Influence of Timing of Nitrogen Fertilizer Application on Grapevines in the San Joaquin Valley

Timing of nitrogen (N) fertilization was evaluated in four mature, furrow-irrigated vineyards in the San Joaquin Valley, cvs. Thompson Seedless and Flame Seedless. The objective was to evaluate the effects of summer or late September N fertilizer applications on N utilization, vine response, and fruit development. Traditionally, N has been applied to soil in late winter or early spring, and information on the effect of fertilization during the growing season is lacking. Ammonium nitrate (50 kg ha-1, N) was applied to the soil either at budbreak, fruit set, veraison, late September (postharvest), or split (fruit set + late September) and compared to an unfertilized control over three years. Nitrogen applied to Thompson Seedless at budbreak increased nitrate levels in petioles sampled at bloom 26% and was less effective than N applied at veraison or late September which resulted in a 54% and 67% increase, respectively. Nitrogen applied at fruit set increased nitrate levels 37%, intermediate to the budbreak and veraison timing. Uptake and utilization of soil-applied N was slower during the period budbreak to bloom compared to the period fruit set to veraison. The inorganic N levels of Flame Seedless petioles were not affected by N fertilization. Total N concentration of fruit juice and arginine concentrations in dormant canes or spurs remained unchanged by fertilization in both cultivars. Delaying fertilization did not affect cane fruitfulness, and stimulation of late season shoot growth was not observed. Fertilization, regardless of timing, reduced soluble solids; however, berry weight, titratable acidity, and pH were not affected. This study indicates that N applied during fruit set, veraison, late September, or split is compatible with vine growth and fruit development.