EFFECT OF FERTILIZATION ON SOIL-PHOSPHORUS IN A LONG-TERM FIELD EXPERIMENT IN SOUTHERN FINLAND

A field experiment was established in 1978 on a loam soil (pH in CaCl2 7.1) to monitor gradual changes in the soil P status as response to d ifferent P fertilization regimes. For 18 years, cereals or grass were cultivated without P fertilization (P-0) or with annual P application of 35 kg ha(-1) (P-1) or 70-79 kg P ha(-1) and 71-83 kg K ha(-1) (P2K) . The effects of the treatments on the crop yield varied yearly. The C hang and Jackson fractionation analysis revealed that fertilizer P not taken up by the plant crops was mostly in the NH4F extract and to a l esser extent in the NaOH extract. The NH4F-extractable P proved also t o be the main P source for plants. However, the changes in the reserve s of inorganic and organic P did not agree very well with the calculat ed P balance in soil (applied P minus plant P uptake). This disproport ion was partly explained by the soil movement from plots to the neighb ouring ones during the experiment. Phosphorus extractable in acid ammo nium acetate or water decreased gradually when no P was applied and in creased with increasing P accumulation. The changes in the inorganic P reserves due to different P fertilization history were reflected a li ttle more sensitively in the water extraction test than in the acid ac etate test.