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UC navies Vegetable Research and Information Center Fertility Management of Dni lrrigated Vegetahbles v soils in Florida, an overall irrigation cycle of 45 min (young crop) up to 1.5 h (mature crop) would be sufficient (assuming today's typical drip tube flow rates) to apply the amount of water ` required by a tomato crop during any one irrigation cycle (Clark et al., 1990; Smajstrla et al., 1985). Irrigation cycles>1.5 h for a mature crop on sandy soils run the risk of leaching nutrients and moving water below the root zone. Longer irrigation cycles can be used effectively on soils ` with high water-holding capacity. More detail on injection calculations and periods is available (Clark et al. 1990; Hochmuth and Clark, 1991). It is very important to design the system and fertilizer injector so that injection and flushing can be achieved in a reasonable amount of time without running the risk of overwatering the crop simply to apply the fertilizer. In some systems, fertilizer is injected continuously (concentration injection) so that all irrigation water applied contains nutrients. This is acceptable as long as no irrigation cycle is so long that nutrients are leached below the root zone. During rainy periods, a bulk injection of a larger amount of fertilizer might be needed to fertilize a crop when no water is required. Water management is integrally linked to fertigation management. Water that moves below the active crop root zone can carry NO3-N (and, in very sandy soils, K) in substantial quantities. One cm of leachate at 100 mg NO3-N•liter 1 would contain 10 kg N•ha". Indeed, one of the major advantages of polyethylene bed mulch (frequently used in conjunction with drip irrigation) is the reduction of NO3-N leaching with precipitation, but that advantage can be negated by excessive drip irrigation. Conversely, in some areas well water used for drip irrigation contains a significant concentration of NO3-N; in regions such as the Salinas Valley of California NO3-N levels of 10-20 mg•liter 1 are common. Irrigating a crop with a total of 30 cm of water at 15 mg NO3-N•fiter 1 would add approximately 45 kg N•ha'1. Nutrient monitoring: The fertigation scheduling approach outlined above should, in most situations, supply adequate nutrition; however, monitoring soil and/or plant nutrient status is the essential safeguard to ensure maximum crop productivity. In conventional production, soil NO3-N testing usually has been +- limited to preplant sampling; since drip irrigation provides the ability to add N at will, more extensive NO3-N monitoring is justified. Traditional soil sampling and laboratory analysis offer the most complete, accurate information, but growers are not likely to go to the effort and expense of this technique on an ongoing basis through a cropping cycle. There are several alternative techniques to aid on-farm nitrogen measurement. One approach is the use of soil solution access tubes, also called suction lysimeters. These devices are simply porous ceramic cups, similar to tensiometer cups, attached to hollow access tubes. The units are installed in the field with the ceramic tips in the active root zone. To collect a sample, a vacuum is applied which draws water from the surrounding soil into the tube. This soil water sample is collected and analyzed for NO3-N content; the vast majority of mineral N is usually in the nitrate form The use of suction lysimetry has serious limitations. There can be large spatial variability; one portion of a field may vary from another and, since NO3-N moves with the wetting front, there can be stratification of NO3-N within the bed. This problem can be minimized by using multiple �„ Page 5