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DIVISION S-4—SOIL FERTILITY & PLANT NUTRITION

Soil Test Phosphorus and Phosphorus Fractions with Long-Term Phosphorus Addition and Depletion

^a Greenhouse and Processing Crops Research Center, Agriculture and Agri-Food Canada, Harrow, ON, Canada N0R 1G0
^b Dep. of Natural Resource Sciences, Macdonald Campus, McGill Univ., 21 111 Lakeshore, Ste-Anne de Bellevue, QC, Canada H9X 3V9
^c Pollution Data Branch, Environment Canada, Place Vincent Massey, 9th Floor, 351 St-Joseph Blvd., Hull, QC, Canada K1A 0H3

^* Corresponding author (zhangt{at}agr.gc.ca).

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 (LP_i) (NaHCO₃–P_i) and moderately labile P_i (MLP_i) (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_i) 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.

Abbreviations: Biocarb-P, biocarbonate P • LP_i, labile inorganic P • M-3 P, Mehlich-3 P • MLP_i, moderately labile P_i • NaOH-P, hydroxide P • P_i, inorganic P • P_o, organic P • Res-P, residual P