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UC news Ve etable Research and Information Center Fertility Management of Drip-Irrigated Vegetahbles The choice of fertilizer suitable for a specific application should be based on several factors: nutrient form, purity, solubility, and cost. The appropriate balance of NO3-N to NH,-N (or urea) is dependent on environmental conditions. In cool weather, 25-50% of applied N should be in the NO3-N form (Hochmuth and Hanlon, 1995). In warm weather, nitrification occurs rapidly, allowing greater use of ammoniacal-N or urea fertilizers, which are significantly less expensive ` than nitrate fertilizers. All common N sources are available in clean, high-analysis, liquid solutions. ` Solubility is an issue with potassium products, as are relative salt index and cost. Potassium chloride is inexpensive and reasonably soluble (solutions >10% K are available), but relatively high in salt index. However, the salt index issue is generally not critical if the application rate is "" limited to the crop requirement. Potassium sulfate is less soluble and more expensive than potassium chloride, but it has a lower salt index. Potassium thiosulfate and nitrate are highly soluble but also are more expensive. Potassium chloride is used commonly for drip injection in ` the West, whereas potassium nitrate is popular in the Southeast. Although a topic of some controversy, there is no definitive research that documents a need to maintain a certain ratio of NX fertilizer in vegetable production. As long as soil-test-predicted amounts of K are added, crop K requirements should be satisfied. Liquid P fertilizers, with the exception of food-grade phosphoric acid, may have impurities which complicate the already difficult task of eliminating chemical precipitation in the drip lines. However, with sufficient knowledge and attention to detail, fertilizer-grade phosphoric acid and ammonium phosphate solutions can be delivered successfully. Fertigation scheduling: Efficient fertigation scheduling requires attention to three factors: crop- and site-specific nutrient •- requirements, timing nutrient delivery to meet crop needs, and controlling irrigation to minimize leaching of soluble nutrients below the effective root zone. Seasonal total N, P and K requirements vary considerably by area and soil type (Hochmuth and Hanlon, 1995; Tyler and 6. Lorenz, 1991). General recommendations should be adjusted based on soil test results to estimate seasonal application rates required for a particular field. In many situations a small percentage of N and K (20-30%), and most or all P, is applied in a preplant broadcast or banded application. 61. Preplant application of N (and K, if needed) is particularly important where initial soil levels are low(Locasio et al., 1982, 1985), or in conditions where early-season irrigation is not required. +- Preplant application of P is common for several reasons. Soluble P sources (e.g. phosphoric acid) are more expensive than granular forms. The potential problem of chemical precipitation in the drip line is avoided. Also, the movement of drip-applied P away from the point of injection is .. governed mainly by soil texture and pH. Movement of P is particularly limited in fine-textured, alkaline soils. During the critical crop establishment period, P banded near the developing seedlings may be utilized more efficiently. When making a preplant application of any nutrient it r. is important that the fertilizer be placed within the wetting zone of the drip system. Once seasonal N, P and K requirements have been estimated and preplant application (if any) v made, the balance of the fertilizer can be delivered through the drip system in multiple applications Page 3 L