Decreasing rice and cowpea yields in alley cropping on a highly weathered oxisol in West Sumatra, Indonesia.
A hedgerow intercropping study was conducted for 7 years on an acid and highly Al-saturated (72%) soil to determine growth and yield responses of tree hedgerows and upland rice and cowpea intercrops. Three tree species (Paraserianthes falcataria, Calliandra calothyrsus and Gliricidia sepium) and a no tree control were planted with three lime application rates (0 and 375 kg ha-1, and liming to 25% Al+H saturation). Annual fertilizer inputs of 20 kg P and 50 kg K ha-1 were kept low to approximate low input farming systems. The trees were pruned 4-6 times per year and prunings applied to the intercrops. Paraserianthes and Calliandra grew vigorously, while Gliricidia grew poorly and was replaced after four years with Flemingia macrophylla. Hedgerow growth and yields were reported in a previous paper [Agroforestry Systems (1994) 27 207-222]. Rice and cowpea yields initially increased with lime and Paraserianthes pruning application, but yields and soil cations (Ca, Mg and K) declined until fertilizer inputs were increased after four years. Thereafter, crop yields increased and soil cations returned to their original levels. Soil C and N were maintained over the 7 years on plots with trees. These results indicate little build-up of nutrient cations due to recycling by the trees and suggest that successful alley farming on such highly weathered soils requires maintenance of soil fertility with external inputs.