Fertilization

Grapes, like other crops, require adequate supplies of all essential plant nutrients for optimum growth and yield. Most soils contain adequate or near-adequate quantities of all nutrients. Nitrogen, phosphorous, potassium, magnesium, and boron are the nutrients most likely to limit grape production in the Midwest. Vineyard fertilizer practices are designed to boost the supply of available soil nutrients to the levels required for optimum growth and fruit production.

For successful soil fertilization, growers must accurately determine the nutrient status of the vineyard. Vineyard sites or sections of the same site may vary in the levels of nutrients available to vines. Growers can choose from several methods to determine their vineyard�s nutrient status. Weak growth, poor leaf color and fruit set, and early defoliation indicate low levels of one or more nutrients. Delayed vine and fruit maturity and excessive vigor suggest an over (or late) application of nitrogen.

Soil tests are helpful for determining the level of nutrients in the soil. Plant petiole analysis, however, is a very important companion analysis that allows a grower the ability to follow the actual amount of nutrients taken up into the vine. Both forms of analysis should be used to maintain a good working knowledge of a vineyard�s fertility.

Soil Analysis

Soil testing is a relatively simple procedure that can be done during the fall or spring. Soil analysis will determine the availability of the primary (N, P, K), secondary (S, Mg, Ca), and micro (Fe, Mn, B, Cl, Zn, Cu, Mo) nutrients. Soil pH in the range of 5.5 to 6.5 is adequate for grape production. American varieties with, for example, Vitis labrusca in the pedigree will be more adaptable to lower pH (high acidity) soil than varieties with a greater amount of V. vinifera. There is very little potential of nutrient elements being tied up or being released at high levels that can cause toxicity to the vines.

Cation exchange capacity (CEC) is an important indicator of the soil�s particles to readily exchange cations (e.g., H+, Ca++, K+) with the plant roots. Generally, as the clay content increases, the more soil particles will be available to hold nutrients. Vineyard soils with 2% to 3% organic matter are considered normal. CEC measures the ability of a soil to retain exchangeable cations (H+, CA++, Mg++, and K+). The percent of organic matter and clay influence the CEC of a soil and ultimately determine the amount of each element available to the plant. There can be different ranges of CEC, depending on the classification of the soil as sand (1 to 5), silt (5 to 20), clay (20 to 30), and organic (30+). The amount of organic matter in a soil can influence soil fertility and tilth.

Nitrogen is generally lacking in Midwestern soils and is customarily applied each growing season at the rate of 50 to 80 lbs. of actual nitrogen per acre. Phosphorus deficiencies are not wide spread in Midwestern soils, with 40 to 50 lbs. per acre considered adequate. Potassium is an element that should be maintained around 300 lbs. per acre. Grapevines use considerable amounts of K for development of foliage, wood, and fruit production. Magnesium, boron (B), and zinc (Z) are considered adequate in the ranges of 200 to 250 lbs., 1.5 to 2% and 8 to 10% per acre, respectively (Table 17).

Liming is beneficial where acidic soils are common. By increasing the soil pH through liming, Ca⁺⁺ and Mg⁺⁺ levels can be increased, toxic levels of micro-nutrients can be reduced, and microbial activity can be increased, causing a release of N, P, K, and B. Additionally, soil structure and tilth have been shown to improve through the use of lime.

The Lime Test Index (LTI) indicates the tons per acre of Ag-ground lime needed to adjust a mineral soil to a specific pH. For example, a vineyard soil that has an LTI = 68 would require 1.2 tons per acre of Ag-lime to raise the soil pH to 6.5. An LTI is derived by multiplying the SMP (Shoemaker, McLean, and Pratt) buffer pH x 10. Coarsely ground material takes longer to react with the soil chemistry, and thus the pH will not change as quickly as when finer ground lime is used.

Soil samples are generally taken either during the fall or early spring, depending on when you would like to apply fertilizer. Some growers feel that by having their soil analyzed in the fall they are able to purchase needed fertilizer at a reduced rate. Others feel that applications of nitrogen and other elements should be made in the spring prior to shoot growth. (See Table 17 for desirable ranges of soil characteristics for grapes.)

