AN ANALYSIS OF COMPETITION BETWEEN INTERCROPPED COWPEA AND MAIZE .1. SOIL N AND P LEVELS AND THEIR RELATIONSHIPS WITH DRY-MATTER AND SEED PRODUCTIVITY

To study N and P competition by intercropped cowpea (Vigna unguiculata (L.) Walp.) and maize (Zea mays L.), a series of experiments was conducted at CATIE in Turrialba, Costa Rica. A semi-erect cowpea cultivar and maize were sown simultaneously in a replacement series with N and P fertilization variables. Cowpea was not able to derive all its N from N2 fixation in mixtures, perhaps because of shading. Strong competition for soil-N by the intercrops was evident under low-N, between 7 and 9 weeks after sowing. It was diminished by N fertilization and also was less later in the season when cowpea was able to fix more N. Dry matter productivity was greater in mixtures than in monocultures. Seed LERs [land equivalent ratios] were greater than unity only under the low-fertilization regimes. Most of these advantages occurred because maize showed little effect from interspecific competition. Greatest seed yields occurred where cowpea showed strong competitiveness, with little yield reduction per plant, and maize yield was not depressed in the presence of cowpea.