Tulare County

Potassium Fertilization

Potassium (K) deficiency is usually confined to small areas in a vineyard--seldom larger than 1 to 3 acres. Deficiency is likely to occur in cut areas, where the K rich surface soil was removed during land leveling, or on very sandy soils that have low native K fertility. Shallow soil areas, poorly drained soils, and root problems caused by soil pests are also more subject to K deficiency.

Vines severely deficient in K tend to have fewer and smaller, tight clusters with unevenly colored, small berries. With Thompson Seedless the lower portion of the bunch may collapse by midsummer, resulting in raisined, immature berries by harvest. Much of the effect of K deficiency on the fruit is the result of reduced vine growth and premature leaf fall.

With furrow/flood irrigation methods, massive fertilizer rates are necessary to overcome the high fixation of K in most California soils. Typical application rates for mild to severe deficiencies range from 1 to 3 lbs of K per vine with potassium sulfate the most commonly used fertilizer. Deep placement of K fertilizer in a concentrated band close to the vine is the recommended application approach. Treatment can correct deficiency for 5 to 10 years, depending on deficiency severity and rate of application. Potassium should be applied late fall to early spring.

Fertilizer efficiency is improved when applied under drip irrigation. With drip, a high concentration of K saturates soil reaction sites in an area of high root density. A fertilizer experiment in drip irrigated vineyard on sandy loam soils in the Southern San Joaquin Valley showed correction of severe potassium deficiency with rates as low as 2 pounds potassium sulfate (45% K) per vine. (It would take 4 to 6 pounds of potassium sulfate per vine to correct a similar deficiency under furrow irrigation.) In this experiment, rates as low as ½ to 1 pound of potassium sulfate per vine increased K levels in tissue and greatly improved visual symptoms of K deficiency compared to unfertilized vines.

A single application of K with drip is just as effective as multiple applications providing the same amount of K is ultimately applied. However, it is often more practical to apply a little K through the drip system on a weekly basis rather than a large amount all at once. For potassium maintenance, 10 to 15 pounds of K per acre can be applied on a weekly basis for five to ten weeks. Fertilization should be completed before veraison (berry softening).

To correct a severe K deficiency in a small area of the vineyard, potassium sulfate at a rate of 2 pounds per vine is placed by hand in a small excavation made in the soil directly under the emitter. This can be followed up with K applied in small amount through the drip system for overall maintenance.

Potassium fertilizer products used in vineyards include potassium sulfate (K₂SO₄) (dry 45% K, liquid 0-0-6); potassium chloride (KCL) (dry 52% K, liquid 0-0-10); potassium nitrate (KNO₃) (38% K + 13% N); liquid potassium carbonate (K2CO3) (0-0-30); and liquid potassium thiosulfate (K₂S₂0₃) (0-0-25+17 sulfur). Potassium sulfate is the most commonly used K fertilizer. Potassium chloride should only be used on well drained soils with no existing salinity problems and at low rates to avoid chloride toxicity. When using potassium nitrate, the nitrogen content must be considered when determining rates. Potassium carbonate has good solubility and is used on acid soils because of its alkaline reaction. Potassium thiosulfate contains two pounds of sulfur per gallon and is used on alkaline soils because of its acid soil reaction. Liquid formulations of potassium are available with varying concentrations of nitrogen.

Boron Fertilization

Boron fertilization methods and rates should be carefully defined because of the small amounts needed for correction while avoiding over-application and possible toxicity. All of the original vineyard trial data published in California involved single, hand-broadcast application intended to be corrective for several years. On the basis of this work, the general recommendation has been to apply 2/3 oz./vine of Solubor 20% B (65% B₂0₃). Experience has shown that this treatment will typically last 4 to 5 years in the San Joaquin Valley. However, higher rainfall viticultural areas and vineyards with limited root systems could possibly develop earlier reoccurrences of deficiency at these rates.

Subsequent to the original work, more economical and convenient application methods have been suggested and used commercially. These include soil spray broadcasting, berm spray (usually included with herbicide spray), foliar spraying, and drip application. Of these, berm spraying in the winter and foliar spraying during the growing season are most common. Either method could be used as a combination with other sprays, eliminating a special spray application. Examples are herbicide, fungicide, and insecticide treatment. Also, such applications involve careful sprayer calibration, important to boron fertilizer as well as pesticide usage.

Berm spraying and foliar spraying were compared with standard hand broadcasting in a 4-year Fresno County study. The results verified the experience of boron application versatility. Generally, the berm spray treatments were the most effective. This is probably because the fertilizer spray was uniformly distributed over the berm surface where roots are more concentrated. Also, the berm area received no irrigation water, which would otherwise leach some of the boron below the root zone. It was concluded that boron rates can be lighter when applied with berm sprays as compared to broadcast application.

It was also found that applying annual, lighter rates on berms provided for more efficient and consistent vine uptake as compared to higher rates every 2 to 4 years. All treatment frequencies (1, 2 or 4 years) with appropriately adjusted rates gave sufficient longevity of correction and could be recommended. However, annual or biannual applications would seem most appropriate in situations with high rainfall and known low boron.

Foliar spraying has been most often used as an emergency measure to correct boron deficiency. However, it can ultimately also provide for root uptake, as the boron ends up on the vineyard floor at leaf fall. It could be useful as a follow-up maintenance tool after soil levels are increased through soil application.

Hand-broadcast application was not as effective as berm spray in the study. However, it is especially useful on small acreages or for spot treating. A single application was shown to last beyond 4 years under winter rainfall averaging 10.5 inches.

On the basis of the work presented here and years of commercial experience, the following tentative guidelines are given. These should be adjusted to local experience and soil, water, and rainfall conditions. Treated vineyards should be monitored with leaf petiole or blade analysis to determine if the application timing and rate is satisfactory.

You will note that the basic recommendation is 1 lb actual B/acre/year with amounts adjusted to frequency of application. It can be modified by local experience, rainfall amounts, irrigation practices, etc. and through tissue analysis monitoring.