SOIL-PHOSPHORUS FRACTIONS UNDER 5 TROPICAL AGROECOSYSTEMS ON A VOLCANIC SOIL

The organic P pool is usually considered a major source of available P in high P-fixing soils of the tropics. Agricultural management practi ces which maintain or increase soil organic P(o) contents would, there fore, help maintain soil fertility over time. The effects of organic a dditions and P fertilization on soil P fractions and yield of maize we re examined after a 10 years rotation involving beans and maize on a t ropical volcanic soil. Five maize cropping systems were analysed: (1) monoculture, alley cropping with Erythrina poeppigiana, alley cropping with Gliricidia sepium and monoculture mulched with E. poeppigiana pr unings, all treatments fertilized with 20 kg P ha-1 and 54 kg K ha-1; and (2) monoculture mulched with E. poeppigiana prunings without ferti lization. Soil P fractions were determined by a sequential extraction procedure. Little differences were found in size and distribution of P pools among treatments fertilized with P regardless of whether they r eceived organic amendments. Mulching without P fertilization showed th e lowest values of NaOH-P(o) (16% v. 22% of total P) and labile P(i), (2.4% v. 4.8%). These results suggest that (1) organic P accretion and mineralization is strongly dependent on inorganic fertilization; and (2) organic additions without synthetic fertilizers may be decreasing the organic P pool, and consequently the soil P fertility. Nevertheles s, absolute values of labile P(i) (resin+NaHCO3-P(i) were quite high ( 52 mg kg-1) in this treatment, and yields of maize were among the high est obtained during most of the 10 years of cultivation.