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UCCE Tulare County, Best Management Practices for Nitrogen Fertilization of Grapes Page 2 of
<br /> support spring growth (1,8,9,10,1 1,13,20,23,24,25,26). Since the grapevine's need for N is most critical in the spring
<br /> and highly dependent on storage, it can be inferred that fertilizer should be applied when the vine can best absorb and
<br /> `incorporate it as part of the N reserve while minimizing N loss from the soil (leaching, denitrification).
<br /> Time of fertilization, using isotopically labeled N, was studied in mature, furrow irrigated, "Thompson Seedless"
<br /> `vineyards in the San Joaquin Valley (20). Application in July or late September(postharvest) resulted in the highest
<br /> concentration of labeled N in both dormant storage tissue and in leaf tissue during rapid spring growth and at bloom.
<br /> Nitrogen applied at budbreak had insufficient time for uptake to become a significant fraction of total N in leaf tissue
<br /> `by bloom. When labeled ammonium sulfate was applied at budbreak to a vineyard on sandy soil, spring rainfall and
<br /> frost protection irrigations severely leached soil N by bloom (19).
<br /> `Nitrogen applications should be made to coincide with periods of rapid N uptake. Studies with potted vines indicated t
<br /> uptake peaks from budbreak to veraison and from harvest to leaf fall (8). Studies in furrow irrigated, mature
<br /> "Thompson Seedless" vineyards indicated the rate of N uptake is more rapid from fruit set to veraison than from
<br /> `budbreak to bloom. The rate of N uptake late in the season is rapid, with newly absorbed N incorporated into storage
<br /> tissue (1.8,20).
<br /> `There is concern that applying N during the periods from bloom until veraison or postharvest could result in greater
<br /> vine growth and production problems than traditional dormant applications. However, studies in the San Joaquin
<br /> Valley have found fertilization during these periods is compatible with fruit development and vine growth(5,18). Fruit
<br /> `soluble solids were similar for all period of N application, although N fertilization generally reduced soluble solids.
<br /> Berry weight, titratable acidity, pH, and total N of fruit juice were not affected by N fertilization, regardless of timing.
<br /> Cane maturity was not affected by fertilization during the growing season. In one trial with "Thompson Seedless", late
<br /> `shoot growth was slightly increased with N applied at veraison and, to a lesser degree,postharvest (5). In viticultural
<br /> areas where winter damage is a concern, more work is needed on the effect of N timing and on the hardening off
<br /> process.
<br /> How Much Nitrogen to Apply
<br /> The principal objective of N fertilization is to maximize crop development rather than vegetative growth. Grapevines
<br /> have a small N demand relative to many other fruit crops. The N requirement for stems, leaves, and fruit measured in
<br /> "Thompson Seedless" grapevines used for raisin production in the San Joaquin Valley was 84 kg.ha 1. Approximately
<br /> 35 kg.ha 1 was removed by the crop and the remainder recycled(27). This would suggest that the annual N demand
<br /> would be approximately 25 to 50 kg.ha 1 depending on crop size. Recent studies in the San Joaquin Valley indicate
<br /> "'Thompson Seedless" yields and fruit quality can be sustained with 25 to 50 kg.ha-1 N applied annually(5).
<br /> The amount of N fertilizer required varies with yield, soil type, and irrigation efficiency. Nitrogen inputs from
<br /> ` irrigation water, crop residue, and the mineralization of soil organic matter must also be considered. Fertilization may
<br /> not be necessary when high levels of NO3 are present in irrigation water, or when legume cover crops are grown.
<br /> Fertilization practices can be monitored by bloomtime petiole sampling for nitrate analysis (4, 6). Vines should be
<br /> ..closely observed, adjusting N rates according to vine vigor and production.
<br /> Application of N in excess of vine requirements increases potential for NO3 pollution and can be detrimental to vine
<br /> growth and production. Excess N is a more common problem in San Joaquin Valley vineyards than N deficiency(4).
<br /> Problems created by excess N include excess vigor, poor bud fruitfulness, excessive berry drop, bud necrosis, delayed
<br /> .. crop maturity, and increased levels of stem necrosis disorder, bunch rot, and leafhopper activity.
<br /> Water Management
<br /> Certainly,rate and timing of N application are important aspects when developing a best management practice. Most
<br /> http://cetulare.ucdavis.edu/puberape/na496.htm 10/11/200
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