Biological nitrogen fixation can supply the N demand of important crops grown in South America, such as soybean and common bean. Soybean is an exotic plant in Brazil and therefore soybean bradyrhizobia diversity is restricted to changes caused by adaptation of inoculant strains to the soil. Some fast-growing indigenous strains are also able to nodulate soybean and were identified mainly as R. tropici, but are poor competitors against Bradyrhizobium. On the other hand, a high level of diversity is found among common bean rhizobia. Selection programs have been conducted to obtain more efficient and competitive strains for both crops and inoculation usually results in grain yield increases. Biological nitrogen fixation is usually limited by environmental factors, as high temperature and low soil moisture, soil fertility problems (such as acidity and deficiency of P and Mo) and agriculture practices (e.g. seed treatment with fungicides, tillage practices).
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We wish to thank CNPq (PRONEX 41.96.0884.00), INCO-EC (ERBIC18CT980321) and CNPq (520396/96-0), for financial support and Dr George G. Brown for his assistance with the English text.
SOYBEAN N2 FIXATION AND FOOD SECURITY FOR SMALLHOLDER FARMERS: A RESEARCH-EXTENSION MODEL FOR SUB-SAHARAN AFRICA
S. Mpepereki, F. Makonese, K.E. Giller
Dept of Soil Science, Univ. of Zimbabwe, P.O.B. MP 167, Mt Pleasant, Harare, Zimbabwe
Nitrogen remains the single most limiting nutrient for crop growth in most developing countries. Exploitation of biological N fixation offers a unique opportunity to harness "free" fertilizer from a relatively low-cost technology. Soybean has been shown to respond well to inoculation with appropriate strains of rhizobia and to fix large quantities of N under field conditions. In parts of Africa including Nigeria, Malawi, Zambia and Zimbabwe, promiscuous soybean has been successfully grown without inoculants demonstrating its potential as a vehicle for conveying the benefits of BNF to poor and marginalized communities (Mpepereki et al. 2000). Legumes however still constitute only a small proportion of planted crops, receive few or no inputs and they are often considered as minor women's crops (Svubure 2000). Soybean, despite its multiple benefits, is even less well known in most parts of sub-Saharan Africa.
A working group of the African Association for Biological Nitrogen Fixation (AABNF), meeting in Accra Ghana in February 2001, proposed a new 21st Century Paradigm for achieving impacts through BNF research that said "research in biological N fixation must be nested into larger understandings of system N dynamics and land management goals before the comparative benefits of N2 fixation may be realistically appraised and understood by society-as-a-whole". The group identified promiscuous soybean as the best legume to provide BNF benefits to the greatest number of rural communities in terms of improving soil fertility, protein nutrition and household incomes. Research must address smallholder farmers' concerns across the diverse agro-ecological zones of Africa and recognize that process research and application of molecular techniques are useful tools to work on recognized constraints within farming systems with the final goal being food security and improved nutrition for poor and excluded communities. Experiences with promiscuous soybean in Nigeria and Zimbabwe where research-extension initiatives have led to widespread adoption of BNF technologies by smallholder farmers could provide useful lessons.
This paper describes the conceptual models developed for research and extension of soybean BNF technologies to smallholder farmers in Zimbabwe and outlines the implementation of a research and extension initiative that has aimed to bring tangible socio-economic benefits to poor and marginalized smallholder farmers.
Soybean has been grown in Zimbabwe's large-scale commercial farms since the 1930s but due to the colonial land tenure system the crop was never promoted among smallholders who were pushed to marginal areas with sandy soils and poor rainfall considered unsuitable for soybean. A general myth was that the crop was too sophisticated for the peasant farmers who also did not have cooling facilities or the know how to handle rhizobial inoculants. Protein malnutrition, general declining soil fertility and a poor resource base against a background of increasing mineral N fertilizer prices, however also ravaged smallholders practicing maize monoculture, following World Bank/IMF-induced removal of government subsidies. Cheaper alternative sources of N inputs were urgently required and soybean BNF was a natural choice because of its multiple benefits.
It was against this background that the smallholder soybean promotion program in Zimbabwe was initiated. A National Soybean Promotion Task Force representing both private and public institutions and coordinated by the University of Zimbabwe was set up. The objectives were to promote soybean BNF among smallholders and to carry out appropriate research to support its successful integration into smallholder farming systems. Conceptual models were drawn up to guide the overall soybean BNF promotion effort.
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