Soil Testing for Grapevines

     As is the case for many perennial woody plants, grape plant nutrition is not best assessed by soil tests.  As long as soil pH and nutrient levels fall within a broad range of acceptable levels grapevines should remain healthy and produce well. This, of course, assumes that all other cultural requirements have been properly addressed. A better way to judge whether the grapevine is taking up sufficient essential nutrients is to carefully observe foliage for nutrient deficiency symptoms and to periodically test the mineral content of petiole tissue (for information on this service contact the lab). 

     A slightly acidic soil pH level of between 5.5 and 6.5 should be maintained for grape plantings. Values slightly outside this range are not cause for concern. Extremely acid conditions (less than 4.5), however, can result in Mg and K deficiencies and alkaline conditions (greater than 7.0) often limit micronutrient availability. It is important to adjust soil pH before planting. The deep-rooted nature of grapevines makes post-planting liming or acidification much less effective.

    Soil tests can not reliably evaluate Nitrogen (N) for plants. Grapes are no exception. If, however, your nitrate (NO3-N) or ammonium (NH4-N) levels are high or vines appear vigorous and covered with dark green foliage , it is unlikely additional N is needed at present. If foliage is light colored and vines show only low to moderate vigor (this may be judged by the extent of pruning required in early spring) a moderate N application may be beneficial. This may be applied in early spring, before bud break. See your recommendations. Nitrogen needs may also be assessed by petiole tissue analysis. Petioles sampled at bloom time should contain between 1.8% and 2.2% N. Mid-August levels should be between 0.8% and 1.0%.

     Phosphorus (P) soil test levels have little bearing on grapevine vigor and production. The vines apparently are able to obtain the phosphorus they need even at very low test levels. Deficiency has only been documented at low P levels and extremely low soil pH levels (less than 4). Only under these conditions should P be considered a necessary part of one’s fertilization program.

     Potassium (K) deficiency is common on very sandy soils low in organic matter (ex. less than 3 %). Such soils have a very low potassium retention capacity and must be periodically replenished. Although soil test levels may reflect this, the grapevine itself (leaf symptoms) and petiole tissue levels are more precise indicators of inadequate K levels. Potassium deficient vines often have leaves whose edges become chlorotic (loss of green color) or even necrotic (dying) beginning in early-summer. This typically occurs first to leaves mid-way along a cane. Petioles sampled in mid-August should contain between 1.1% and 2.0 % K. If your soil test recommends that potassium be applied and your soil is an acid sandy one, take care to follow the recommendation and not overfertilize.

     Magnesium (Mg) deficiency, like potassium, occurs on acid sandy soils with low organic matter levels.  Excessive potassium fertilization on such soils can compound the problem. Liming with dolomitic limestone (before planting to a depth of 2 feet at the recommended rate) is the best solution. Post planting Mg corrections may be accomplished with dolomitic limestone soil applications or magnesium sulfate (Epsom Salts) foliar sprays. Usually after mid-season, magnesium deficient vines show leaves with chlorotic spots between veins. Those lowest on the cane are first to show symptoms and as the season progresses the condition extends upward  to younger leaves. Petioles sampled in mid-August should contain between 0.2% to 0.5% Mg. A soil test may indicate the possibility of Mg deficiency, but leaf symptoms and petiole analysis are better indicators.

     Calcium (Ca) is rarely a concern with grapes. Those soils requiring lime to supply magnesium will receive sufficient calcium with that application. Deficiencies of calcium have not been observed in Massachusetts vineyards.

     Micronutrient (Mn, Fe, B, Zn, Cu) soil test levels have not been correlated to grapevine vigor and yield. Your soil test results are printed for your information only. To better judge their sufficiency levels in your vines consider mid-August petiole analysis.

*******

     Grapevine nutrition is only one of the many factors to be considered in the creation of a healthy and productive vineyard. Primary soil requirements are good internal drainage, an open (porous) soil surface, sufficient organic matter levels, and deep rooting capability. Proper site characteristics include good air and water drainage (landscape position). Properly attend to all cultural aspects ranging from stock plant selection, through planting, proper pruning, the use of mulches and cover crops, insect and disease control, to cultivation. Only then will the efforts you make in providing proper vine nutrition through soil and petiole tissue tests be effective.