Grapes
Trees, vines and woody ornamental plants tend to be sensitive to Cl and high salts. Chloride and salt sensitivity can result from irrigation waters as well as from Cl and high salt indexed fertilizers. As a general guide, irrigation water containing 70 to 140 ppm Cl will cause sensitive plants to show slight to moderate injury, whereas the range for moderately tolerant plants is between 140 to 350 ppm. Note: Actual sensitivity varies widely depending on variety and rootstock (California Fertilizer Association, 1990).

In cases where high rates of K are applied to overcome K fixation, SOP is the preferred product over KCl due to potential Cl injury. Cook (1960) recommended that SOP be used at rates of 1,500 lb/A. Forshey (1969) suggested that SOP is preferable on saline soils for grapes and bush fruits, at low as well as at high rates of application. Christensen (1981) recommended SOP on soils with existing salt problems as well as for shallow, poorly drained soils, at high rates on young vines, or at high rates during the growing season. Sulfate of potash is effective in correcting K deficiency without the danger of Cl build up.

Research gives a better understanding of the K requirements in winegrape production. Studies by Matthews and Anderson (1992-93) show that K availability, applied as SOP, interacts with soil water conditions and the type of rootstocks that are used. On clay soils, SOP applied at the rate of 8 lb/vine maintained more favorable petiole K levels from bloom through harvest than the untreated comparison under standard irrigation. When irrigation rates were increased from 10 gal/vine/week to 40 gal, SOP increased petiole K levels further. The soil water content during fruit ripening can be an important factor in maintaining canopy K status. In another study, results confirmed that rootstocks differ in their capacity for K uptake and that there seem to be specific rootstock-scion interactions that contribute to vine growth and yield (Table 1). One year after SOP was applied at the rate of 8 lb/vine, yield and clusters per vine increased up to 60 percent for all rootstocks except 5C.

Table 1.
Potassium, applied as SOP, affects yield of Chardonnay grown on different rootstocks.

Peacock and Christensen (1985) reported that SOP is a good K source for grapes grown under drip irrigation. They increased K in the petioles and reduced foliar K deficiency symptoms by placing dissolved SOP directly below the emitter, simulating injection into the irrigation system.

In evaluating the longevity of SOP responses in three California vineyards, Christensen (1975) found a good response from a single 1 lb/ vine application which lasted 4 to 10 years. He concluded that most growers could expect correction to last 6 to 8 years; less in the case of severe deficiency.

Edelbauer (1979) working with nutrient solutions, found that yields were reduced if 'Gruner Veitliner' vines were supplied with greater than 62 percent of their K as KCl. If 25 percent or less came from KCl and the remainder from SOP, there was no adverse affect on yield or juice quality.

Recommendations (lb/A) for SOP in the San Joaquin Valley have been as high as: severe deficiency-2,300 to 2,700; moderate deficiency-1,350; mild deficiency-1,000. These applications should be effective for at least five years (Christensen et al., 1978).

In a single year field trial, it was shown that SOP, applied at a 36 lb/A K2O rate as a solution through drip irrigation, held higher tissue K levels than the control grapes fertilized with soil applied K (Waldon, 1995). The field trial indicated that this higher K allowed greater sugar production earlier. Resulting lab analysis showed an 11 percent increase in sugars using an enhanced SOP solution.