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

have not been studied. Because of the possible conflicting
uses that legume leaves have on the farm, it
is important to quantify the economic b~nefits separatelyand
together. ­
Similar to other field crops, grain legumes require
soil nutrients for them to grow as well as fix N from
the atmosphere. The growth is a response to the soil
type, fertilization and soil water availability. On
sandy soils that are commonly found in the communal
areas, the nutrition of N fixers that contribute to
the successful symbiosis has not been . emphasized.
Higher cowpea yields from homestead fields (Table
3) are a result of better soil fertility management.
Studies have sho~ that they respond well to P application
(Giller, 2001). However, the P, K, micronutrients
and lime requirements for cowpea in a
maize/cowpea rotation have not been worked out.
Fertilization of the legume in a legume/cereal rotation
is important if productivity is to improve from
the current low levels. At present, there is scant information
on the effective and efficient way of using
organic and inorganic fertilizers on legume-cereal
rotations (Giller, 2001). The current practice of adding
mineral N reduces the N-fixing capacity of the
legumes in these farming systems (Table 7). For the
different agro-ecological zones, rates of P application
need to be worked out. The economics of the
first application .as well as the residual P effects on
both the cereal and the legume in rotations, as well
as in intercrops for the different soil types, are required.
This information will be important and useful
in targeting legumes properly on the farm. Work
on row spacing of maize and cowpea show improved
yields with wider spacing but the wider
spacing leads to low plant populations. This leads
to low biomass production and hence less effective
utilization of the BNF from legumes.
Soil nutrients interact with the available moisture ..
As reported by Muza and Mapfumo (1999), soil nutrient
and water interactions have a large effect on
the overall biomass production of legumes. Cowpea
has the advantage of a deep rooting system that
makes it adaptable to different agro-ecological
zones. However different varieties have different
attributes so proper targeting is important for effective
use of cowpea in farming systems. Grown in
rotations with maize, cowpea has been reported to
reduce weed pressure in the residual season
(Kamau et al. 1999). Similar findings have been reported
in intercrops, except that the weed suppression
in intercrops is in the first season.
Cowpea utilization
Most cowpea grown. is utilized as boiled beans for
either direct consumption or as relish. It remains a
cheap source of protein especially during the dry
winter season. In Shurugwi a study done at a school
positively correlated the consumption of cowpea to
the high tum out and class performance of primary
school pupils (SDARMP, 1997). Other less commonly
used dishes include porridge, scones/bread,
and there is potential for more. For the benefit of the
communities these other benefits in addition to soil
fertility technologies have proven to be important in
technology acceptability. In the case of cowpea, the
health effects of the dishes have to be considered to
see how these could be made part of the diet of
HIV / AIDS affected persons. Cowpeas provide both
calories and protein (Venter et al. 1997). For food
security, indigenous and traditional crops need to
be improved since their important contribution has
largely been ignored in recent years.
Constraints in cowpea production
Fertilization. Constraints on using legumes effectively
in soil. fertility management in the smallholder
farms are varied. Low perception about minor
crops, little biomass from a small area planted,
seed availability problems, lack of exposure to information
on their production and little information on
the potential benefits of using the legume crop in
maize-based farming systems are some of the reasons
why legumes are little used in fertility management
(Rusike et al. 2000). Cowpea grown in intercrops
benefits from the fertilizer applied on the
maize. The nitrogen from basal and topdressing
maize fertilizer reduces the amount of N fixation by
the legumes. This reduces the benefits from the
cowpea and the potential N addition to the nutrient
budget through BNF. Farmers sometimes also complain
about the higher labour demands with legume
crops compared to the cereals (Jeranyama et al.
2000). Soil fertility management based on rotations
can be used to come up with integrated soil fertility
management strategies that have the potential to
improve he livelihoods of people in the smallholder-farming
sector of Zimbabwe. Use of combinations
of organic and inorganic nutrient sources
can produce better crop yields and improve the soil
organic matter levels in the long term (Murwira et
al. 2002 ; Nhamo et al. 2001).
Marketing. The cowpea market is underdeveloped.
The whole product chain has not .been developed
and supported enough to benefit the farmers. Seed
sources identified in the study are mainly local and
little commercial seed finds its way to the farmers.
Interested farmers are therefore faced with the uncertainty
of growing unproved seed. A large proportion
of the farmers keep some of their harvest for
seed for the commonly grown legumes. Considering
that some of the grain is consumed by the family,
seed availability could be one of the root causes
of the low areas for cowpea (Rusike et al. 2000). A
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Grain legumes and GreenManures for Soil Fertility in Southern Africa