Soil test phosphorus and phosphorus fractions with long-term phosphorus addition and depletion

The fate of fertilizer P in soil during crop production has to be determined to evaluate the long-term economic value and sustainability of fertilizer practices. We assessed changes in soil test P and soil P fractions with continuous P fertilization and soil P depletion under continuous corn (Zea mays L.) in a Ste. Rosalie clay soil (humic Gleysol; fine, mixed, frigid, Typic Humaquept). Soil samples were analyzed for Mehlich-3 P (M-3 P) and P fractions using a modified Hedley's procedure. Soil M-3 P values remained constant in spite of crop removal in soil not receiving fertilizer for 10 yr. Continuous P fertilization at rates from 44 to 132 P ha -1 yr -1 increased linearly soil M-3 P, with 6.3 kg P ha -1 of net P addition required to increase M-3 P by 1 mg P kg -1 . Residual fertilizer P in soil resulted from the continuous P addition were found predominately in labile inorganic P (LPj) (NaHCO 3 -P 1 ) and moderately labile P i (MLP,) (NaOH-P i ). Increased P rates favored soil P transformation from LP i to MLP i , indicating enhanced soil P retention. With P depletion, soil M-3 P declined in plots previously receiving 132 kg P ha -1 yr -1 , with 4.2 kg P ha -1 crop P removal decreasing soil M-3 P by 1 mg P kg -1 . Continuous crop removal of soil residual P (Res-P) resulted in decreases in soil LP i and increases in MLP i , an indication of increased retention of Res-P with time. However, moderately stable P i (HCl-P,) remained constant, both with continuous P addition and P depletion. Conversion of residual fertilizer P to less available P forms in soil was a slow process and thus the fate of the Res-P should be taken into consideration when developing soil nutrient management plans.