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 NaHCO 3 -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 NaHCO 3 P) were increased, as was NaOH-extractable P i in all soils. On the most P-deficient soil (Conestogo SiL), where corn grain yields were increased by fertilizer P, P fertilization also increased HCl-P i , residual P (H 2 O 2 -H 2 SO 4 extractable P) and labile Po (NaHCO 3 -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 SiL. 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.