By Erica Lundquist, Ph.D.
Former LCWC Viticulturist

Many Lake County soils contain high levels of magnesium (Mg) which can cause potassium (K) deficiency and reduced vigor and yield in grapevines. High Mg soils are derived from rocks that once made up the ocean crust. These rocks have been metamorphosed to produce serpentine rock during the collision of continental and ocean plates. Few Lake County vineyards are planted on true serpentine soils (which are shallow soils containing serpentine minerals over serpentine rock), but many are on material at least partly derived from serpentine parent materials and therefore contain high Mg. Some serpentine soils also contain high and potentially toxic levels of nickel, chromium and cobalt. In Lake County high Mg soils are found in some of the valley soils as well in some alluvial soils in elevated landscape positions. (Alluvial soils are derived from material transported by water.) I will discuss both serpentine and high Mg soils together in this note.

The easiest way to identify a high Mg soil is to look at the ratio of calcium (Ca) to Mg on the cation exchange capacity. (See box on terminology below.) Soil analysis reports frequently list the amount of Ca and Mg both as ppm (parts per million) and as the percent saturation on the cation exchange. Use the latter figures to calculate the Ca:Mg ratio. For example if the percent saturation of Ca is 50% and that of Mg is 25%, the Ca:Mg ratio would be 2:1.

Soils begin to show high Mg problems at an exchangeable Ca:Mg ratio of 1:1 or lower, that is when they have less Ca than Mg. Soils not influenced by serpentine minerals usually have substantially more exchangeable Ca than Mg, however it is not necessary, nor is it economically feasible, to adjust the entire soil Ca:Mg level to 2:1 or above.

Terminology Ions are atoms or molecules that have a charge when dissolved in water. Cations are positively charged ions, e.g. Ca2+, K+, NH4+. (Anions, e.g. NO3-, are negatively charged.) Soil particles are negatively charged. This charge attracts and holds cations. A soil’s cation exchange capacity, C.E.C., is the total amount of cations that it can hold exchangeably. The cations are exchangeable in the sense that they exchange with cations dissolved in soil water. Most of the negative charges that make up the C.E.C. are on clay particles and organic matter.

Problems associated with high Mg soils may include the following; reduced vine growth and crop load, K deficiency, poor soil structure and slow water infiltration, and cracking clays. Potassium deficiency in grapevines often becomes obvious after veraison when demand for potassium in fruit clusters is high and is manifested as leaf burn, defoliation, reduced ability to tolerate water stress, and slow sugar accumulation. Even though some high Mg soils have sufficient K levels, it appears that excess Mg interferes with K uptake.

Ca or K to alleviate high Mg problems Both Ca and K additions may help to alleviate problems from high Mg, but soil and grapevine petiole tests will help you to decide which one to focus on. Look at soil analyses that you already have, or take a sample and send it to a lab for analysis. (See Soil Sampling Box below). Take petiole samples at bloom to determine whether vines are able to take up sufficient potassium. If K levels are borderline at bloom, test again at veraison. (Details on petiole sampling can be found in ‘Assessing Vine Nutrient Needs’ in the Vineyard Notes Archives on the LCWC website.) Roland Meyer, U.C. Cooperative Extension Soils Specialist, gave some rules of thumb for assessing Ca vs. K needs. In general, a soil with K levels lower than 100 ppm is low in K, and if petiole levels indicate K deficiency, K fertilization is needed. If this is combined with a Ca:Mg ratio that is somewhat out of line, e.g. 1:2 or higher on the Mg side, then Ca addition is likely to help. Once the K levels are much above 150 ppm, K fertilization is not likely to help, and the focus should be on Ca addition. In the latter case, petiole tests and vine symptoms may still indicate K deficiency, but it is likely due to an exchangeable Ca:Mg ratio that is strongly weighted toward Mg, e.g. 1:2 or more.

Soil Sampling It is important to sample soil consistently to make year to year comparisons and evaluate the effectiveness of treatments. It is best to use a soil probe so that you can take many cores scattered throughout the vineyard. Organic matter and nutrients are generally concentrated in the surface ‘topsoil’ layer, so sampling the top 12” will give a good idea of the soil’s fertility. Sample 15 to 30 cores for a 5 to 20 acre vineyard block that has a uniform yield history. If the block has areas with different vine productivity, sample these areas separately. Mix the cores thoroughly and take a subsample of the amount of soil the lab requires. It is probably best to sample in the vine row where roots are concentrated. If you are trying to determine the effect of materials applied through the drip system, obviously your samples should be under the drippers. (You may want to sample under the drippers and away from the drippers for comparison.) Take good notes on when, where and how you sampled because you may want to repeat the sample to determine the effects of fertilizer application.

