EFFECT ON MAIZE GROWTH OF THE INTERACTION BETWEEN INCREASED NITROGEN AVAILABILITY AND COMPETITION WITH TREES IN ALLEY CROPPING

Maize growing next to Erythrina hedgerows had 44% lower biomass (p < 0 .01) and 35% lower N content (p < 0.1) than maize growing in the middl e of the alleys. Maize growing next to Gliricidia hedgerows had the sa me biomass but 56% higher N content (p < 0.1) than maize growing in th e middle of the alleys. However these differences did not develop unti l 2 months after sowing of the maize. Spatial variability in soil nitr ogen mineralization and mulch nitrogen release did not explain any of the differences in growth or N uptake of the maize with respect to dis tance from the trees. It is hypothesized that the slower growth of the maize next to the Erythrina trees after 2 months is due to increasing light and/or nutrient competition from the trees as the trees recover from pollarding. The apparent lack of competition from Gliricidia may be due to different rates of regrowth or different shoot and root arc hitecture. A theoretical model is described demonstrating that if a cr op is to take advantage of the higher nutrient availability under alle y cropping it must complete the major part of its growth before the tr ees recover significantly from pollarding, and start competing strongl y with the crop.