EFFECT OF NITROGEN-FERTILIZER ON YIELD OF PERENNIAL PASTURES IRRIGATED WITH SALINE WATER

This study examines the interaction between salinity and nitrogen (N) fertiliser in perennial pastures growing in the Shepparton Irrigation Area of northern Victoria. In a greenhouse experiment, perennial ryegrass was irrigated with 5 concentrations of sodium chloride (100-6000 mg NaCl/L) and fertilised with 5 concentrations of N (25-800 mg/L). Nitrogen at 200 mg/L was optimum for dry matter yield; at N concentrations >200 mg/L, yield declined. Dry matter yield was greater at 750 mg NaCl/L than at 100 mg NaCl/L but was reduced with further increases in salinity. Significant interactions between salinity and N were recorded for dry matter, and for chloride, Na, potassium, and magnesium concentrations in perennial ryegrass. In a field experiment, the growth of mixed perennial ryegrass-white clover pastures irrigated with saline water [less-than-or-equal-to 3000 mg/L of total dissolved solids (TDS)] also responded to N applications. With no added N, yield (2-year average) decreased from 16 630 to 11 277 kg DM/ha (32% decrease) with an increase in salinity from 100 to 3000 mg TDS/L. With 150 kg N/ha, the average yield was 17 681 kg DM/ha at 100 mg TDS/L and 14123 kg DM/ha at 3000 mg TDS/L (20% decrease). Form of N (calcium nitrate, ammonium nitrate, or urea) had no significant effect on yield at 150 kg N/ha. Application of urea at 150 kg N/ha to pastures irrigated with water of salinity up to 3000 mg TDS/L would be economically justifiable.