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Winegrape Fertilization Practices for Oregon

A vineyard nutrition management program should complement the soil’s ability to provide the nutrients needed to sustain adequate vigor and produce the desired quantity and quality of crop. Soil conditions and production systems can vary considerably from vineyard to vineyard. Therefore, fertilization practices should be customized for individual vineyards and blocks within vineyards, and should be based on a thorough knowledge of the existing conditions.

Soil Characteristics

Every vineyard should be mapped for soil characteristics. A starting point is the soil type descriptions found in your county soil survey map. Such a map is a useful general guide for the soil associations and soil types within a site, but can not be used for the management of that site. The soils in any one association ordinarily vary in slope, depth, drainage, and other characteristics that affect their management. Develop a map of your vineyard that locates variations in soil types, depth, drainage, water holding capacity, slope, and other notable characteristics.

Soil analyses should be done for each recognizably different area within your vineyard. Soil nutrient content does not rapidly change for most nutrients, so analyses are generally not necessary to do more frequently than every 5-10 years unless major applications of fertilizer or lime are made.

A fertilization program must also be based on the production system that is being used. The relative nutritional needs and efficiency of nutrient uptake varies among grape varieties, clones, and rootstocks. Vine spacing, and the nutritional needs and/or contributions of cover crops must also be considered.

Monitor grapevine nutrient status annually with separate petiole analyses of each block, variety, rootstock, or other significantly different area of the vineyard. Petiole analyses should be conducted at the same time every year, using the same procedure, so that the results can be used to monitor trends in nutrient status. The changing trends in nutrient status are more important than single season results, which can be influenced by seasonal climatic differences or localized episodes of stress from factors such as drought or overcropping.

Keep records on all fertilizer applications; include product, rate and timing in your records. Follow up with written comments on the observed response to the fertilizer application.

Oregon Vineyards

Soil tests of Oregon vineyards frequently indicate low levels of phosphorus and boron, sometimes low potassium, and usually a relatively low pH. Keep in mind, however, that soil tests rarely are representative of the entire rooting depth of grapevines. Nutrient content and pH vary with soil depth. Interpreting soil tests in combination with the results of petiole analyses and observations of grapevine vigor provide the most complete picture of the nutrient status of your vineyard.

Grapevine petiole analysis results (Table 1.) from Oregon State University’s Central Analytical Lab indicate that nutrient deficiencies were relatively infrequent in Oregon vineyards. Only nitrogen (38%) and boron (14%) were commonly deficient, and petiole nitrogen levels are generally ignored in favor of observations of grapevine vigor and crop production. Phosphorus, potassium, magnesium, and zinc were not commonly deficient.

Table 1. Summary of Oregon winegrape petiole
analysis reports, 1986-1995.
% Samples Deficient
Nitrogen 38
Phosphorus 5
Potassium 4
Boron 14
Calcium 1
Zinc 7
Magnesium 4
Source: Oregon State University

Vineyard fertilization practices in Oregon match the petiole analysis results fairly well. It was estimated by the Oregon Agricultural Statistics Service (Table 2.) that 23% of the grape acreage in 1995 received nitrogen fertilizer, 10% received phosphorus, and 9% received potassium. No figures are available for micronutrient applications, but boron and zinc are commonly applied.

Table 2. Estimated fertilizer
primary nutrient applications
to Oregon vineyards in 1995.
% Acres lb/acre
Nitrogen 23 27
Phosphorus 10 32
Potassium 9 43
Source: Oregon Agricultural Statistics Service.

Nitrogen

Nitrogen (N) is the most commonly needed fertilizer element in vineyards. Grapevines, however, do not have as high a nitrogen requirement as many other crops. Nitrogen fertilization always raises the concern of encouraging excessive vigor that can result in shading and reduced fruit quality. A common approach to nitrogen fertilization on relatively fertile Oregon vineyard soils is to fertilize new vines with 20 to 30 lbs. of actual nitrogen per acre during the first two years. Once vines are established, no nitrogen is applied until decreased vigor is observed. Then, a conservative nitrogen fertilization rate (25 to 30 lbs. N/acre) is applied and the vine response is closely observed. This may be a sensible approach, but keep in mind that vine growth and yields are usually reduced before symptoms are clearly expressed. Fertilization programs must also consider the nutritional requirements of annual and permanent cover crops.

The decision of which type of nitrogen fertilizer to use is primarily dependant on cost and the rate at which the nitrogen becomes available from the fertilizer product. The nitrate form of nitrogen found in calcium nitrate (15.5% N) is immediately available to the plant. It is also the most expensive dry fertilizer source of nitrogen. Ammonium nitrate has half of its 33% nitrogen in the readily available nitrate form. The other half is in the ammonium form which must undergo conversion to nitrate by soil microbes, requiring from 1-2 weeks. Urea fertilizer (46% N) also must be converted to the nitrate form before it is available to the vine. To prevent nitrogen loss from volatilization, urea and ammonium nitrate fertilizers should be drilled or incorporated at least two inches deep. Urea can be incorporated by rainfall or irrigation following application, but rain does not prevent volatilization loss when dry ammonium nitrate is applied to the soil surface. It must also be noted that urea and ammonium nitrate are acid-forming in the soil, while calcium nitrate does not acidify the soil. Monitor topsoil pH when these nitrogen fertilizers are used on a regular basis.

