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

ENHANCING THE CONTRIBUTION OF LEGUMES AND BIOLOGICAL
NITROGEN FIXATION IN CROPPING SYSTEMS:
EXPERIENCES FROM WEST AFRICA
Abstract
BERNARD VANLAUWE, ANDRE BATIQNO,
Tropical Soil Biology and Fertility Institute of CIA T, PO Box '30677, Nairobi, Kenya
RJ CARSKY, J DIELS, N SANGINGA, and S SCHULZ
/ITA Nigeria, c/o Lambourn, 26 Dingwall Road, CroydonCR9 3££, UK
The lack of adoption of improved soil fertility management options to counteract soil fertility decline has led to major
changes in the research and development paradigm, leading to the currently widely adapted concept of Integrated Soil
Fertility Management. Legumes in general and biological N fixation specifically have a potentially important role to
play in ISFM strategies. Four examples are summarized of attempts to enhance the soil fertility status using legumes in
various agro-ecozones of the West African savanna. These case studies cover the technical aspects of the various legumebased
systems but also focus equally on the evaluation, adaptation, and adoption processes.
Alley cropping with leguminous hedgerows is a first example. The technology was proven to be technically sound and
generated a lot of process work in agroforestry systems. Especially important to note is that the impact assessment phase
was not in synchrony with the technology development phase, which excluded any useful feedback and delayed the identification
of the appropriate niches for this system. These were unfortunately found to be geographically limited. The
Mucuna cover cropping system is a second example. As with alley cropping systems, the inclusion of Mucuna in a
cropping system was observed to significantly enhance crop yield. Contrary to alley cropping, however, impact assessment
was implemented during the testing and evaluation phase and useful feedback loops led to clearer insights about
the specific role Mucuna could play in farmers' fields . This role was more associated with its ability to suppress Imper­
·ata cylindrica weeds than with improving the soil fertility status, thereby also limiting its niche for adoption. As a
third example and a reaction to the lack of widespread adoption of the former two technologies, dual purpose grain legumes
- cereal rotations are evaluated. Such systems, using improved legume germplasm that provides net N benefits to
the cropping system besides grains, significantly enhance cereal yield and supply the farmer with immediate products
that can be consumed or sold. This technology shows a lot ofpromise and.is currently spreading in the Northern Guinea
savanna zone of Nigeria, but required the creation of local processing skills and/Or markets for th~ grains and intensive
interaction between breeders, soil fertility specialists, and farmers. A last example deals with the role ofcowpea in rotations
in the dry savannas. As with soybean, improved germ plasm of cowpea can also be used to enhance the soil fertility
status while yielding immediate benefits to farmers.
In conclusion, several aspects are highlighted that need to be considered when aiming at enhancing the contribution of
legumes and biological N fixation to cropping systems. These include the need for immediate benefits and the role of
multipurpose germplasm in providing these, the need to identify niches and the role of markets, and the need for multidisciplinarity
and full participation of all stakeholders. Finally, some potential routes for future research are indicated.
Key words: Grain legume, green manure legume, forage legume, dual or multi-purpose legume, biological N fixation,
cropping system, impacts, west Africa
Introduction
The soil fertility status of the soils in sub-Saharan
Africa (SSA) is generally believed to be poor due to
poor inherent soil quality and inappropriate soil
management practices. Such statements are usually
backed-up by a demonstration of highly negative
nutrient balances for the major plant nutrients and
the existence of wide gaps between yields obtained
under well-managed compared to on-farm conditions
for the major crops. These facts are generally
also applicable to the West African savanna ,a.groecozone.
Although soil fertility replenishment has
been on the research and development agenda for
several decades in SSA as this is believed to have
substantial impacts on the livelihoods of the rural
population, relatively little has been achieved. The
reasons for this are plenty and beyond the scope of
this paper but importantly, the paradigms underlying
the soil fertility research and development
agenda have continuously changed to attempt to
deal with the issue· of non-adoption of improved
Grain Legumes and Green Manures for Soil Fertility in Southern Africa 3