Short-term Sesbania fallows have now become a central agroforestry technology which is currently being tested as a major component of soil fertility recapitalisation strategies in Kenya. As a result of the large (2-3 fold) yield responses of subsequent maize crops to 2 year Sesbania fallows, many farmers in eastern Zambia (where land availability is not a major constraint) and western Kenya (where off-farm labour allows generation of income) have adopted this technology, and there is increasing interest from farmers in adjacent regions. Smaller yield benefits are seen over several seasons after the fallow. Recent results have demonstrated that even shorter term fallows yielded substantial residual benefits to maize for at least two cropping seasons.

Sesbania sesban is known to modulate and fix nitrogen freely, given suitable conditions. However, there is little direct evidence to indicate how much N actually comes from N2-fixation (to balance N removal by subsequent crops), or to what extent nitrate-N is captured from depth or simply prevented from leaching from the surface soil horizons. The N-rich litter of Sesbania results in effective N supply and substantial responses in crop growth but the fast N supply is not in synchrony with crop demand which may result in substantial losses during the early crop phase and such litter contributes little to building the soil organic matter status.

Although there is widespread current interest in Sesbania fallows, a number of potential problems associated with reliance on a single species have also been highlighted. In particular the susceptibility of Sesbania to attack by root-knot nematodes and by the Mesoplatys beetle is causing increasing concern, particularly in regions where important crops are susceptible (for instance tobacco in Malawi) where Sesbania may serve as a secondary host. Other problems include the intense insect damage which often limits seed production in Sesbania. Single provenances have been widely propagated and disseminated to farmers, leading to concerns should new pests and diseases arise.

Despite the large yield responses to Sesbania fallows, its litter decomposes rapidly as it is rich in nitrogen with little lignin or polyphenols, and thus it is unlikely to contribute to long-term enhancement of soil organic matter status. Experiments to manipulate decomposition rates by mixing litters of different qualities have demonstrated that it is possible to alter rates of decomposition and soil organic matter build-up by using litters of species rich in reactive polyphenols which can complex with proteins to form precursors of soil organic matter.

To explore the potential benefit of short-term fallows comprised of mixtures of fast-growing legume tree species to improve soil fertility, crop production, reduce risk and nutrient losses, and provide additional products. Complementary patterns of shoot and root growth, nutrient exploration, synchrony of nutrient release and building of soil organic matter will be assessed and modelled.

• Guidelines on soil fertility improvements by mixed fallow systems quantified on farmers' fields and promoted.
• Sources of nutrient acquisition quantified and complementarity of tree mixtures evaluated and documented.
• Nutrient interactions of organic residues in tree mixtures experimentally measured and reported.
• Models and sustainability of mixed species fallows evaluated, tested and improved; recommendation domains in two target countries examined and verified.