Nitrogen applications should be completed around fruit set, but no later than veraison, to allow for uptake and utilization by the vines during the growing season. Mid- to late summer applications of nitrogen will encourage the vines to continue vegetative growth into the fall season when plants should begin to harden off prior to dormancy.

You can collect samples by using either a soil probe, spade, or shovel. Be sure to take samples in your vineyard along a Z- or X-shaped pattern in the field or block of vines to assure representative samples. Avoid sampling soil only in the middle or along the edges of the vineyard. In order to have a representative sample, be sure to use a clean plastic bucket and take a generous amount of soil from each subsample. To adequately evaluate your vineyard soil, be sure to sample at two different depths starting at just below the surface to 8 inches, then from 8 to 16 inches. This will allow you to make any necessary adjustments in soil fertility or pH or both.

When submitting a soil sample for analysis, you should fill out a Soil Test form. This form will instruct the individual processing your soil sample as to the type of test(s) you want to have conducted. Information given on the form will provide the location of the soil sample, recent history of fertilizer and lime applications, depth of where the sample was taken, crop status, intended crop, crop age (if planted), and whether irrigation is applied.

Plant Analysis

In plant analysis of grapes, leaf petioles are the parts sampled. Chemical analysis of the petioles indicates the level of nutrients in the entire vine. Samples are normally collected between July 1 and August 30 which corresponds to veraison. Contact your county Extension office for a listing of potential laboratories where you can send your samples for analysis. Be sure to provide complete information on the container as to the date, location, and variety sampled.

To be most efficient, petiole analysis should be conducted over a period of years as part of the management program. A single analysis is useful in diagnosing a nutrient problem or in determining the nutrient status of the vine at the time sampled. However, one analysis cannot indicate what the nutrient status may be a year later.

Petiole analysis continued for three to five years in a given vineyard will help establish trends and changes in nutrient element levels. The direction and nature of the trends are then interpreted and used in planning annual fertilizer and cultural programs. This information can greatly help growers maintain vineyards at their peak of production year after year. The recommendations for nutrients in grape petioles are shown in Table 18.

Primary Nutrients

Primary nutrients are essential for sustaining vegetative growth, yield production, plant vigor, and winter hardiness of vines. Nutrient levels can be assessed by soil and/or plant tissue analysis.

Historically, grapes grown in the Midwest were petiole sampled during mid-July to the end of August. Research from California and New York indicated that sampling for N during June would provide a more accurate example of the N that is available for plant growth and development during the growing season. This may, in the future, become a more acceptable practice for Midwest grape growers.

Nitrogen

The nutrient element that is often low or deficient in a vineyard is nitrogen. Grape leaves will exhibit a light-green to yellowish-green color as nitrogen in the vine drops to low or deficient levels. Leaf discoloration will appear in the older leaves as the nitrogen is translocated from older to new emerging leaves. Vines will have poor vegetative growth and reduced fruit set where nitrogen is deficient.

Nitrogen is generally applied in split applications at budbreak or post-budbreak and during bloom. Application rates vary with vine vigor and other factors. In general, most vineyards should receive between 40 and 80 pounds actual nitrogen per acre each year. If ammonium nitrate (33-1/2% nitrogen) is used, 120 to 240 pounds of the material will be broadcast per acre over the vineyard row area. Less-vigorous vines should receive higher rates. In small vineyards, nitrogen fertilizer can be applied annually at the rate of 1/2 to 1/3 pound of a 33-1/3% nitrogen carrier, or its equivalent, around each vine. Do not concentrate the fertilizer at the base or allow fertilizer to touch the vine.

Most nitrogen forms can be used equally well in vineyards. The choice depends largely on the cost per pound of nitrogen applied, with one possible exception. On soils with a high pH, where grape leaves show symptoms of manganese or iron deficiency, sulfate of ammonia is preferred. This form of nitrogen tends to increase soil acidity, which makes more manganese available to the vine roots. Do not use sulfate of ammonia on low-pH soils.

