How To Monitor Potassium Fertilization In Grapevines

Monitoring grapevine nutrition and undertaking appropriate fertilization is important in maintaining healthy vines and maturing grapes of a good quality. This is especially challenging with the diversity of soils and climates where grapevines are grown in the U.S. While grapevines require a number of different nutrients to maintain health and production, the nutrient needed in the largest quantity is potassium (potash).

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Vines’ Need For Potassium

The potassium (K) needed by grapevines is typically acquired from the soil reserves of K. As long as the soil reserve is adequate we are in good shape. However, when soil reserves are low or when the crop load is especially heavy, K deficiencies can occur. Potassium deficiencies usually occur in sandier soil areas or in soils where the vine’s ability to acquire K is limited by soil compaction, very dry soils, poor water drainage, or soil pests (nematode and phylloxera).

A grapevine’s ability to acquire K from the soil is influenced by the grape variety (for own rooted vines) and by the rootstock (for grafted vines). Some rootstocks do a better job of acquiring K from the soil than do own-rooted vines (‘Freedom’, ‘St. George’, ‘99R’, ‘1616C’,‘039-16’) while other rootstocks have less of an ability to acquire K (‘420A, ‘110R’, ‘140Ru’, ‘1103P’).

When grapes are harvested, potassium is removed from the field with the crop; therefore, the soil reserve of K is continually depleted. This is not a problem until the soil reserves become too low. So, how much K is removed when a grape crop is harvested?

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While this varies by variety, typically about 5.4 pounds of K are removed per ton of green grapes (or about 20 pounds of K per ton of raisins). This means that for a 10-ton per acre green crop (or 2.5-ton per acre raisin crop), 54 pounds of K per acre are removed.

Identifying Fertilization Needs

There are three approaches that growers can use to identify K fertilization needs. First is visual monitoring of the vines for deficiency symptoms, second is annual tissue (petiole) analysis, and third, soil analysis.

Equipped with some basic knowledge and photos of typical nutrient deficiency symptoms, most growers can make a preliminary diagnosis in the field. Vine tissue (petiole) analysis can be used to detect deficiency situations before they occur and should be done every season at bloom time and/or at veraison (berry softening). Soil analysis is not a very reliable means of determining fertilization needs, but should be used to determine if the K deficiency is due to low soil reserves or to other factors (i.e. soil compaction, poor water drainage, soil pests, etc.).

If a K deficiency is confirmed, then fertilization should be started. However, most research has shown that vine health and crop yield are impacted only when K is truly deficient. This is not to say that marginal levels of K identified in vine petiole analysis should be ignored, but rather that the vineyard has a propensity for low K and should be closely monitored.

Fertilization

Potassium fertilization should be limited to vineyard areas with confirmed K deficiencies. These vineyard areas should be mapped for later (usually winter) treatment. The mapping can be done with row/vine counts or using a GPS instrument.

Soil Applications

High rates and concentrated placement of K fertilizers are needed. For the quickest response, apply the fertilizer in a single heavy application rather than in small amounts. In furrow irrigated vineyards, a dry form of K fertilizer is banded in the soil about 24 inches from the vines at 8 to 10 feet deep. In drip irrigated vines, a small amount of fertilizer can be placed on the soil surface under drip emitters (a shoveled out pocket in the soil helps ensure that the fertilizer stays in place). Using this approach allows the fertilizer to be in the soil in an area of high root concentration and frequent water application.

Dry forms of fertilizers include potassium sulfate (sulfate of potash) [43% K] and potassium chloride (muriate of potash) [51% K]. While potassium chloride is sometimes less expensive, it does require additional water applications to leach the chlorides so the vines will not be adversely affected.

Irrigation System Applications

Injecting potassium through a drip system (fertigation) is appropriate when a very large portion of a vineyard is in need of K fertilization. Liquid forms of potassium sulfate, potassium chloride, or potassium thiosulfate can be used. Remember that potassium chloride will need leach applications.

Avoid the use of dry, mixed (complete, nitrogen, phosphorus, and K) fertilizer products. They are expensive and inefficient sources of K.

If you need nitrogen, use an N fertilizer material; if you need potassium, use a K fertilizer material — it’s less expensive.

It is important to identify the vineyard area(s) in need of fertilization and to confirm the need for fertilization. With the increased cost of K fertilizers in recent years, one can spend $800 to $1,200 per acre correcting this deficiency.

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