Document(s) 36 of 36

Résumé

Des études conduites dans l'est de la Zambie à partir de 1987 ont révélé qu'une courte durée de jachère améliorée avec le Sesbania sesban constitue une alternative agroforestière préférable à la jachère naturelle pour accroître la fertilité des sols dégradés. La rotation du S. sesban pendant 2–3 ans a doublé ou triplé le rendement par rapport au système de culture sans engrais. En outre, les jachères à base de Sesbania produisent de 10 à 35 t de bois de chauffage ha^-1 après 1–3 ans. Les jachères améliorées de Sesbania sont plus appropriées pour les paysans pauvres pour connaître les avantages des fertilisants. Il apparaît que les jachères améliorées avec Sesbania constituent une réponse à l'appel à une agriculture durable qui peut nourrir une population sans cesse croissante, sans causer trop de dommages à l'environnement.

The backdrop

More than 90% of Zambia's staple food, maize, is grown by small-scale farmers. The country's food security rests with these producers and is threatened when such farmers are not given the means to deal with problems of severe land degradation and, by consequence, declining food production and soil fertility; high population growth; shorter fallow periods; high cost of fertilizers; erratic rainfall; and scarcity of fuelwood. Also, the biodiversity of the Miombo woodlands is threatened by agricultural expansion. Each of these problems is related to, and even exacerbates, the others.

Finding a solution in Sesbania trees

Sesbania sesban is widespread in southern, Central, and eastern Africa. It is widely distributed along major lakes, rivers, streams, and marshes. After trying many different tree species and provenances, researchers with the Zambia – International Centre for Research in Agroforestry (ICRAF) Agroforestry Research Project in Chipata selected S. sesban as a potential species for short-rotation fallows, as it is fast growing, vigorous, and easy to propagate and to remove from the soil; produces high-quality biomass; nodulates easily; and fixes N. The ICRAF researchers found that growing Sesbania in depleted fields or on fallow lands for 2 or 3 years and then introducing a hybrid maize crop after the fallow period produced exciting and encouraging results. Without N fertilizers, maize yields were 2.3 t ha^-1 after 1 year of Sesbania fallow; 5.6 t ha^-1, after 2 years; and 6.0 t ha^-1, after 3 years. Continuous maize crops gave only 1.5 t ha^-1.

Sesbania fallows also produced 10–35 t of fuelwood ha^-1 after 1–3 years. Such on-farm production of fuelwood eases the burden of labour carried by women and children, as well as reducing the pressure on natural woodlands.

When the Sesbania trees are removed (usually by hand), roots are left in the soil: 1.2 t ha^-1 of root mass after 1 year; 1.7 t ha^-1, after 2 years. Most (90%) of the roots were in the top 50 cm of the soil, and as they slowly decayed they provided nutrients to crops. A few roots reached the water table, at a depth of 7.5 m, where they could access nutrients and water.

A cost–benefit analysis showed that Sesbania fallows are more profitable than continuous cropping, even allowing for the land taken out of maize production. In a farm setting, on severely depleted soils, maize yields increased from 0.15 t ha^-1 to 4.0 t ha^-1 after 2 years of Sesbania fallow. However, for wide evaluation on farms, nursery-raised, inoculated, bare-rooted seedlings, rather than potted ones, must be used to reduce establishment costs.

There is considerable enthusiasm among farmers in Chipata, Katete, and Chadiza, where 200 farmers established Sesbania fallows from bare-rooted seedlings in the 1994/95 season. Currently, more then 1 000 farmers in eastern Zambia alone are participating in an evaluation of improved fallows. Trials at 100 sites with various soil and climatic conditions will show researchers what biophysical conditions — rainfall, soil fertility, and soil composition — are needed to grow S. sesban. On-farm nurseries have been set up at farmer training centres and in farmers' fields to supply seedlings to farmers who wish to try the technology. Furthermore, several farmers have established their own farmer-designed trials with Sesbania in fallow fields. This indicates that the technology is poised to take off.

Acknowledgments

We gratefully acknowledge the Swedish Agency for Research Co-operation with Developing Countries for the continued funding of the Zambia – ICRAF Agroforestry Research Project in Chipata, Zambia, and the International Development Research Centre, Canada, for covering the travel and related costs of presenting this work at the cover-crops symposium in Cotonou, Benin. We also thank Damary Odanga for preparing a poster for the symposium and Alice Aduol for typing the manuscript.

Document(s) 36 of 36