Estimating Nitrate Leaching in Lettuce Production

In Salinas Valley, there is potential for nitrate leaching to occur in intensive lettuce production. Lettuce is a high value crop and sidedress fertilizer costs represent a small fraction (< 5%) of total crop production costs. To ensure maximum yields, there is a tendency for fertilizer application rates to exceed crop nutrient requirements. As a result, there is potential for significant nitrate leaching if excessive irrigation is applied during the growing season.  Little research has been conducted to quantify nitrate leaching in lettuce production under a range of N fertilization rates. The goal of this project was to estimating nitrate leaching using suction lysimeters and to identify best management practices that can reduce nitrate leaching in lettuce production.  The trial was conducted at the Hartnell East Campus Research Facility Salinas, CA on a Chualar loam soil. Romaine lettuce was planted on June 23.  UAN 32 was applied through drip irrigation at 11 (zero sidedress), 84, 168, 252 and 336 kg N ha^-1. Irrigation water was applied at 116% of ET which was representative irrigation amounts applied in the area. To measure leachate, suction lysimeters were placed below the root zone at 60 cm deep in the plant seedline in each plot; suction was maintained at 20-30 kPa using small suction pumps. Leachate was collected 24 hours after each irrigation; the quantity of nitrate leached was estimated from the concentration of nitrate in the lysimeter leachate and water movement in soil which was estimated by soil water measurements made with a neutron probe (before and after each irrigation) and accounting for crop evapotranspiration (ET_c). No significant differences were seen in lysimeter leachate nitrate concentrations until 56 days after germination, after which one or more fertilizer treatments had significantly greater nitrate-N than the zero sidedress treatment. Between 18-32% of the N leached during the season occurred with the water applied to germinate the crop. Maximum yield was obtained at 168 kg N ha^-1, but the total nitrate-N ha^-1 leached at this fertilizer rate was 78 kg N ha^-1 which was also equivalent to the 252 and 336 kg N ha^-1 fertilizer rates. Over 45 kg nitrate-N ha^-1 was leached from the zero sidedress treatment which indicates that mineralization of soil organic matter can also contribute significant nitrate that can be leached by over irrigation.