Synergistic effect of inorganic N and P fertilizers and organic inputs from Gliricidia sepium on productivity of intercropped maize in Southern Malawi

In Malawi, N and P deficiencies have been identified as major soil fertility constraints to maize (Zea mays, hybrid NSCM 41) productivity. In this study, we evaluated the effect of three rates of N and P fertilizers on maize performance in monoculture and maize intercropped with the nitrogen fixing legume gliricidia (Gliricidia sepium) in replicated field trials run for four years (2002/03-2005/06 seasons) at Makoka, in southern Malawi. Significant season-to-season variation was found in stand loss, ears per plant, stover yield, grain yield and thousand kernel weight (TKW), which was related to distribution of rainfall received during the growing season. All variables were significantly higher in the gliricidia/maize intercrop compared with monoculture maize. During the four consecutive cropping seasons, grain yields of maize increased by 343% (i.e. from 0.94 tons ha(-1) in unfertilized sole maize to 4.17 tons ha(-1) in gliricidia/maize intercropping). Optimum synergistic effect on grain yield (38% increase over unfertilized gliricidia/maize) was obtained when half recommended N and P rates were combined with gliricidia indicating interspecific facilitation. Response surface modelling showed that the optimum combination of factors for maximum grain yield (4.2 t ha(-1)) in monoculture maize was 80 kg N ha(-1), 31 kg P ha(-1) and 917 mm seasonal rainfall. In the gliricidia/maize intercrop, the stationary point had no unique maximum. Ridge analysis revealed that the estimated ridge of maximum grain yield (5.7 t ha(-1)) in the intercrop is when 69 kg N ha(-1), 37 kg P ha(-1) is applied and a seasonal rainfall of 977 mm is received. The total P uptake in the intercrop (14.3 kg ha(-1)) was significantly higher than that in maize monoculture (6.6 kg ha(-1)). P uptake was significantly (P = 0.008) influenced by P fertilizer rate. Therefore, we conclude that combining inorganic N and P fertilizers with organic inputs from gliricidia has positive and synergistic effects on maize productivity in southern Malawi.