An edaphic evaluation of a mixed cropping system based on cocoa grown in combination with oil palm in southwestern Nigeria

This study was carried out in one of the plots at the Gambari Experimental Station (GES) jointly initiated by the Cocoa Research Institute of Nigeria (CRIN) and the Nigerian Institute of Oil Palm Research (NIFOR) in 1965. The experimental design, mainly agronomic, was randomized blocks consisting of three spacing arrangements: pure stands of cocoa and oil palm; avenue planting of cocoa and oil palm and hollow square planting of cocoa and oil palm. Three decades after planting, soil samples were collected from the 0-15 cm and 15-45 cm layers under pure stands of cocoa, pure stands of oil palm, avenue planting and hollow square planting. The soil samples were analyzed for particle size distribution, bulk density, total porosity, moisture content, pH, organic matter, nitrate-nitrogen, available phosphorus, calcium, magnesium and potassium. Most of the structural, chemical and nutrient properties of soil had mean values that were significantly higher under hollow square treatment than under cocoa and oil-palm pure stands and to some extent under avenue planting. The results and their implications for soil productivity and management for sustainable tree cropping in the Nigerian cocoa belt are discussed.