California Agriculture

Increased regulatory activity designed to protect groundwater from degradation by nitrate-nitrogen (NO₃-N) is focusing attention on the efficiency of agricultural use of nitrogen (N). One area drawing scrutiny is the way in which growers consider the NO₃-N concentration of irrigation water when determining N fertilizer rates. Four drip-irrigated field studies were conducted in the Salinas Valley evaluating the impact of irrigation water NO₃-N concentration and irrigation efficiency on the N uptake efficiency of lettuce and broccoli crops. Irrigation with water NO₃-N concentrations from 2 to 45 milligrams per liter were compared with periodic fertigation of N fertilizer. The effect of irrigation efficiency was determined by comparing an efficient (110% to 120% of crop evapotranspiration, ET_c) and an inefficient (160% to 200% of ET_c) irrigation treatment. Across these trials, NO₃-N from irrigation water was at least as efficiently used as fertilizer N; the uptake efficiency of irrigation water NO₃-N averaged approximately 80%, and it was not affected by NO₃-N concentration or irrigation efficiency.