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Effects of conservation agriculture on soil quality and productivity in contrasting agro-ecological environments of Zimbabwe C. Thierfelder & P. C. W all 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. Several Correspondence: 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 Soil Use and Management

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Page 1: Effects of conservation agriculture on soil quality and productivity in contrasting agro-ecological environments of Zimbabwe

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

Page 2: Effects of conservation agriculture on soil quality and productivity in contrasting agro-ecological environments of Zimbabwe

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

Page 3: Effects of conservation agriculture on soil quality and productivity in contrasting agro-ecological environments of Zimbabwe

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

Page 4: Effects of conservation agriculture on soil quality and productivity in contrasting agro-ecological environments of Zimbabwe

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

Page 5: Effects of conservation agriculture on soil quality and productivity in contrasting agro-ecological environments of Zimbabwe

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

Page 6: Effects of conservation agriculture on soil quality and productivity in contrasting agro-ecological environments of Zimbabwe

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

Page 7: Effects of conservation agriculture on soil quality and productivity in contrasting agro-ecological environments of Zimbabwe

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

Page 8: Effects of conservation agriculture on soil quality and productivity in contrasting agro-ecological environments of Zimbabwe

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

Page 9: Effects of conservation agriculture on soil quality and productivity in contrasting agro-ecological environments of Zimbabwe

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

Page 10: Effects of conservation agriculture on soil quality and productivity in contrasting agro-ecological environments of Zimbabwe

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

Page 11: Effects of conservation agriculture on soil quality and productivity in contrasting agro-ecological environments of Zimbabwe

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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