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UCCE Tulare County,Best Management Practices for Nitrogen Fertilization of Grapes Page 3 of critical, however, is the efficient management of irrigation water to minimize leaching and denitrification. Irrigations must be accurately scheduled and applied, and irrigation systems must be properly designed, operated, and maintained. Water management in vineyards has improved with increased knowledge of crop water requirements and evapotranspiration(ET). The relative constancy of evaporative demand in the San Joaquin Valley and the availability of long-term ET records have resulted in the development of practical and efficient drip irrigation schedules (16). Water budgets that consider ET, water storage in the root zone, and allowable depletion between irrigations have improved efficiency of high volume irrigation. Advantages With Drip Irrigation Drip irrigation, when managed properly, can achieve high irrigation efficiency primarily by minimizing water flow below the root zone but also from reduced surface evaporation and runoff(_1.5,16). The potential for increased irrigatioi _efficiency is greatest on sandy, well drained soils, texturally nonuniform fields, and hilly terrain. Drip irrigation wets a limited area of the root zone. Nutrients applied through the system are placed where roots are _concentrated and uptake is supported by high soil-water matrix potentials when irrigations are properly scheduled; N uptake is impeded by under or over irrigation. This provides an ideal environment for uptake of N and other nutrients applied through the system(7,15). Fertilization through a drip system is relatively easy and offers an opportunity for _multiple applications. Multiple applications, however, do not necessarily improve uptake and utilization of N. The value of multiple ,applications with a drip system was studied in the San Joaquin Valley in a mature, "Thompson Seedless" raisin vineyard and using isotopically labeled ammonium sulfate(17). Irrigations were accurately scheduled and monitored t( meet vine requirements while minimizing leaching. To test the value of single versus multiple applications,40 kg.ha-1 —N was applied as follows: single using 40 kg.ha 1 N(lx) applied 4 April 1984; 20 kg.ha 1 applied 4 April 1984 plus 2C kg.ha-1 four weeks later(2X); eight weekly applications of 5 kg.ha-1 N(8X)beginning 4 April 84. The results,presented in Table 1, show no clear advantage in multiple applications. The percent N derived from fertilizer in leaves, roots, trunk, and canes did not vary with application technique. Multiple applications of N through a drip system would probably improve N uptake efficiency when irrigations are not managed efficiently, or on very sandy soils. Also, multiple applications using small amounts of N should be considerer to reduce the potential for salt injury or N toxicity. Future Directions —Cover crops affect cycling of N in the root zone and the N status of the grapevine (21,22). Winter cover crops can reduce nitrate leaching to groundwater by taking up excess nitrate in the soil during an otherwise fallow winter period. Mineralized N from incorporated cover crop residues can then become available to the grapevine during summer `months, thus contributing to more efficient nitrogen cycling. To manage excess N, summer cover crops, such as sudan or perennial grass sods, compete with the vine for NO3 during periods of rapid uptake and reduce the N status of the ,vine. Our research team is currently evaluating the effect of various floor management systems on the N status of the vine .and the annual dynamics of the inorganic N pool in the root system. Non-tillage and reduced tillage methods are included in these studies along with various cover cropping systems: annually seeded winter growing grasses and legumes; reseeding winter annual grasses and legumes; perennial grasses and legumes; and summer annuals. Our .objective is to further the understanding of how to best manage N in grape culture. Table 1. The % N derived from fertilizer in leaves samples 7/18/94, 9/20/84 and 5/7/85; a root, trunk "http://cetulare.ucdavis.edu/puberape/ng496.htm 10/11/200