SOIL PROPERTIES AND CROP PERFORMANCE ON A KAOLINITIC ALFISOL AFTER 15YEARS OF FALLOW AND CONTINUOUS CULTIVATION

A long-term field experiment was established on a kaolinitic Alfisol i n Ibadan, Nigeria, in 1972. The land was cleared manually from seconda ry forest and used for (i) continuous no-till cropping with maize (Zea mays L.) and maize/cassava (Manihot esculenta Crantz) intercropping, (ii) planted fallow of guinea grass (Panicum maximum Jacq.), leucaena (Leucaena leucocephala de Wit), and pigeon pea (Cajanus cajan Millsp.) , and (iii) natural bush regrowth in a randomized complete block desig n with three replications. At the end of 15 years, the fallow plots we re cleared manually and cropped with maize for three years. The chemic al and physical soil properties and crop performance of the newly-clea red plots were compared with those under 15 years of continuous cultiv ation. A total of 26 woody species were identified on the bush regrowt h plots. Above-ground biomass accumulation of the bush plots was 157 M g ha(-1) containing 1316 kg N ha(-1). Guinea grass, leucaena and natur al bush regrowth plots had comparable organic C concentrations (approx imately 20 g kg(-1)) in the surface soil (0 to 10 cm) after 15 years. The organic C concentration in the surface soil under pigeon pea was t he lowest (9.5 g kg(-1)) among the four fallow treatments. Soil under 15 years of continuous no-till maize with and without residue mulch, r espectively, contained approximately half (10 g kg(-1)) and a quarter (5.7 g kg(-1)) of the organic C under natural bush or guinea grass fal low. The levels of exchangeable Ca, K, Mg and effective cation exchang e capacity (ECEC) were lower in the soils under continuous cultivation than in those under natural bush and planted fallow. Soil acidificati on occurred in soils under continuous cropping as depicted by the lowe r pH values and greater exchangeable Al and Mn concentrations compared to the fallow plots. Grain yield of maize (3 to 5 Mg ha(-1)) without fertilizer application in the plots newly cleared from natural bush, g uinea grass and leucaena fallow was comparable with that of continuous no-till maize with residue mulch and chemical fertilizer (N, P, K, Mg , Zn) applications. Among the four fallow treatments, maize grain and stover yields were the lowest in plots cleared from pigeon pea fallow.