High application rates can stimulate excessive growth, which may result in the appearance of deficiency symptoms of other nutrients. For example, if supplies of potassium, magnesium, or other nutrients are low in a given vineyard soil, excessive nitrogen application rates could result in the appearance of deficiency symptoms of one or more of these elements.

Phosphorus

Midwestern vineyards tend not to have serious phosphorus (P) deficiency problems. Generally, adequate amounts of P can be found when soils are tested. In the event that a soil analysis indicates a lack of P, then superphosphate or blended fertilizer with P included can be applied at a rate based on soil and/or leaf tissue analysis.

FIGURE 110. Potassium deficiency symptoms characterized by yellowing of leaf margins.

Potassium

Grapevines will often show signs of potassium (K) deficiency when heavily cropped and little or no additional K has been added. A dull, dark green color will appear on the leaves. In mid- to late summer, leaves may have a bronze color, especially on the west-facing side of the trellis. Some leaves may have dark spots or blotches. This symptom often has been characterized as black leaf of grapes (Figure 110).

Marginal chlorosis, browning, and dying may occur as the deficiency becomes more severe. Other possible symptoms include brown dead spots or areas throughout the leaf. In severe cases, more than half of the leaves on a vine may show these symptoms. Severe potassium deficiency greatly reduces vine vigor, berry size, and crop yield.

Symptoms of potassium deficiency generally develop in mid-shoot leaves followed by older basal leaves. Potassium carriers include potassium sulfate (sulfate of potash), containing about 50% K2O; potassium chloride (muriate of potash), containing about 60% K2O; and potassium nitrate, with 44% K2O.

Foliar sprays of potassium sulfate or potassium nitrate can be effective to temporarily reduce a severe K deficiency. Potassium compounds tend to be fixed in the soil surface, although less than phosphate. This fixation, which makes potassium unavailable to plant roots, generally is greater in a clay soil with pH near 7.0 than in a sandy soil with a pH near 5.0. Therefore, potash application rates may need to be greater and more frequent on clay than on sandy loam soils, especially if the pH is above 6.5.

Response to potash fertilizers is greatest when applications are made in 2-foot bands beneath the trellis. Broadcasting potash over the entire vineyard is less efficient and less economical.

Either soil or foliar applications of sulfate or nitrate of potash should be made where potassium is low. Soil application rates should be based on a soil test, foliar analysis, or both. In general, 100 to 400 pounds per acre have been adequate.

If foliar sprays are indicated, use a solution containing 6 to 10 pounds of either carrier per 100 gallons. Apply at a rate of 200 to 300 gallons per acre of mature vineyard. Foliage applications are of primary value, but they provide only a temporary solution to a potassium problem. Soil applications have a more lasting effect. Make one or more applications as soon as the need is determined. Avoid excessive rates of potash, which can lead to magnesium deficiencies in the vineyard and high pH must and wine.

Magnesium

Magnesium (Mg) is most likely to be deficient in vineyards with low pH (acidic) soil or in situations where excessive amounts of potassium have been applied. The lower the soil pH, the more Mg is tied up in the soil particles and not available for plant use. Increased levels of potassium will cause the displacement of Mg++ cations on the soil surface, which causes a reduction in available Mg ions.

Symptoms of Mg deficiency develop on the older leaves first. Chlorosis (yellowing) appears between the veins of the leaves while the veins remain green (Figure 111). As a vine becomes more severely affected, the interveinal chlorosis intensifies in older leaves and spreads to younger leaves toward the terminals of canes. The younger terminal leaves may not exhibit symptoms until the entire vine is extremely deficient.

FIGURE 111. Magnesium deficiency symptoms in a red (L) and a white (R) cultivar.

Where the soil pH is below 5.5, apply dolomitic lime (high in magnesium) at the rate of two to four tons per acre. This will increase soil pH and correct a magnesium deficiency. Complete correction could occur within a few months or may not be achieved until one or two years after the application.

