Soil fertility in spatially variable soils in smallholder areas of

southern Africa

Justice NyamangaraDepartment 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 dynamicvariability in soil fertility, crop yields and nutrient useefficiencies 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

Study sitesRainfall: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 Murewa and Gokwe respectively) and hence the manure application rates at the farm level were low.

Wealthy farmers applied more manure (3.5-9 t ha-1) to their fields in Murewa, compared to the intermediate (up to 1.5 t ha-1) and resource-constrained (<1 t ha-1) farmers.

RG3 farmers in Gokwe applied significantly less mineral NPKS fertiliser (<100 kg ha-1) compared to RG1 (>250 kg ha-1) and RG2 (up to 150 kg ha-1) farmers.

Other nutrient resources used mostly in Murewa but on a limited scale were compost, leaf litter and anthill soil, and these were targeted to homefields.

Resource-endowed Intermediate Resource-constrained

Min

era

l F

ert

iliser

Inpu

t (k

g h

a-1

)

0

50

100

150

200

250

300

350Homefield

Outfield

a b

Resource-endowed Intermediate Resource-constrained

Ma

nu

re (

t h

a-1

)

0

1

2

3

4

5

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).

There were differences for total soil N and SOC across resource-endowment classes and field types in each wealth category but the differences were not significant.

However, total SOC and N were higher in homefields compared to outfields in Murewa, and the opposite trend was observed in Gokwe South.

MurewaO

.C (

g k

g-1

)

0

2

4

6

8

10

12

14Homefield

Outfield

Gokwe

Resource-endowed Intermediate Resource-constrained

O.C

(g

kg

-1)

0

2

4

6

8

10

12

14

a b

a b

Soil Organic Carbon

CEC and all exchangeable bases were also higher in homefields compared to outfields, and were largely similar for RG1 and RG2 farmers and much lower for the RG3 farmers.

Soil pH was higher in homefields compared to outfields in Murewa but the difference was only significant for RG1 farmers where soil pH was extremely acidic in outfields.

Murewa Gokwe

CE

C (

cm

ol ckg

-1)

0

5

10

15

20

25

30

35Homefield clay

Outfield clay

Homefield sand

Outfield sand

a b a b c

Cation Exchange Capacity

Available P was

particularly responsive to

management and

decreased sharply from

the RG1 group (>20 mg

kg-1) to < 5 mg kg-1 in the

RG3 group in Murewa.

In Gokwe available P was

significantly higher in

fields of the RG1 farmers

than the RG2 and RG3

farmers.

MurewaA

vaila

ble

P (

mg

kg

-1)

0

5

10

15

20

25

30

Homefield

Outfield

a b

Ava

ilable

P (

mg

kg

-1)

Gokwe

Resource-endowed Intermediate Resource-constrained

0

5

10

15

20

25

30

a b

Available soil P content

The observed decrease in SOC, total N and available soil P with decrease in resource-endowment in sub-humid conditions has been reported elsewhere in Zimbabwe (Mtambanengwe and Mapfumo, 2005; Zingore, et al., 2006).

Attributed to differences in the nutrient resources available to the different classes of farmers.

Murewa

Tota

l N

(g

kg

-1)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Homefield

Outfield

Gokwe

Resource-endowed Intermediate Resource-constrained

Tota

l N

(g

kg

-1)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

a b

a b

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!!