Phosphorus concentrations in subsurface water as influenced by cropping systems and fertilizer sources

Over-fertilization of agricultural soils with P has been linked to water pollution, and it is becoming apparent that subsurface P losses can be substantial. The silty-loam Gleysol chosen for this study contained 146 mg Mehlich-3 P kg(-1), which exceeds the critical limit (66 mg Mehlich-3 P kg-1) for Quebec. Equal quantities of fertilizer P (45 kg P ha(-1)) were applied from inorganic (triple superphosphate) and organic (composted cattle manure) sources to corn-corn and corn-soybean rotations. We evaluated crop production, the Mehlich-3 P and degree of soil P saturation levels, and the P concentration in subsurface water. Crop yields were unaffected by the fertilizer source, and only 31 to 67% of the P applied over a 2-yr period was exported, suggesting that the P fertilizer rate, chosen to match the P requirements of the corn crop, was excessive. After 2 yr, surface (0 to 15 cm) soils contained 149 to 199 mg Mehlich-3 P kg(-1) and the Mehlich-3 (P/Al) saturation ratio (DSPSM3) ranged from 0.10 to 0.14. Subsurface water from piezometers installed to a 60-cm depth contained between 0.3 and 1.7 mg total P L-1, and particulate P was the dominant P form at most sampling dates. Mehlich-3 P was positively correlated with dissolved reactive P, whereas the DSPSM3 was positively correlated with the total P and particulate P concentrations in subsurface water. We conclude that these soil test parameters could be useful indicators of subsurface P losses from Quebec soils.