Crop production in an intercropping system with tropical leguminous trees

Abstract

Leucaena leucocephala, Calliandra calothyrsus, Gliricidia sepium. Prosopis juliflora and Cassia siamea are among the most important leguminous trees in the tropics. Although their potential role in agroforestry systems has been recognised, integration with arable crops is rare. The rapidly increasing populations in the tropical countries, especially Kenya where the birth rate is currently estimated to be 4.3 per cent per annum have resulted in a great demand for land leading to environmental degradation and a decline in soil fertility. In an attempt to improve or sustain productivity in Kenya without costly inputs such as chemical fertilizers which are too expensive for peasant farmers, an investigation into low input crop production in an intercropping system using leguminous trees was undertaken. Special emphasis was placed on the use of tree prunings as sources of manures either from the intercropped t~ee or brought in from other areas. Three field experiments were conducted in Kenya. The effects of manures on soil environment and crop production and also the effects of intercropping Prosopis and Cassia with crops on yield and soil nutrients was examined. Greenhouse and laboratory experiments in Oxford investigated in more detail the amount of N fixed and made available to millet in association with the trees under different management systems. The laboratory experiment concentrated on the relative rates of decomposition and release of nitrogen from tree foliage. Application of manures resulted in an average increase in maize yield by between 11 and 22 per cent and greatly increased the level of soil N and of organic C. Leucaena gave the highest maize yield and soil N but least organic C. Soil N was found to be a reliable predictor of grain yield. The manures conserved soil moisture by lowering soil surface temperatures. Decomposition of manures was fastest in Leucaena and slowest in Terminalia. The laboratory investigation also showed that Leucaena had the fastest rate of decomposition and highest rate of release of N while Cassia had the slowest and least. Intercropping with Prosopis resulted in an average increase of 13 and 4 per cent in grain yield of maize and sorghum respectively but no increase in bean; it significantly improved soil N in all the treatments. The spacing did not affect yields until the third season, but affected the dry matter yield per ha of Prosopis and Cassia. Close in-row spacing of Cassia resulted in intra and inter specific competition before coppicing. The amount of N fixed by Leucaena and the soil N were significantly affected by the treatment. The management of the hedges and of the trees mixed with millet was crucial in determining their effects on crops. Manured pure millet and millet mixed with Leucaena gave the highest yields, growth, and soil and foliar N contents. Significant differences occurred amongst the tree species in the amount of N fixed and made available to millet.