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UC Davis Vegetable Research and Information Center Fertility Manacemenl of Drio-lrricated Vecetahbles Unlike NO3-N, available soil P and K exist in chemical equilibrium with slowly available forms. The most appropriate soil test procedures vary depending on location and soil characteristics. In ` the Southeast, acidic sandy soils usually are assayed for P and K using the Mehlich I soil test (Hanlon et al., 1990). Neutral or alkaline mineral soils in the West are extracted more appropriately by the bicarbonate and ammonium acetate techniques for P and IC, respectively ` (Reisenauer, 1983). Mineral soils in the West commonly contain sufficient K for maximum vegetable production and, where heavy fertilization has been employed over years, sufficient P as well. Soil supply of K is generally more limited in the Southeast; however, P has accumulated to ` high levels in many soils used repeatedly for vegetable production. Crop nutrient requirements: Vegetable crops differ widely in their macronutrient requirements, and in the pattern of uptake over the growing season. In general, N, P and K uptake follows the same course as the rate of crop biomass accumulation. Fruiting crops such as tomato, pepper and melon require relatively little nutrition until flowering, when nutrient uptake accelerates, peaking during fruit set and early fruit bulking. As fruits mature, macronutrient requirement declines. Non-fruiting crops such as celery, lettuce, and cole crops have slow nutrient uptake through the first half of the season, the rate of nutrient acquisition accelerating until just before harvest. Fertilization recommendations, based on research conducted regionally or locally, vary considerable among areas of the country; it is important to recognize these regional difference when formulating a fertigation program Nutrients applied through drip irrigation systems: Although most nutrients can be injected successfully into drip irrigation systems, the most often- applied nutrients are N and K. Crop requirements for N and K are large relative to other elements, and fertigation provides a system to supply the requirements of the crop in a scheduled fashion during the season. The leaching potential for N on most soils, and for K on sandy soils, �- makes split applications of these nutrients through a drip system attractive for improving fertilizer efficiency. In addition, there are few precipitation and clogging problems associated with N or K injection. Phosphorus, Ca, Mg, and micronutrients can be injected successfully into drip irrigation systems if precautions are taken to mitigate against chemical precipitation. Analysis of irrigation water for Ca, Mg, Fe, pH, carbonate, and bicarbonate is important for predicting chemical precipitation problems; the risk of precipitation increases with increasing pH or increasing concentration of these materials. Acids may need to be injected with the fertilizer to maintain high nutrient +. solubility during fertilizer injection. Nutrient sources: .. A variety of fertilizer materials can be injected into drip irrigation systems. Common N sources include urea-ammonium nitrate solutions, ammonium nitrate, calcium nitrate, and potassium nitrate. Potassium can be supplied from potassium chloride, potassium sulfate, potassium thiosulfate, or potassium nitrate. The choice of phosphorus products is more limited; phosphoric acid or ammonium phosphate solutions are used most commonly. Mono ammonium- or mono potassium phosphate are available, but are used infrequently. L L. Page 2 I L