS and Hereford. Organic matter input from
crop residues slightly increased the difference in SOC between
CA and the conventional plots at Chikato. This agrees with
Chivenge et al. (2007) who demonstrate that reduced tillage
has a greater impact on clay soils than high input of organic
matter because of textural protection whilst the opposite
applies to sandy soils.
At Chikato and in some years at Hereford, DS
outperformed RS, which is remarkable as both systems are
very similar. The seeder used in DS has a ripper tine to open
the furrow, and so the machine opens a furrow, seeds, applies
fertilizer and closes the ripline immediately, while in the RS
treatment, the ripline is left open while seed and fertilizer are
placed manually in the furrow, which is then closed, probably
leading to losses in soil moisture, especially if the ripline is
not closed immediately. Another factor that could affect the
germination of maize crops is the uneven seeding depth on
the RS treatment.
Differences in rainfall use efficiency between seasons,
especially on sandy soils, suggest waterlogging effects, which
were experienced in the very wet seasons in Chikato and
HRS (Thierfelder & Wall, 2009). CA increases infiltration
that will be positive in drier years but will have negative
effects on crop yields in very wet years if there is an
impediment to drainage. Future predictions on climate
change for southern Africa suggest that this region will be
more affected by drought, reduced average rainfall and
increases in temperature (Lobell et al., 2008). Our data show
higher infiltration rates on CA fields that may help farmers to
mitigate the effects of mid-season dry spells (Thierfelder &
Wall, 2010a). However, there are different critical moisture
limiting phases in the life cycle of a maize crop such as at the
initial crop establishment, the time at silking ⁄ tassling and at
Harvest year
2005 2006 2007 2008 2009 2010
Mai
ze g
rain
yie
ld (
kg/h
a)
0
1000
2000
3000
4000
5000
6000
7000
8000
Conventional ploughing, maize (CP)Ripline seeding, maize (RI)Direct seeding, maize (DS)
b
a
abNS
NS
NS
aab
b
NS
Figure 4 Effect of two conservation agriculture and one conventionally ploughed cropping system on maize yields at Henderson Research
Station, 2005–2010; NS, not significant; mean bars labelled by the same letter in each year are not significantly different; vertical error bar
represents SEDs in each year.
10 C. Thierfelder & P. C. Wall
ª 2012 The Authors. Journal compilation ª 2012 British Society of Soil Science, Soil Use and Management
grain filling, all of which can have major effects on crop
yields. Unfortunately, owing to our experimental design, we
could only include the time before silking ⁄ tassling.CA cropping systems are likely to work best when
adequate fertilizers and weed control are applied to achieve
sufficient biomass and grain yields. Farmers in transition
from conventional to CA systems should, therefore,
concentrate available resources such as mineral fertilizer,
manure and compost on smaller areas first to gradually
increase productivity and soil quality before expanding to
other areas within their farms.
Conclusions
Results from two contrasting on-farm sites and a research
station trial indicate greater infiltration on CA plots with
residue retention than on conventionally ploughed control
plots. Other soil indicators, such as greater aggregate stability
on clay-rich soils and lower soil dispersion rates on sandy
soils, highlight improvements in soil quality as a consequence
of no-tillage and crop residue retention. However, it is
difficult to separate the effects of each component as we
compare the combined effects of CA with conventionally
ploughed system. This could be overcome if there were fully
factorial component trials. Soil carbon increased more on CA
than on conventionally ploughed control treatments. The
results suggest that more soil carbon can be maintained on
soils with higher clay contents because of the protection of
the carbon within soil aggregates. In areas with sufficient
rainfall, the greatest advantages of CA for farmers will
probably not be short-term increases in crop yields but rather
longer term increases in soil fertility that may lead to more
stable crop yields. At Chikato, where moisture and fertility
are more limiting, the additional return occurred sooner,
while it took longer at Henderson Research Station and
Hereford. Differences in rainfall use efficiency between
seasons especially on the very sandy soils with impeded
drainage in Chikato show that CA performance will be
negatively affected in very wet seasons – something also seen
in the 2007 ⁄ 2008 cropping season at HRS. However, in the
better aggregated and more fertile clay soils at Hereford,
higher rainfall led to higher overall yields. Our results are
based on relatively high fertilizer levels. Where farmers want
to shift to CA and cannot afford such high rates, it will be
better in the first instance to concentrate available resources
on smaller areas, and then over time, farmers may gradually
increase the area under CA to thus increase soil fertility at
the farm level.
Acknowledgements
Funding from the Ministry of Economic Cooperation (BMZ)
of the German Government and the International Fund for
Agriculture Development (IFAD) is gratefully acknowledged,
as is the technical support of extension officers and
researchers from Hereford Farm, Zimuto Communal Area,
and Henderson Research Station for assisting in data
collection and trial management as well as the Institute of
Agriculture Engineering, Zimbabwe, and Dr Neal S. Eash
from the University of Tennessee, USA, for assisting in soil
analyses. Special thanks go to Brian Sims for valuable
comments and editing of drafts of this paper.
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