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




Justice Nyamangara
Department of Soil Science & Agricultural Engineering
University of Zimbabwe
Box MP167, Mount Pleasant, Harare, Zimbabwe
Presentation outline
 Introduction
 Objectives
 Hypotheses
 Methodology
 Results
 Discussion
 Conclusions
Introduction
Average nutrient
  application rates in
  2002/3 (kg/ha arable
  land)
SSA       –    9
Latin America-73
South Asia – 100
E & SE Asia- 135
(FAO, 2004)

Average Grain Yield
  Africa – 1 t/ha
  World – 3 t/ha
(Africa Fertiliser Summit,
  2006)
Soil fertility and yield potential across
resettlement 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
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.
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
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).
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).
   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.
Objectives
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 agro-
    ecological regions.
Hypotheses
   Farmer resource endowment is positively
    related to soil fertility status.

   Gradients of decreasing soil fertility from
    the homestead occur irrespective farmer
    resource-endowment.
Methodology
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.
   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.
Results
   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 more
Manure (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
   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).
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 in
O.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
   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 and
CEC (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
Homefield
                        30         Outfield     Murewa


                        25
                                                               a           b
                                                                                         Available P was
                                                                                          particularly responsive to
Available 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-constrained




Available soil P content
1.2
                                             Murewa
                                                                      Homefield
                    1.0                                               Outfield


                    0.8

                                                                                       The observed decrease
Total 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
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.
   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.
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
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.
Thank you!!

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

  • 1. Soil fertility in spatially variable soils in smallholder areas of southern Africa Justice Nyamangara Department of Soil Science & Agricultural Engineering University of Zimbabwe Box MP167, Mount Pleasant, Harare, Zimbabwe
  • 2. Presentation outline  Introduction  Objectives  Hypotheses  Methodology  Results  Discussion  Conclusions
  • 4. Average nutrient application rates in 2002/3 (kg/ha arable land) SSA – 9 Latin America-73 South Asia – 100 E & 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 across resettlement 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. Objectives 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 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.
  • 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.
  • 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 more Manure (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 in O.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 and CEC (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 to Available 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-constrained Available soil P content
  • 22. 1.2 Murewa Homefield 1.0 Outfield 0.8 The observed decrease Total 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.