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

logical processes in the given agro-ecosystems. Despite
the advocacy for integrated nutrient management
and ecological approaches to agriculture (e.g.
Giller and Cadisch, 1995i Breman, 1998), little or no
research work in Zimbabwe and other parts of
Southern Africa has focused on natural weeds as an
organic nutrient resource that can be exploited by
smallholder farmers for their management of soil
. fertility. This study, therefore, focused on selfregenerating
N2-fixing indigenous legumes. These
legumes are considered an under-utilized component
of an organic resource pool that may be readily
available to smallholder farmers in many parts of
Africa. Assessing and manipulating the diverse N2­
fixing herbaceous legumes in local agro-ecosystems
provide a good starting point to meet these challenges.
Based on results of a study initiated in December 2001,
this paper explores the concept and scope for indigenous
fallows (Indifallows). The general objective was
to make an appraisal on the potential for indigenous
legumes to contribute towards combating the problem
of poor soil fertility which underlies rural poverty in
Zimbabwe and other parts of Africa.
The Indifallow concept
The Indifallow concept is based on harnessing biological
nitrogen fixation (BNF) of herbaceous annual
legumes native to or naturalized under given agroecological
environments in order to improve the N
economy of natural fallows at minimal establishment
and management costs. While it is traditional
practice to fallow unproductive land, effectiveness
of fallows as a means for soil fertility restoration has
often been compromised by the reduction in fallow
periods as land becomes limiting and also the poor
quality of the plant biomass generated in the fallowing
phase. Efforts to improve the quality of faHows
through agroforestry tree crops such as Leucaena,
Sesbania and Acacia spp. have often been hindered
by high establishment costs and lack of immediate
benefits to the farmer (Cook and Gmt, 1993; Kwesiga
and Coe, 1994; Snapp et al. 1998). Through use
of self-regenerating and well-adapted indigenous
annual legumes, constraints related to seed costs
and availability, nursery management and biomass
management are minimized. Thus a focus on BNF
of indigenous herbaceous legumes will not only
help to provide a basis for integrating weeds as a
potential source of N and soil organic matter in
cropping systems, but also to improve and maintain
the biodiversity in smallholder agro-ecosystems.
Most studies on weed management in smallholder
fanning systems have been concerned with the adverse
effects of weed competition on moisture and
nutrient uptake by crops, and the labour costs involved
in managing the weeds. As a result, the
strategy has been to eradicate weeds, mainly by depleting
the seed bank (weed plants are killed before
flowering). Clean weeding, which involves maintenance
of weed-free fields until the end of the cropping
season, is a common practice among smallholder
farmers in Zimbabwe. This is still driven by
extension recommendations based on 'green revolution'
technologies. Although this weed management
approach has become a tradition, it may compromise
tl}e long-term sustainability of these cropping
systems· thro_ugh loss of bio-diversity and reduced
organic matte'i- inputs. It also leads to the dominance
of pernicious weeds that are by definition difficult
to control by hand weeding and cultivation.
An analysis of 'green revolution' technologies in
sub-Saharan Africa has shown that they are largely
incompatible with the socio-economic environment
on smallholder farms (Quinones et al. 1997). It is imperative
that the current weed management regimes
be revised to match the demands for integrated nutrient
management and reduce labour requirements.
This may reduce the burden on women and children
who usually provide labour for key agricultural
activities such as weeding and seedling establishment
and transplanting in agroforestry. Technologies
with minimal labour demands are likely to
be particularly appropriate for the poorest farmers
who are often women in single-headed households,
or families where key members providing labour
have been lost due to AIDS. As we explore the feasibility
and merit of indifallows from a soil fertility
perspective, the key question is whether such legumes
do exist in smallholder farming systems and
under what soil conditions. .
Study Sites
The research was conducted in three communal
(smallholder) areas found in different eco-zones of
Zimbabwe, namely Chikwaka (31° 30' Ei 17° 40' S)
in Natural Region II, Chinyika (32° 25' Ei 18° 15' S)
in NR III and Zimuto (30 0
52' E; 19° 50' S) Communal
Areas in NR IV. Natural Region II receives over
750 mm of rainfall annually between November and
March while NR's III and IV receive 650-750 mm
and 450-650 mm of unimodal rainfall per annum
respectively. The soils in all sites are granite-derived
sand to loamy sands, Haplic Lixisol/ Arenosols according
to the F AO classification. The sites were
mainly chosen based on their being representative
of most smallholder farmmg areas. Chikwaka and
Zimuto are old Communal Areas where cultivation
by smallholders has been going on for over 70
years. The a:verage household landholding in Chikwaka
and Zimuto was 3 ha, while in Chinyika, a
resettlement area established in 1982, the landholding
was 6 ha per household.
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Grain Legumes and Green Manures for Soil Fertility in Southern Africa