Complete fertilizers, those containing nitrogen, phosphorous, and potassium (N-P-K) are a more expensive source of nitrogen fertilizer because you are paying for P and K that your vineyard may not necessarily require. Foliar fertilizers usually are the most expensive source of nitrogen, and often contain many additional elements that do not require supplemental applications. Foliar fertilizers are usually not the best choice for nitrogen fertilization because the relatively large amounts of nitrogen required are difficult to supply with the dilute formulas that are necessary. Organic materials, such as manure, grape pomace (acid-forming), or an annual cover crop turned under, can be a good source of nitrogen as well as provide other soil-improving benefits. Be aware that organic sources vary in their nitrogen content and the rate of nitrogen availability. Compare the cost of the nitrogen they contain and their application to the cost of applying dry nitrogen fertilizers.

Nitrogen fertilizers traditionally have been applied in late winter or early spring so that it would be in the root zone at bud break. We now know that new vine growth in the spring is primarily dependent on nitrogen stored in the wood and roots. Therefore, the most efficient time to apply nitrogen has been shown to be from fruit set to the post-harvest period.

Phosphorus

Grapevine phosphorus (P) deficiency has not been a problem in Oregon despite the sometimes low soil P content. Several factors contribute to this: grapevines have a good ability to extract P from the soil, P is very mobile in the vine, and crop removal of P is relatively small. Generally, P fertilization is not necessary, but if soil and petiole tests indicate very low P levels you may consider a trial application in a portion of your vineyard. Apply triple superphosphate (0-45-0) at the rate of 1,500 pounds per acre in a band close to the vine. Observe the treated vines over the next several seasons to determine if there was any response to the fertilizer application.

Potassium

Grapevines have a relatively high need for potassium (K), comparable to nitrogen, and much of the potassium is removed from the vineyard in the fruit. Potassium deficiencies, however, were only seen in 4% of the petiole samples tested by O.S.U. over a ten year period (Table 1). The reasons are that many Oregon soils have adequate levels of K, potassium is resistant to losses from leaching, and deficiencies are generally confined to small (less than 1 to 3 acres) areas in a vineyard. However, levels of K often decline considerably from the topsoil to subsoil layers. This can lead to temporary deficiencies in nonirrigated vineyards, particularly during the fruit ripening period when considerable K is accumulating in the fruit. Overcropping a vine also can lead to a temporary K deficiency during fruit ripening.

If a potassium deficiency appears, first try to determine the cause of the deficiency before deciding a course of action. The temporary deficiencies caused by drought or overcropping probably can be ignored if soil tests from the deficient area indicate that adequate K levels are present. If soil K levels are quite low, it may be due to an overabundance of calcium (Ca) or magnesium (Mg). These three elements compete for fixation sites on soil particles, and a large excess of any one element can cause reduced availability of one or both of the other elements. This situation is difficult to correct, requiring massive applications of K fertilizer to correct an excess Ca or Mg problem.

If potassium fertilization is warranted, potassium sulfate (0-0-51) is an effective fertilizer source. Because potassium is rapidly fixed by the soil, the quickest response can be achieved by applying the fertilizer in a single heavy application. Apply the fertilizer in a concentrated band to the root zone at a rate of 3-5 pounds per vine, in 6-8 inch furrows, 18-24 inches from the vine.

Avoid unnecessary applications of potassium. High K levels can lead to high K content in fruit and elevated must pH. Extremely high K levels may induce a magnesium deficiency. Remember, K deficiencies tend to be localized in relatively small areas; spot treat these areas, not the whole vineyard.

Boron

Boron (B) deficiencies are relatively common in Oregon (Table 1) because of naturally low levels in our soils. Adding to the low soil boron problem, B is very immobile in the plant, which sometimes makes it unavailable when and where it is in critical need by the vine. Boron is needed for early shoot growth in the spring, and plays an important role in pollination and fruit set. Boron deficiencies have been associated with: drought the preceding fall or early winter, cold weather combined with cold wet soils in the spring, and pruning in late fall or early winter.

Unlike the other previously discussed mineral nutrients, boron fertilization is most effectively achieved with a soluble B foliar-applied fertilizer. Because boron is so important to grape production and B fertilizer is relatively inexpensive, it is recommended that boron foliar applications routinely be made to most Oregon vineyards. A post-harvest application that wets the buds is the best way to prevent the shoot-stunting symptom sometimes seen in the spring. Pre-bloom sprays seem to be an effective way to get B into flower parts. Use foliar applications at an annual rate of one pound of actual boron per acre to maintain adequate B levels without building up excesses. A note of caution about B; there is a narrow range of B levels between deficiency and excess (toxicity) for grapevines. A spray concentration of 0.4 lbs. actual B per 100 gallons of water should be safe for pre-bloom or other growing-season sprays. The post-harvest spray can be up to 0.8 lbs. actual B in 100 gallons of water.