Heavy soils with high C.E.C. require greater amounts of K and Ca fertilizers than sandy soils with low C.E.C. In addition some types of clay will fix K (tie it up in an unavailable form). Corrective applications of K for a heavy soil should be on the order of 1/4 – 1/2 lb K2O/ vine. Check petiole K levels a year after the corrective application to see if the application was sufficient. Maintenance levels of K fertilizer may be on the order of 50-100 lbs/ acre. Potassium fertilizers are soluble, and in general can be chosen based on their price. High applications of KCl, 0-0-60, or muriate of potash, may lead to high concentrations of chloride which can be toxic to vines. Other forms of K fertilizer such as potassium sulfate (0-0-50) are more expensive, but do not contain chloride.

Very high rates of Ca are needed to alter the Ca:Mg ratio, and somewhat greater amounts are needed with a higher C.E.C. To give an example, in one experiment, a soil with a high C.E.C., 40 meq/100g, 115 tons of gypsum per acre were needed to change the Ca:Mg ratio from 1:4.2 to 1:0.5 to a six inch depth1. While applying this amount of gypsum may not be economically feasible, a beneficial effect can be achieved by applying a high rate of gypsum to a limited volume of soil where vine roots are active. In addition less gypsum is required with a smaller C.E.C. and when the Ca:Mg ratio is more balanced (ratio of 1:1 or less Mg). Lime should be used only to correct the acidity of the soil when the pH is less than 6.5. This will usually require only 1-4 tons per acre 6” of soil depth. Lime should not be used to correct the Ca:Mg ratio because it is very insoluble, especially at the usually high pHs (pH 7 and greater) of high Mg soils. Gypsum is much more effective than lime for correcting the Ca:Mg ratio of high Mg soils because it can be applied at very high rates in a limited area without changing the soil pH. Other, more soluble forms of Ca are too expensive to use at high rates.

Research has shown foliar application of K to be ineffective at decreasing grapevine deficiency symptoms or increasing tissue K levels2. When applying the K to soil, the objective is to increase K availability where roots are active. Most of the applied K will be held on the soil’s C.E.C., and relatively little will be in solution and leach through the soil. Broadcasting K is ineffective because the fertilizer is often placed where there is little contact with active roots. Roland Meyer recommends placing the K fertilizer in a basin under the drip emitter, or trenching it in where roots are active. (See discussion on trenching next paragraph.) By placing the fertilizer in a basin, most of the applied irrigation water flows over the fertilizer and helps to dissolve and leach it down to the vine roots. (Rain will also move the potassium down, but an even greater amount of water moves through the profile in a small area under the drip emitters.) Potassium applied through the drip system may also be effective, however with the generally low irrigation requirement in this area, it may be difficult to apply all of the K required. Longer irrigation sets to apply more K may lead to K spreading over the soil surface where roots do not grow. Another drawback is that materials that can be applied through the drip system are more expensive than dry forms.

The imbalance of Ca and Mg in the soil appears to influence root function and nutrient uptake, and foliar applications of Ca clearly will not improve the root environment. Because very large amounts of Ca are needed to affect the Ca:Mg ratio, it is better to concentrate the gypsum in a limited zone of soil where roots are active. Therefore, incorporating it into the soil to a depth of two feet or so will be beneficial. This can be done by excavating under the dripper (with a shovel or by angling a trencher under the emitter), or it can be done by trenching near the vine row. Again there is a more concentrated flow of water in the drip zone to move the Ca down, and roots will be most active in this area. If trenching is too expensive, creating a basin for gypsum placement under the dripper may be the next best alternative.

Both Ca and K can be applied almost any time of the year because they are held on the soil C.E.C. and will not be rapidly leached through the soil profile. Fall application is often convenient and allows the nutrients to dissolve and move with winter rain and to be available for spring growth.

When soils are dry, K uptake is greatly reduced. In vineyards that are deficit irrigated (vines receive less than their full water demand) the soil will continue to dry throughout the growing season. Therefore deficit irrigation or severe cut back on irrigation prior to harvest may contribute to late season K deficiency.

If soils are nitrogen or phosphorous deficient, addition of these nutrients may improve the effect of K and Ca addition.

1. Meyer, Roland D. 1997 “What are serpentine soils and why are they difficult to farm?” from Pope Valley Viticulture Day
2. Christensen, Peter, 1989. “Foliar Fertilization”, Vine Lines from Fresno County.

Acknowledgements My thanks to Roland Meyer for his assistance with this note.