INFLUENCE OF FERTILIZER, IRRIGATION, AND NONGROWING SEASON PRECIPITATION ON SOIL NITRATE-NITROGEN UNDER CORN

Corn (Zea mays L.) production requires large amounts of N fertilizer, which may be lost due to leaching or denitrification. Prediction of the amount lost would be helpful in minimizing these losses. A field experiment was conducted to investigate the effects of fertilization, irrigation, and non-growing season precipitation on soil NO3-N under corn in southwestern Quebec from 1984 to 1987. The high rate of fertilization (400 kg N ha-1, 300 kg P2O5 ha-1, 400 kg K2O ha-1) over the normal rate (170 kg N ha-1, 100 kg P2O5 ha-1, 170 kg K2O ha-1) significantly increased soil NO3-N during the growing season, and a residual effect of fertilizer rate on soil NO3-N levels was noted in 2 of 3 yr during the non-growing season. The high rate of fertilization, however, did not increase NO3-N accumulation in the soil profile over the normal rate of fertilization in successive years. Irrigation significantly decreased soil NO3-N during the growing season. At the normal rate of fertilization, gains of soil NO3-N over winter were found with lower winter precipitation. At the high rate of fertilization, however, losses of soil NO3-N over winter occurred in 2 of the 3 yr. The changes in NO3-N in the rooting zone over winter depended upon the precipitation received during the preceding winter and the amount of NO3-N in the soil the previous fall.