CHANGES IN THE FORMS AND DISTRIBUTION OF SOIL-PHOSPHORUS DUE TO LONG-TERM CORN PRODUCTION

Long-term fertilizer-P application affects soil-P distribution and forms. These effects must be characterized to use fertilizer P most efficiently. In three southern Ontario soils of varying texture, we determined changes in soil organic P (P-o) and inorganic P (P-i) caused by fertilizer P application (0-90 kg broadcast P ha(-1) yr(-1) during 10 yr of corn production. Soil P was characterized by (1) annual measurement of 0.5 M NaHCO3-extractable P (Olsen-P) and (2) sequential extraction from soil taken at the beginning of the experiment and after receiving 0 to 90 kg broadcast P ha(-1) yr(-1) for 10 yr. Fertilizer P increased Olsen-P concentrations in all soils. The increases were proportional to the cumulative amount of P applied. Based on all three soils, 16 kg P ha(-1) was required to increase Olsen-P concentrations by 1 mg L(-1) soil. After 10 yr of 90 kg broadcast P ha(-1) yr(-1), labile P-i fractions (resin P and NaHCO3 P) were increased, as was NaOH-extractable P-i in all soils. On the most P-deficient soil (Conestogo Sit), where corn grain yields were increased by fertilizer P, P fertilization also increased HCl-P-i, residual P (H2O2-H2SO4 extractable P) and labile P-o (NaHCO3-P-o and NaOH-P-o). A P balance was calculated, which accounted for additions to, removals from, and changes in the total P status of the 0-20 cm layer. When no broadcast P was applied, there was an unaccounted-for input (possibly from the subsoil), of 20.9 kg P ha(-1) yr(-1) on the Conestogo Sit. When 90 kg broadcast P ha(-1) yr(-1) was applied to the Fox SL, the coarsest soil studied, there was a deficit of 30.9 kg P ha(-1) yr(-1) and elevated Olsen-P concentrations in the 25-36 cm depth, suggesting downward movement on fertilizer P. It appears that subsoil P was involved in the P cycle of these two soils.