NITROGEN AND CHLORIDE CONCENTRATION IN DEEP SOIL CORES RELATED TO FERTILIZATION

Shallow-rooted, high-value vegetable crops are normally heavily fertil ized with nitrogen. Improving farmers' management practices requires a simple method to monitor nitrogen loading below the root zone, and ir rigation efficiency, In fields with low nitrogen and water use efficie ncies, alternative Best Management Practices (BMPs) should be initiate d and evaluated to reduce nitrogen loading to the ground water while m aintaining yields. The objective of the study was to estimate the exte nt of nitrate-nitrogen leaching below the root zone of shallow-rooted onions, and deep-rooted chile and alfalfa, using chloride in the irrig ation water as a tracer. Soil samples were taken from seven fields in 15cm increments to 180cm at the end of the 1994 growing season. The sa mples were analyzed for nitrate-nitrogen and chloride. Irrigation effi ciency ranged from 70 to 76% for the chile fields, 77-80% for onions a nd was 97% for alfalfa. Nitrogen loading below the root zone of chile fields varied from 290 kg ha(-1) per year for sandy loam soils to 64 k g ha(-1) for clay soils. Nitrogen loading below the root zone of onion s varied from 199 kg ha(-1) per year for a loamy sand field to 161 kg ha(-1) per year for a clay field. The nitrogen loading below the root zone of a sandy loam alfalfa soil was only 42 kg ha(-1) per year becau se of the low leaching fraction. Results indicated that irrigation eff iciencies are reasonable but nitrogen applications amounts need to be decreased by using alternative BMPs. (C) 1997 Elsevier Science B.V.