Soil fertility changes under an oil palm-based intercropping system

Published information on nutrient dynamics in relation to soil fertility management for oil palm-based cropping systems is still scarce in Nigeria. An intercropping experiment with oil palm (Elaeis guineensis Jacq), soybean (Glycine max), maize (Zea mays) and cocoyam (Xanthosoma sagittifolium) was therefore conducted from 1992-1994 to evaluate changes in soil nutrient levels. The experimental design was a randomised complete block with three replicates. Treatments consisted of: (A) sole oil palm; (B) sole soybean; (C) sole maize; (D) sole cocoyam; (E) oil palm + soybean; (F) oil palm + maize; (G) oil palm + cocoyam; (H) oil palm + soybean + maize; (I) oil palm + soybean + cocoyam; (J) oil plam + soybean + maize + cocoyam. A pre-cropping composite soil sample was taken at a depth of 0-30 cm in 1992. Soil sampling for analysis was conducted on plot basis after harvesting for soil pH, organic matter, total nitrogen, available P and exchangeable K, Mg and Ca. There was no significant change in pH after 3 cropping years, but soil organic matter declined from 10-51% in treatments G and I. The decline in total N ranged from 50-70% in treatments A and J. At the end of the experiment the P level increased up to 71% in treatment E. Exchangeable K declined from 32 to 62% in treatment F and H, respectively. The observed decline in N and K could be due to crop removal and leaching. No significant changes were observed for Ca and Mg. Intercropping depressed the sex ratio in oil palm by between 6 and 17%. Soybean seed yield was highest (1.5 t ha(-1)) in the sole plot and lowest (0.7 t ha(-1)) in soybean + oil palm t maize t cocoyam. Grain yields from sole maize and maize intercropped with oil palm were superior to yields from the other intercropping treatments. A similar trend was observed for cocoyam corm yield. The significance of this finding for sustainable agriculture is that intercropping encourages the maximization of land use, stability in yield and profit. Furthermore, intercropping of cereal with grains provides balanced energy and protein diets for farm families.