Effect of fertilization on soil phosphorus in a long-term field experiment in southern Finland

Abstract

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 different P fertilization regimes. For 18 years, cereals or grass were cultivated without P fertilization (P0) or with annual P application of 35 kg ha-1 (P1) 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 Chang and Jackson fractionation analysis revealed that fertilizer P not taken up by the plant crops was mostly in the NH4F extract and to a lesser extent in the NaOH extract. The NH4F-extractable P proved also to be the main P source for plants. However, the changes in the reserves of inorganic and organic P did not agree very well with the calculated P balance in soil (applied P minus plant P uptake). This disproportion was partly explained by the soil movement from plots to the neighbouring ones during the experiment. Phosphorus extractable in acid ammonium acetate or water decreased gradually when no P was applied and increased with increasing P accumulation. The changes in the inorganic P reserves due to different P fertilization history were reflected a little more sensitively in the water extraction test than in the acid acetate test.