Alley cropping on an Ultisol in subhumid Benin. Part 2: Changes in crop physiology and tree crop competition

From 1986–92 alley cropping with Leucaena leucocephala (Lam.) de Wit and Cajanus cajan (L.) Millsp. was compared to a no-tree control, and fertilisation with 90, 39 and 75 kg ha−1 N, P and K vs. no fertilisation were studied in maize and cassava grown intercropped or in rotation. Light transmission measurements and row position effects on food crop yield did not indicate any decisive shading effect in tree-crop competition when trees were pruned 2–3 times a year. In the last three years, with NPK in solecropping the crop growth rates (CGR) of maize reached a peak of 100 g m−1 wk−1 around 70 days after planting (DAP). CGR in unfertilised Cajanus alleys was as high as in corresponding fertilised treatments but delayed by one month. For cassava, the highest growth rates of 60–130 g m−1 wk−1 lasted from 60 to 190 DAP with NPK in solecropping. The harvest index (HI) of cassaca reached 40–50% in all intercropping and unfertilised solecropping treatments. The HI in solecropping was reduced by fertilisation to 30–40% due to excessive leaf growth at 60–190 DAP. The increase in number of thickened roots starting at 30 DAP in solecropping was delayed by one month in intercropping. The reason for significantly higher cassava root yields in Cajanus alleys as compared to no trees in the fertilised solecrop was attributed to the enhanced root formation at 85 DAP due to a slower release of nutrients subject to leaching such as N and K from tree prunings than from mineral fertiliser.