SOIL-PHOSPHORUS DYNAMICS IN A LONG-TERM FIELD EXPERIMENT AT ASKOV

Inorganic and organic soil P (P-i, P-o) fractions were followed monthly for 15 months in a 100-year-old, fertilization and crop-rotation experiment with the Rubaek-Sibbesen, macroporous resin method, the Olsen method, and the Hedley fractionation method. Resin P, and Olsen P had similar levels and variation patterns. They increased in spring after fertilization, decreased during summer and autumn, and increased again in winter after repeated slurry applications. Resin P-o decreased in spring and peaked in summer. The variation in time of the Hedley P-i and P-o fractions was relatively smaller and was neither related to season nor to fertilization. Un-manured soil contained much less total P than NPK and slurry-treated soils, but the differences in total P-i were greater than those in total P-o. Neither total P-i nor total P-o concentrations differed between NPK and slurry treatments, indicating that P-o in animal manure is quickly mineralized. All P-i and P-o fractions were smaller in unmanured than in fertilized treatments. These differences were relatively largest for resin P-i and resin P-o, i.e., the most labile fractions, and decreased for the medium and less labile P-i and P-o fractions. The reactions by resin P-i, Olsen P-o and resin P-o to seasons and treatments indicate that these fractions are estimates of the most labile pools of P-i and P-o in soil, which make them relevant for shortterm studies. The medium and less labile P-i and P-o fractions of the Hedley fractionation method seem more relevant for long-term studies.