If the soil pH is above 6.5 and no liming is needed, correction is best achieved through foliage applications of magnesium sulfate (Epsom salts). Even if dolomitic limestone has been applied and Mg deficiency symptoms are severe, foliage application provides a quick but temporary solution.

For foliar sprays, mix magnesium sulfate at the rate of 16 pounds per 100 gallons of water. Two applications usually are adequate. Apply the first shortly after bloom and the second two weeks later. Each spray application requires about 200 to 300 gallons of the mixture per acre to adequately cover the vines.

If longer-lasting effects are desired, magnesium sulfate can be applied to the soil. This is recommended on high pH soils where magnesium deficiency has resulted from excessive potash applications. In these cases, magnesium sulfate may be applied alone or mixed with other fertilizer materials at the rate of 100 to 500 pounds per acre broadcast over the entire vineyard floor. In small vineyards, apply magnesium sulfate at 1/4 to 3/4 pound per vine. Magnesium sulfate or dolomitic limestone can be applied any time of year.

Secondary Nutrients

Secondary nutrients, although not required in large amounts, are essential for good plant growth and vigor. Over-application can occur when adjusting levels of calcium (Ca), manganese (Mn), iron (Fe), boron (B) or copper (Cu). If too high a concentration of secondary nutrients is applied either by foliar or soil application, then there is a risk of increasing each of the elements to toxic levels.

Secondary nutrients Mn, Ca, and Fe are normally not found to be deficient. Soil and/or leaf tissue may indicate that these elements are deficient. A foliar application of a deficient nutrient(s) can be sprayed on for immediate effect. This type of fertilization is a temporary means of correcting a deficiency and should not to be used in a long-term fertility program.

Calcium

Calcium is generally not found to be deficient in soils that are well limed. Unless the pH is allowed to drop below 5.8, there is little chance for Ca to be tied up in the soil. Additionally, lime that is high in Ca can add adequate amounts to the soil. Calcium can be applied as finely ground limestone or hydrated lime, or in fertilizer mixtures.

Boron

Boron deficiencies have been observed in grape plantings in the form of poor fruit set. Clusters will tend to be small, and (shot) berries will not fully develop on the rachis. Terminal buds may not break in the spring, and ends of shoots sometimes are distorted. A plant tissue analysis can determine if there is a deficiency in B, and appropriate amounts can be applied in a foliar spray. Boron availability should not be a problem if soil pH does not become alkaline. Borax or borate, B carriers, can be sprayed on in the spring when needed.

Manganese

Manganese deficiency symptoms first appear as interveinal chlorosis, or yellowing, of the younger terminal leaves. A Mn deficiency may occur when the soil pH is 7.0 or higher and can be corrected by applying fertilizer-grade manganese sulfate at one to two pounds per vine, or 250 to 500 pounds per acre, depending on vine size and severity of the deficiency. Manganese chelate (EDTA), or its equivalent, can be used in place of manganese sulfate. Always read the product label before applying these materials.

A foliar application of Mn can be sprayed on for immediate effect. This application is considered to be a temporary means of correcting a deficiency and not for long-term adjustment of the nutrient level. Mix manganese sulfate at the rate of 4 pounds plus 2 pounds of hydrate lime per 100 gallons of water. Manganese chelate is mixed at the rate of 1 pound per 100 gallons of water. Spray at the rate of 200 to 300 gallons per acre. Two applications usually will provide season-long control of manganese symptoms. Apply the first just after bloom or when symptoms first appear and the second two weeks later.

Iron

If deficiency symptoms develop or a petiole analysis indicates a need, apply iron in the form of iron chelates.

Copper

Copper is required in minor amounts and, if needed, can be applied at no more than 4 to 6 lbs. of copper sulfate per acre in a foliar spray in early spring before budbreak.

Zinc

Zinc is required in minor amounts and, if needed, can be applied in a foliar spray at 5.5 lbs. of zinc chelate per acre in early spring before budbreak.

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