Evaluation of Best Management Practices for Reducing Nitrate Leaching in Commercial Lettuce Fields.

Ground water on the central coast of California frequently has nitrate-N concentrations above the drinking water standard of 10 mg L^-1  in regions intensively farmed with cool season vegetables. To assure high yields and quality of lettuce, the major cool season vegetable grown on the central coast, growers typically apply rates of fertilizer N greater than crop uptake and more irrigation water than the consumptive use requirement of the crop.   Two tools available to growers, the quick soil nitrate test and evapotranspiration data could help producers improve water and fertilizer nitrogen management of lettuce.   We conducted 5 large-scale replicated trials in commercial lettuce fields during the 2008 and 2009 seasons to compare grower practices to a best managed practice (BMP) which included the use of the quick soil nitrate test for determining N fertilizer requirements and scheduling irrigations using daily evapotranspiration data and a water use model for lettuce.   Trials were conducted in 6 to 11 ha fields, having soil textures ranging from silty clay to sandy loam.   Strips of 49 m in width x field length were managed by the growers’ standard practice or using the BMP treatment.  Treatments were replicated 3 times at each field site.    Water and nitrogen fertilizer application were significantly reduced under  the BMP regime, averaging 136 kg of N ha^-1 and 29 cm of water compared to an average of 197 kg of N ha^-1 and 35 cm of water under  the grower standard treatment.   Nitrate-N concentrations of leachate sampled with suction lysimeters 60 cm below the soil surface ranged from 105 to 178 mg L^-1 and were not significantly different between treatments; however, estimated losses of nitrate-N were least in the BMP treatment due to improved water management.