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

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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 inltration 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 rst 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 signicant 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 inltration, 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 benets are

    however scarce. The benets 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: c.thierfelder@cgiar.org

    Received January 2011; accepted after revision March 2012

    Soil Use and Management doi: 10.1111/j.1475-2743.2012.00406.x

    2012 The Authors. Journal compilation 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 & Stocksch (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 sufcient supporting

    scientic 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 t. Site-specic 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 9495% 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 20042010.

    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 farmers

    eld. By the 2009 2010 cropping season, this had beenexpanded to twelve replications in the community. Treatment

    characteristics were as follows:

    1. The conventional control treatment (CP) with ploughing

    at a shallow depth (1015 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 rst CA treatment was a ripline-seeded (RS)

    treatment, which was subsoiled in the rst 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 eld at Hereford and HRS

    (Table 2) or stored next to the demonstration plots after

    harvest at Chikato and spread on the elds 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 2.53.0 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 rst CA treatment, residues were

    retained from the previous harvest or supplemented with

    Hyparrhenia grass at Chikato to achieve 2.53.0 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

    2012 The Authors. Journal compilation 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 of35 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), SC513and SC403 (hybrids) in 2005 06 and 2006 07] was sown at37 000 plants ha on all plots in a particular year at respectiveseeding 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 signicant 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 23 times per season).

    Hereford Farm. Here, the programme started with

    replications at four sites in 2005 06, which increased to eightin 2007 08. All sites were managed with a maizesoyabeanrotation 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 prole 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 028 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 7E )70 1.45 5 4.2 1.6 37 0.06 83 13 4Bs )105 1.67 2030 4.5 1.7 44 84 14 2Bt >115 1.73 2030 4.3 7.0 38 66 15 19

    Chikato 020 1.32 4.6 2.52 20 0.26 95 3 2

    Hereford 020 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 052009 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 bythe same letter in column at each site are not signicantly different at P 0.05 probability level (LSD-test).

    Effects of CA on soil quality in Zimbabwe 3

    2012 The Authors. Journal compilation 2012 British Society of Soil Science, Soil Use and Management

  • rst year onwards. Maize plant populations at Hereford were

    denser than in Chikato (44 444 ha) because of the higherrainfall in this area. Soyabeans [Glycine max (L) Merr] were

    sown at 333 333 plants ha. Fertilizer levels for maize werethe 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 and7 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 cropemergence, 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 in20062008, PAN 67 in 2009 10) were sown at each seedingdate (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 fth

    replication was added in the 2006 2007 crop season. The plotarea was 160 m2. The treatments at HRS were as follows:

    1. Shallow ploughing (1015 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 1015-cm-

    deep ripline opened by an an...

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