EFFECTS OF NITRIFICATION INHIBITORS AND TIME AND RATE OF SLURRY AND FERTILIZER N-APPLICATION ON SILAGE MAIZE YIELD AND LOSSES TO THE ENVIRONMENT

Field experiments with silage maize during eight years on a sandy soil in The Netherlands, showed that dicyandiamide (DCD) addition to autum n-applied cattle slurry retarded nitrification, thus reducing nitrate losses during winter. Spring-applied slurry without DCD, however, was on average associated with even lower losses and higher maize dry matt er yields. Economically optimum supplies of mineral N in the upper 0.6 m soil layer in spring (EOSMN), amounted to 130-220 kg ha-1. Year to year variation of EOSMN could not be attributed to crop demand only. A ccording to balance sheet calculations on control plots, apparent N mi neralization between years varied from 0.36 to 0.94 kg ha-1 d-1. On av erage, forty percent of the soil mineral N (SMN) supply in spring, was lost during the growing season. Hence, the amounts of residual soil m ineral N (RSMN) were lower than expected. Multiple regression with SMN in spring, N crop uptake and cumulative rainfall as explanatory varia bles, could account for 79 percent of the variation in RSMN. Postponem ent of slurry applications to spring and limiting N inputs to economic ally optimum rates, were insufficient measures to keep the nitrate con centration in groundwater below the EC level for drinking water.