Influence of Different Irrigation Amounts On Plant Vigor and Fruit Quality in Two California Vineyards On Contrasting Soil Types.

Two Merlot vineyards, each containing at least two contrasting soil types, were investigated to monitor soil moisture variations and plant water stress under different irrigation regimes corresponding to 80, 66 and 50 percent of the grower’s usual water supply on each soil type. One of the goals of this study was to evaluate differences in evapotranspiration (ET) of grapevines planted on two contrasting soils in a single vineyard block, using surface renewal methods, and to evaluate site-specific corrective measures. A second goal was to measure differences in fruit quality between soil sites and between irrigation treatments. Both vineyards were located in the Sacramento-San Joaquin Delta of northern California. Vineyard 1 contained soils with contrasting morphology. An Entisol on recent alluvium induced high vine vigor, lower grape yields and delayed fruit ripening. A nearby Alfisol with compact clayey subsoil supported less vigorous growth, lower plant water potential but higher fruit yields and earlier ripening. Vines in vineyard 1 were 5 years old, had VSP trellising with machine pre-pruning after harvest, and final cordon pruning in March. Vineyard 2 was a 10-year old Merlot vineyard on a nearly level terrace, containing two strongly contrasting Alfisols. The first soil had loamy topsoil over very clayey, compact subsoil with minimal gravel. Vines were vigorous, plant water potential values were lower and harvest weights were higher than those recorded on the second soil. The second soil had sandy loam topsoil over sandy subsoil and contained 35 to 85% gravel and cobbles. Plants displayed lower vigor and had lower yields than those on the first soil. Plants in vineyard 2 had a VSP training system, and were cordon pruned in early January. Measured ET of vines on the four sites showed marked differences, reflecting the contrasting soil properties.