Zinc

Zinc (Zn) deficiencies can be a serious problem in grapes, causing poor fruit set and stunted shoots with small, misshapen leaves. Deficient levels of zinc have occasionally been seen in Oregon petiole samples, but usually are localized within a small portion of a vineyard. Low Zn levels are generally associated with sandy soils and soils with high pH or high P levels; none of these conditions are common in western Oregon vineyards. Clay soils with a high magnesium content also can be low in available Zn.

Foliar application of zinc is the most effective method for treating Zn deficiency. Neutral zinc products containing 50-52% Zn, or zinc oxide (75-80% Zn) are both effective as foliar sprays. Use 4-5 pounds per acre of neutral zinc or 2-3 pounds per acre of zinc oxide in dilute applications of 100-150 gals/acre. Both of these materials are not very soluble and require good agitation and occasional flushing of sprayer lines to prevent clogging. Chelated zinc products are fully soluble in the spray tank, and are the preferred form when low volume or concentrate foliar sprays are applied.

Zinc spray applications are most effective in improving fruit set when applied during the period of two weeks prior to bloom up to full bloom. If foliar deficiency symptoms persist or reappear, a second application may be necessary.

Soil pH

Excessive soil acidity can reduce growth and yield of grapevines, and potentially cause fruit quality problems. Western Oregon vineyard soils are naturally acidic, with a pH generally in the range of 5.2 to 6.0. Soil pH can decline over time due to the acidifying effects of urea or ammonium fertilizers and sulfur used for powdery mildew control. Therefore, many of our soils are below the optimal pH range (6.0 to 6.5) for grapevines. Watch for rising Manganese (Mn) levels in your annual petiole analysis as an indicator of declining soil pH.

Low soil pH is not a simple or quick situation to correct, especially in an established vineyard.

Soil pH is increased by the application of lime in the form of ground limestone (calcium carbonate) or dolomitic lime (calcium carbonate and magnesium carbonate). Lime should be spread evenly over the soil surface and incorporated (turned under), which is difficult, if not impossible, in an established vineyard. It is most effective to adjust pH prior to planting, when deep mixing of lime is possible.

The soil pH test indicates if lime is needed. The lime requirement (SMP) test determines how much lime should be applied to adjust the pH to the desired level. Accurate lime recommendations cannot be made without performing an SMP or similar lime test procedure. Refer to your soil test analysis for the SMP buffer value. This value is used with the SMP lime requirement table (Table 3) to determine the quantity of lime to apply to raise the soil pH to a target level. If quantities greater than one ton/acre are needed for an established vineyard where incorporation of the lime is not possible, apply the total lime requirement over several years. When planning lime applications, consider that your lime source is also providing calcium (Ca), and magnesium (Mg) if you use dolomitic lime. The amounts of available Ca, Mg, and K in the soil are interrelated; an extreme excess of any one of them can cause deficiencies of the others.

Table 3. SMP Lime Requirement
Tons/acre of 100-score lime needed
to raise pH of surface 6 inches of soil to a target pH.
SMP Buffer 5.3 5.6 6.0 6.4
6.7
6.6 1.1
6.5 1.0 1.7
6.4 1.1 2.2
6.3 1.5 2.7
6.2 1.0 2.0 3.2
6.1 1.4 2.4 3.7
6.0 1.0 1.7 2.9 4.2
5.9 1.4 2.1 3.3 4.7
5.8 1.7 2.5 3.7 5.3
5.7 2.0 2.8 4.2 5.8
5.6 2.3 3.2 4.6 6.3
5.5 2.6 3.6 5.1 6.8
5.4 2.9 3.9 5.5 7.3
5.3 3.2 4.3 6.0 7.8
5.2 3.6 4.7 6.4 8.3
5.1 3.9 5.0 6.9 8.9
5.0 4.2 5.4 7.3 9.4
4.9 4.5 5.8 7.7 9.9
4.8 4.8 6.2 8.2 10.4
This table adapted from Oregon State University Extension
Publication EC 1478, Soil Test Interpretation Guide.

Conclusions

A vineyard nutrition management program should be based on a thorough knowledge of the specific conditions and circumstances within the varied sites and blocks of your vineyard. Utilize soil tests and petiole analysis to monitor the nutrient status of the soil and grapevines. Keep records of vine growth, production, and fruit quality on a block-by-block basis. Apply fertilizer nutrients only when there is a demonstrated need; if there is doubt, conduct a small trial application and evaluate the vines’ response.
This paper was presented at the 1997 annual meeting of the Oregon Horticultural Society by former Extension faculty, Dr. Ed Hellman