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Soil fertility in spatially variable soils in smallholder areas of southern Africa
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Soil fertility in spatially variable soils in smallholder areas of southern Africa


This study compared the effects of: …

This study compared the effects of:
i) Farmer resource endowment
ii) Field location in relation to homestead,
on soil fertility status in two smallholder areas located in contrasting agroecological regions

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  • 1. Soil fertility in spatially variablesoils in smallholder areas ofsouthern AfricaJustice NyamangaraDepartment of Soil Science & Agricultural EngineeringUniversity of ZimbabweBox MP167, Mount Pleasant, Harare, Zimbabwe
  • 2. Presentation outline Introduction Objectives Hypotheses Methodology Results Discussion Conclusions
  • 3. Introduction
  • 4. Average nutrient application rates in 2002/3 (kg/ha arable land)SSA – 9Latin America-73South Asia – 100E & SE Asia- 135(FAO, 2004)Average Grain Yield Africa – 1 t/ha World – 3 t/ha(Africa Fertiliser Summit, 2006)
  • 5. Soil fertility and yield potential acrossresettlement types (Bindura, 2006/7)  Fertiliser response higher in new resettlements.  In old resettlements fertiliser response poorer than communal areas.  Control yield in new resettlement areas >1.5 t/ha
  • 6. Nitrogen  Nitrogen the most limiting nutrient in southern Africa (Sanchez et al., 1997; Nyamangara et al. 2000).  Major source of mineral N is Ammonium Nitrate and Urea.  Manufactured Zimbabwe, SA, some imported.  Expensive - In Zimbabwe, plant consumes 10-20% of electricity.
  • 7. Phosphorus  P second most limiting nutrient in southern Africa.  In Malawi, most common compound fertiliser – NP  In Zimbabwe – P mined locally – Low in Cd, important for horticulture industry
  • 8. Other nutrients K –rel. OK but cases of deficiency and crop response (maize) have been reported in high potential areas of central Malawi, Eastern Zambia and NE Zimbabwe. Micronutrient deficiencies (esp. Zn, B) and crop responses also in high rainfall areas (Zimbabwe, central Malawi, eastern Zambia) on sandy acidic soils (Mugwira & Nyamangara, 1998; Zingore et al., 2008; TSBF-CIAT, 2008).
  • 9. Zimbabwe smallholders areas Soil fertility decline is a major constraint to increasing crop productivity on smallholder farms. These are characterised by varying soil fertility between and within farms as well as across agro-ecological zones Farmers typically apply most nutrient resources to fields closest to homesteads -has led to gradients of decreasing soil fertility from the homestead in some cropping systems (Tittonell et al., 2005), large enough to affect crop response. However, cases soil fertility gradients increasing from homefields to outfields have also been reported in the Central Highlands of Ethiopia (Haileslassie, et al., 2007).
  • 10.  Soil fertility also varies considerably between farms, mainly driven by differing access to nutrient resources between farms of different wealth classes and use large amounts of fertilisers. Improved understanding of the spatial and dynamic variability in soil fertility, crop yields and nutrient use efficiencies is necessary. Although the occurrence of soil fertility gradients has been documented, this has been mainly in sub-humid conditions where there is a general shortage of arable land for expansion.
  • 11. ObjectivesThis study compared the effects of:i) Farmer resource endowmentii) Field location in relation to homestead,on soil fertility status in two smallholder areas located in contrasting agro- ecological regions.
  • 12. Hypotheses Farmer resource endowment is positively related to soil fertility status. Gradients of decreasing soil fertility from the homestead occur irrespective farmer resource-endowment.
  • 13. Methodology
  • 14. Rainfall:Study sites Murewa: 800- 1000 mm, Gokwe South: 450 – 650 mm Soils: Murewa: Granitic sands and Red clays, Gokwe S: Kalahari sands Farming syst.: Mixed - dominated by maize (+ cotton in Gokwe) Fields are individually owned and managed but are also communally grazed in winter.
  • 15.  34 & 23 farmers were selected in Murewa and Gokwe, respectively, and classified into 1) resource-constrained (RG1), intermediate (RG2) and resource-endowed (RG3) (Mtambanengwe and Mapfumo 2005; Zingore et al. 2007a). The farmers in the different wealth categories were asked to select the most productive and least productive maize fields. The distance of each field from the homestead and cattle pen was measured and the field nearest to the homestead was designated ‘homefield’ and the one furthest ‘outfield’. A structured questionnaire was used to collect soil fertility management practices used and main crops grown by the selected farmers on the home- and outfields. At silking stage (ca. 10-12 weeks after emergence) soil samples were taken to assess soil fertility status.
  • 16. Results
