Phosphorus Cycling in Unfertilized and Fertilized Agricultural Soils

Abstract Surface soil samples (0–50 mm depth) were taken from several grassed and cropped, unfertilized and P‐fertilized soils at monthly intervals for 2 yr, to investigate seasonal variations in amounts and forms of P and relative importance of inorganic and organic P as sources of plant available P. Although no consistent seasonal variation in inorganic P content was observed for the unfertilized soils, amounts increased after fertilizer P addition. Organic P content was higher in the winter (Oct.–Mar.) than spring months (May–June) for both unfertilized and fertilized soils. Consequently, mineralization of organic P during the growing season, which contributed similar amounts of P (20–74 kg P ha −1 ) as added in fertilizer (13–100 kgP ha −1 ), was not inhibited by fertilizer P addition. Organic P variation was mainly due to changes in moderately labile organic P, with more labile and resistant pools remaining constant. The most labile organic P pool was maintained at a constant level possibly by mineralization and formation from moderately labile organic P. Little change in P content of unfertilized subsurface soil (50–150 mm) was observed, although inorganic and available P contents increased slightly following fertilizer P application. Available P (Bray‐I P) was closely correlated with organic P in unfertilized soils and with inorganic P in fertilized soils. Slopes of these relationships were related to phosphatase enzyme activity and P sorption maximum for organic and inorganic P, respectively. The importance of organic P as a source of available P in both unfertilized and fertilized soils was demonstrated. The need to include organic P in soil‐P fertility tests, especially with the increasing use of reduced‐tillage practices, is emphasized.