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UC Davis Vemetable Research and Information Center Fertility Management of Drip-Irrigated Veoetahbles r lysimeters per field, but that also greatly increases the effort required, and the cost. Interpretation of results is also problematic. Extensive field data is lacking, but in general, a root zone soil solution NO3-N concentration >75 mg•liter I indicates that sufficient N is available to meet immediate plant needs. A lower NO3-N concentration cannot be interpreted directly as N deficiency, given the difficulty of obtaining a sample representative of the whole root zone. Plant i tissue analysis would be warranted to confirm crop N status. Another simple technique for estimating soil nitrate concentration is the `quick test' procedure '— described by Hartz (1994). This test has the advantage of measuring NO3-N in a composite soil sample representative of the root zone, compared to the site-specific measurement of a suction lysimeter. Soil and the NO3-N extracting solution are measured volumetrically, eliminating the need to dry or weigh soil. The moisture content of soil will affect the test, but moisture content of drip-irrigated soils generally will fall in a relatively narrow range, so the impact will be minor in most cases. Adjusting the test based on soil texture (hence, water-holding capacity) will improve accuracy. In general, soil NO3-N values above 20 mg•liter t indicate sufficient available N to meet immediate plant needs. •- Reliance on soil NO3-N testing is most appropriate early in the crop cycle, when crop N uptake rate is low and the detection of substantial residual NO3-N can lead to reduced additional N fertigation. By mid-season, crop uptake rates increase and soil NO3-N concentration correspondingly will change more rapidly. Also, once an extensive root system is developed, many crops can take up N in excess of crop needs (luxury consumption); low late-season soil NO3-N does not necessarily reflect N deficiency. From mid-season until harvest, plant tissue +- analysis should be the primary indicator of N status, although soil testing still may be used to identify fields where NO3-N levels remain high enough to delay additional N application. Conventional plant tissue analysis, in which tissue is dried, ground and analyzed chemically in a laboratory, is the most accurate way to determine crop nutrient status. Through decades of research, sufficiency guidelines have been developed for most important vegetable crops. These +... guidelines have been published for vegetables in Florida (Hochmuth et al., 1991) and California (Reisenauer, 1983). Although not specifically developed for drip irrigation, these standards are still generally applicable. Unfortunately, laboratory analysis of dry tissue is relatively costly, and i.. the time lag between sampling and obtaining results can be significant. In recent years there has been increasing interest in on-farm tissue testing, particularly for monitoring drip-irrigated fields. On-farm monitoring usually involves the analysis of NO3-N and K content of petiole sap; sap .� analysis for PO4-P is uncommon. Measurement techniques include colorimetric methods, NO3-N or K test strips (Hochmuth, 1994), or ion-specific electrode (Hartz et al., 1993; Vitosh and Silva, 1994). Although all methods can be used successfully, the ion-specific electrode is the most commonly used approach. Table 2 lists petiole sap NO3-N and K sufficiency ranges developed under Florida conditions. These values are similar to those developed in California. The appropriate protocol for tissue collection, handling, and analysis is discussed by Hochmuth 6„ (1994). L Page 6 L