  • 17.  Cattle ownership in both study sites was low for Zimbabwe (5.4 and 5.6 cattle per household in 5 Murewa and Gokwe respectively) and hence the manure application rates at the farm level were low. 4  Wealthy farmers applied moreManure (t ha-1) 3 manure (3.5-9 t ha-1) to their fields in Murewa, compared to the 2 intermediate (up to 1.5 t ha-1) and resource-constrained (<1 t ha-1) 1 farmers. 0  RG3 farmers in Gokwe applied Resource-endowed Intermediate Resource-constrained significantly less mineral NPKS 350 fertiliser (<100 kg ha-1) compared Homefield Outfield to RG1 (>250 kg ha-1) and RG2 (up 300 to 150 kg ha-1) farmers.Mineral Fertiliser Input (kg ha -1) a b 250 200  Other nutrient resources used mostly in Murewa but on a limited 150 scale were compost, leaf litter and anthill soil, and these were 100 targeted to homefields. 50 0 Resource-endowed Intermediate Resource-constrained
  • 18.  Farmers in Murewa own small farms (1-3 ha) and continuously cultivated their fields, while in Gokwe South farmers owned larger farms (5-10 ha) and frequently fallowed their fields. Fields in Murewa had been under cultivation for longer periods (~30 years) compared with Gokwe (~15 years). Outfields were generally larger homefields (16.7% in Murewa; 31.9% in Gokwe South).
  • 19. Murewa 14 Homefield Outfield  There were differences 12 for total soil N and SOC 10 a b across resource- endowment classes and O.C (g kg-1) 8 6 field types in each wealth 4 category but the 2 differences were not 0 significant. 14 Gokwe 12 a b  However, total SOC and 10 N were higher inO.C (g kg-1) 8 homefields compared to 6 outfields in Murewa, and 4 the opposite trend was 2 observed in Gokwe 0 Resource-endowed Intermediate Resource-constrained South. Soil Organic Carbon
  • 20.  CEC and all Homefield clay 35 Outfield clay exchangeable bases Homefield sand were also higher in 30 Outfield sand homefields compared to outfields, and were 25 a b a b c largely similar for RG1 and RG2 farmers andCEC (cmolckg-1) 20 much lower for the RG3 farmers. 15  Soil pH was higher in 10 homefields compared to outfields in Murewa but 5 the difference was only significant for RG1 0 Murewa Gokwe farmers where soil pH was extremely acidic in outfields. Cation Exchange Capacity
  • 21. Homefield 30 Outfield Murewa 25 a b  Available P was particularly responsive toAvailable P (mg kg )-1 20 15 management and 10 decreased sharply from the RG1 group (>20 mg 5 kg-1) to < 5 mg kg-1 in the 0 30 Gokwe RG3 group in Murewa. a b 25 Available P (mg kg )-1 20  In Gokwe available P was 15 significantly higher in 10 fields of the RG1 farmers 5 than the RG2 and RG3 0 farmers. Resource-endowed Intermediate Resource-constrainedAvailable soil P content
  • 22. 1.2 Murewa Homefield 1.0 Outfield 0.8 The observed decreaseTotal N (g kg-1)  0.6 a b in SOC, total N and 0.4 available soil P with 0.2 decrease in resource- endowment in sub-humid 0.0 Gokwe conditions has been 1.2 a b reported elsewhere in 1.0 Zimbabwe 0.8 (Mtambanengwe and Total N (g kg-1) Mapfumo, 2005; Zingore, 0.6 et al., 2006). 0.4 0.2  Attributed to differences 0.0 in the nutrient resources Resource-endowed Intermediate Resource-constrained available to the different classes of farmers. Total Soil N
  • 23. Discussion The amount of manure produced and applied to fields has declined compared to previous years where up to 80 t ha-1 were applied (Mugwira and Murwira, 1997) due to decreasing cattle number (droughts, land pressure). Farmers in Gokwe applied manure to their fields once every 2 - 3 years, similar to findings of Ahmed et al. (1997) who reported that smallholder farmers in semi- arid areas of Zimbabwe applied manure once every 3 - 5 years to their maize crop. Farmers cited the manure scarcity due to low cattle ownership (~ 6 cattle per household in Gokwe) as the main reason behind the practice.
  • 24.  RG1 farmers often have access to livestock manure and resources to purchase mineral fertiliser. The higher soil fertility status in homefields in Murewa and outfields in Gokwe S. implied that the farmers in the contrasting agro- ecological zones used different management strategies. In Murewa, where land holdings are small and land for expansion unavailable, farmers concentrated their nutrient resources in homefields, a practice that has also been reported elsewhere (Tittonell, 2006; Zingore et al., 2007; Mtambanengwe and Mapfumo, 2006; Vanlauwe et al., 2006). However, in Gokwe where land holding is large and land for expansion available, farmers quickly move to another field, further way from the homestead once fertility has declined (Mapedza et al., 2001). Soil fertility will be higher in the relatively younger outfields compared to the older homefields which are continually cultivated.
  • 25. Conclusions Resource-endowed farmers have access to more soil nutrient resources resulting in higher soil fertility status in their fields compared to their poorer counterparts. Besides farmers’ access to resources and management strategies, land availability and farming system have an influence in the direction of soil fertility gradients within farms. Soil fertility gradients need to be considered in developing fertiliser recommendations and in targeting crops to be grown
  • 26. Acknowledgements AFRICARE-Zimbabwe, Regional Universities Forum (RUFORUM) and the Tropical Soil Biology and Fertility Institute of CIAT (TSBF-CIAT) for providing funding for this work. Farmers in Gokwe S. and Murewa districts for their cooperation Department of Agricultural Technical and Extension Services (Agritex) for coordinating field activities.
  • 27. Thank you!!