Grain Legumes and Green Manures for Soil Fertility in ... - cimmyt

Introduction
Legumes playa signifiCant role in the improvement
of nitrogen budg~ts through biologiCal nitrogen
fixation (BNF) and cycling of other nutrients on the
farm (Giller and Wilson, 1991; Giller, 2001) . Cowpea
(Vigna unguiculata (L). Waip.) can be considered a
cheap legume to grow in that its fertility and rainfall
demands are low. Hegewald (1990) found cowpea
to produce acceptable yields on acidic oxisols. However,
the economiCs and yield benefits of BNF in
maize/cowpea rotation have not been fully explored
(Shumba et al. 1990). Its role in increasing
and maintaining soil fertility is not clearly defined.
Often farmers do not have a planned P and N fe-rtilization
strategy for these rotations. The fertilization
of cowpea grown in rotation with maize has not
been studied thoroughly, especially the economics
of fulfilling the P requirements of the legume. There
is need to evaluate the different fertilization practices
in relation to their agro-economic effectiveness
for different farmer domains.
Soils in the smallholder farming sector of Zimbabwe
are inherently poor in fertility. Deficiencies in both
macro- and micronu trients have been reported in
these sandy soils (Grant, 1981; Mashiringwani, 1983;
Nyamapfene, 1991). Farmers use different strategies
to either add and/or recycle nutrients on their
farms. Use of cattle manure, composts, mineral fertilizers,
crop or grass residues, grass fallows, grain
legumes and green manures in cereal/legume rotations
are loosely practiCed · by farmers as ways of
managing soil fertility (Nhamo et al. 2002). Such
practices on sandy soils are important in the management
of the most limiting nutrients and soil organiC
matter and there is potential to improve their
efficiencies. In all, the practices have been to add
whatever is available to the soil or nothing at all.
This results in addition of much less, just enough or
more than required nutrients for the'field crops.
With these practices, the use of both organic and
mineral fertilizers has no scientific basis. This has
rendered optimum crop production and profit levels
difficult to attain on sandy soils.
Besides the soil fertility contribution, cowpea provides
the needed proteins in rural households
through both the pea and the leaves that are used as
relish. Traditionally, cowpea porridge was an important
and nutritious dish making part of the diet
for the farming· communities. It is a multiplepurpose
legume which can be used for human food
and livestock feed (Johnson, 1970; Rao and
Mathuva,2000).
In the smallholder farming systems of Zimbabwe
the cultivation of tradition legumes, including cowpea,
is not emphasized. Current uses of cowpea for
. improving household food security and soil fertility
vary from area to area. The reasons for this variability
are not clear. The aims of this study were to determine
the current cultivation practices, perceptions
of farmers on the benefits and constraints of
effectively utilizing cowpea in their farming system,
and to evaluate the role cowpea could play in improving
soil fertility and hence household food security.
Materials and Methods
A survey was conducted in three communal areas,
Chihota (Mashonaland East Province), Zimuto
(Masvingo Province) and Shurugwi (Midlands
Province) representing natural regions II, III and IV
of Zimbabwe respectively. Chihota receives annual
rainfall of between 800 and 1000 mm whereas Shurugwi
and Zimuto receive 600-800 mm and 450-600
rum respectively. The rainfall distribution within
and across season if variable, and in all the areas
mid-season droughts are a common feature.
Farmers in Chihota, Shurugwi and Zimuto rely on
agriculture for food and to generate income to sustain
their families. Most families have financial constraints
and limited agricultural inputs are purchased
for use in the production of both legumes
and cereals.
Within each of the· areas, a formal questionnaire was
administered to collect information on cowpea practices.
The questionnaire captured information on
household characteristics, crop production practiCes,
and cowpea placement in the farming systems,
'current constraints and opportunities for in-·
creased productivity. The semi-structured questionnaire
was administered to a sample of sixty households
in each of the three communal areas. Welltrained
enumerators carried out data collection ~ The
data collected was captured and analyzed using
SPSS (Statistical Package for Social Sciences).
Results
Like most communal areas in Zimbabwe, maize
dominates other crops and most of the land was put
under this staple food crop in Ch,ihota, Shurugwi
and Zimuto. Tables 1 and 2 show the area under
several non-legume and five legume crops grown in
the study areas. Minor traditional crops like millets
(rapoko) were also commonly cultivated on small
areas in the study areas. A few farmers grew cash
crops including cotton, tobacco and paprika.
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Grain legumes and Green Manures for Soil Fertility in Southern Atnca