Development of efficient, cost effective P management strategies for highly weathered tropical soils is limited by our understanding of the fate of fertilizer P and the availability of organic P. A 4-year field study on an Ultisol was conducted to examine P fertilizer management options. Four levels of fertilizer P were applied to each of four crops during the first 2 years. The cumulative P fertilizer input for each treatment after 2 years was 0 (control), 155 (low), 310 (medium), and 930 (high) kg P ha−1. The residual benefits of these inputs were measured throughout the next 2 years, during which no additional P was applied. A sequential P fractionation procedure (extraction with Fe oxide impregnated filter paper (FeO), 0.5 M NaHCO3, 0.1 M NaOH, 1.0 M HCl, concentrated HCl (Con-HCl), and H2SO4 digestion) was used to measure progressively more recalcitrant inorganic (Pi) and organic (Po) P fractions. Total soil P (sum of all P pools) in the control, which received no P fertilizer, was 1780 mg P kg−1. Despite this high amount of P, soybean dry matter yield and P uptake increased with P inputs. Dry matter yield and P uptake declined during the first year of the residual phase for all P treatments. In the high P treatment, approximately 9, 54, and 37% of the added fertilizer P was recovered from labile (FeO and NaHCO3 Pi), moderately labile (NaOH Pi), and recalcitrant (Conc. HCl Pi and H2SO4 P) pools, respectively, 3.5 months after the first P application. The distribution of fertilizer P among the pools remained relatively constant throughout the 4-year experiment, suggesting that the observed decline in plant available P, after P additions ceased, was not caused by conversion of P to more recalcitrant forms. Across all P treatments, FeO and NaHCO3 were correlated highly (r2 > 0.95) with Mehlich-1 extractable P but not with soybean yield or P uptake.
Total Po (excluding Po in H2SO4 P) was 18% of total P. The NaHCO3 Po pool declined at the same rate as soil organic C and total N, suggesting that NaHCO3 Po and C mineralization may be coupled. In the control, NaHCO3 Po was correlated with labile Pi and soybean dry matter yield and P uptake, suggesting that mineralized Po is an important source of plant P in unfertilized systems.
