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Phosphorus Cycling in Unfertilized and Fertilized Agricultural Soils¹

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.

¹ Contribution of the Dep. of Agronomy, Oklahoma Agric. Exp. Stn., Oklahoma State Univ. Published with the approval of the Director as paper no. 4611 of the Journal Series. In cooperation with an agreement with the Water Quality and Watershed Research Laboratory, USDA-ARS, Durant, OK 74702. Agreement no. 58-7B30-8-22.

² Soil Scientist, Southern Plains Watershed and Water Quality Laboratory, USDA-ARS, P. O. Box 1430, Durant, OK 74702-1430.

Received for publication October 1, 1984. Accepted for publication January 25, 1985.

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