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UC Dads Ve etable Research and Information Center Fertility Management of Dnplmaated Veaetahbles r over the growing season. From crop nutrient uptake characteristics one can apportion fertigation to meet nutrient requirements by crop growth stage. To calculate fertilizer application on the ` basis of daily or weekly need, one must account for the relative rate of crop development, which is dependent on temperature. Total seasonal crop nutrient requirements are relatively independent of environmental conditions. For example, an early spring melon crop will need roughly as much ` total N as a mid-summer or fall crop, even though the length of the growing seasons (planting to harvest) will differ significantly. Ideally, a crop-specific fertigation template could be developed using growing degree day(GDD) information. In practice this is not often done. Alternatively, historical information on crop phenology can be used to construct general fertigation schedules (Table 1); these schedules are based on research and commercial grower experience in Florida (Hochmuth, 1992). For some crop groups, such as cucurbits, extrapolation of the nutrient program for one crop to another crop is possible. These schedules assume the soil will be supplying little of the crop K requirement. It is important to keep in mind that the actual fertilizer requirement may be considerably less, depending on soil test K levels. Similarly, crops grown on soils with high N supply capacity(high organic matter, significant residual mineral N content, etc.) also may require substantially less N .. fertilizer. These schedules also can be compressed or expanded depending on the length of the growing season. Application of N and K in excess of crop requirements can have significant adverse consequences in addition to the added fertilizer expense. Nitrate contamination of groundwater has become a serious environmental issue in some areas, and excessive fertigation increases NO3-N leaching loss ■- (Pier and Doerge, 1995; Thompson & Doerge, 1995). Heavy N application, particularly when NHL-N predominates, can induce blossom-end rot in crops like tomato and pepper, and stimulate 1. vegetative growth at the expense of fruit yield. Excessive K fertilization has been shown to reduce specific gravity of potato and size of strawberry fruit (Hochmuth et al, 1993; Albregts et al., 1996). r— Nutrients can be injected at various frequencies (daily to bi-monthly), depending on system design constraints, soil type, and grower preference. Frequent injection might be needed on sandy soils that do not retain large amounts of nutrients, and for growers who want to minimize injection pump size and cost. Fertigation frequency, in most situations, is not as important as achieving a correct rate of application of nutrients to the crop during a specified period (Cook and Sanders, 1991; Locascio and Smajstrla, 1989). Some growers find it easy to fertigate with each irrigation using automated controls, which places small amounts of nutrients at risk of leaching during any single leaching event. Since leaching is possible with drip irrigation, nutrients applied in any irrigation must not be subjected to excessive irrigation either during that application or in subsequent irrigations. L, It is possible to inject nutrients in noncontinuous (bulk) or continuous (concentration) fashion. For bulk injection, drip irrigation systems always should be brought up to operating pressure before injecting any fertilizer or chemical. Fertilizer should be injected in a period such that L enough time remains to permit complete flushing of the system without overirrigation. On sandy Page 4 L