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 for ms. These effects must be characterized to use fertilizer P most effic iently. In three southern Ontario soils of varying texture, we determi ned changes in soil organic P (P-o) and inorganic P (P-i) caused by fe rtilizer P application (0-90 kg broadcast P ha(-1) yr(-1) during 10 yr of corn production. Soil P was characterized by (1) annual measuremen t of 0.5 M NaHCO3-extractable P (Olsen-P) and (2) sequential extractio n from soil taken at the beginning of the experiment and after receivi ng 0 to 90 kg broadcast P ha(-1) yr(-1) for 10 yr. Fertilizer P increa sed Olsen-P concentrations in all soils. The increases were proportion al 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-extractab le P-i in all soils. On the most P-deficient soil (Conestogo Sit), whe re corn grain yields were increased by fertilizer P, P fertilization a lso increased HCl-P-i, residual P (H2O2-H2SO4 extractable P) and labil e P-o (NaHCO3-P-o and NaOH-P-o). A P balance was calculated, which acc ounted for additions to, removals from, and changes in the total P sta tus of the 0-20 cm layer. When no broadcast P was applied, there was a n 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) w as applied to the Fox SL, the coarsest soil studied, there was a defic it of 30.9 kg P ha(-1) yr(-1) and elevated Olsen-P concentrations in t he 25-36 cm depth, suggesting downward movement on fertilizer P. It ap pears that subsoil P was involved in the P cycle of these two soils.