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
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
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.