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 practices which maintain or increase soil organic P(o) contents would, therefore, help maintain soil fertility over time. The effects of organic additions and P fertilization on soil P fractions and yield of maize were examined after a 10 years rotation involving beans and maize on a tropical 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 prunings, all treatments fertilized with 20 kg P ha-1 and 54 kg K ha-1; and (2) monoculture mulched with E. poeppigiana prunings without fertilization. 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 received organic amendments. Mulching without P fertilization showed the lowest values of NaOH-Po (16% v. 22% of total P) and labile Pi, (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. Nevertheless, absolute values of labile Pi (resin+NaHCO3-Pi were quite high (52 mg kg-1) in this treatment, and yields of maize were among the highest obtained during most of the 10 years of cultivation.