By Erica Lundquist, Ph.D.
Former LCWC Viticulturist

On June 29 and 30, 2004, the American Society for Enology and Viticulture sponsored a symposium on the Soil Environment and Vine Mineral Nutrition prior to their regular annual meeting. The following is the first part in a series summarizing the presentations at this symposium. The proceedings from the symposium will be published in the winter, so contact me then if you would like a copy of some or all of the papers presented.

Malcolm Sumner a leading researcher on soil acidity from the University of Georgia, Athens, and Daniel Roberts of Integrated Winegrowing in Sebastopol, California gave presentations on this topic.

Acid soils were defined as soils with a pH below 5.5. The main harmful consequence of this low pH is increased aluminum solubility (as Al3+), which becomes toxic to vines. Aluminum toxicity causes roots to be stunted with few fine roots. This restricted root growth leads to drought stress, reduced uptake of phosphorous, potassium, calcium and magnesium, and lower yields. Phosphorous availability is often very low in acid soils. Daniel Roberts gave guidelines for "very acid soils" as having pH below 5.0, 100-1000 mg/kg Al3+, and less than 5 mg/kg P.

While soil acidity is most frequently due to natural processes, there are management practices which contribute to soil acidification. These are the use of sulfur (for powdery mildew control), because it oxidizes to sulfuric acid, removal of base cations with harvested grapes, and to a lesser extent, use of ammonium based fertilizers.

To ameliorate the problems of acid soils, lime can be applied to surface layers. Lime is very insoluble and will not leach down to lower layers. Therefore it is only effective as deep as it can be thoroughly incorporated, often no more than one foot. To ameliorate soil at lower depths gypsum can be applied at the surface. It will dissolve into calcium and sulfate ions, which will leach down into the soil profile. The calcium will replace harmful aluminum on the cation exchange capacity. The sulfate (in contrast to sulfuric acid) does not acidify soil. Soil labs can give recommendations on the amount of lime and gypsum to apply to raise the soil pH. The calcium in calcium nitrate can also contribute to amelioration of subsoil pH. However, because nitrogen must be applied at low rates to avoid excessive vine vigor, calcium nitrate can only make a small contribution to raising soil pH.

Daniel Roberts gave specific management recommendations for vineyards on acid soils. He recommends liming the surface soil to a pH of 5.7 to 5.8, and using gypsum to ameliorate Al in the subsoil. Phosphorous (P) deficiency is frequently a problem, and he has found that ammonium based fertilizers such as diammonium phosphate, 12-26-26, are most effective at supplying P. Phosphorous is taken up best when there is good water availability, however to achieve high wine quality, it is often advisable to apply very little irrigation between set and veraison. Therefore at veraison low petiole P levels (0.05-0.1%) are common. Because of P deficiency, small vines yielding 2-3 lbs of fruit per vine are expected. To compensate for low yield Daniel Roberts recommends planting at close spacing, 3-4 feet by 4-7 feet. Slow growth of the vines is expected with full yield by the sixth year. Sufficient levels of P in petioles at bloom are 0.1-0.15%, and at veraison are 0.05-0.1%. At these veraison levels the vines frequently lose basal leaves. Different varieties vary in sensitivity to P deficiency. Syrah is most sensitive, showing deficiency at 0.2% P, and Cabernet Sauvignon is less sensitive, showing deficiency at 0.1% P. The rootstocks 110R, 1103P, St. George, and AXR are better at withstanding Al toxicity than are other rootstocks.

This presentation was by Alfred Cass of Cass & Associates in Santa Rosa. Because Alfred Cass could not be present, Malcolm Sumner gave his presentation.

Soil mineral nutrients are taken up only as dissolved forms by plants. The two main ways that they move to roots are by mass flow and by diffusion. In mass flow, the nutrients are carried along with water taken up by the plant. Only very soluble nutrients such as nitrogen, are taken up primarily by mass flow. Most other nutrients are taken up by diffusion. In this case, plants actively take in the particular nutrient. This creates a narrow sheath around the root with a low concentration of that nutrient. Nutrients diffuse from the higher concentration in the soil solution to the lower concentration around the root. Diffusion occurs over a very limited distance (millimeters or less), so fine root growth (or mycorrhizae- see presentation by Paul Schreiner in Part II) are important to maintain adequate nutrient supplies.

Soil cultivation leads to declining soil organic matter levels, which in turn reduces soil aggregation and a large proportion of large soil pores. Soil compaction is also caused by using heavy equipment in the vineyard when soil is moist. The main effects of loss of large soil pores and compaction are to lose the range of soil moistures at which roots can function effectively. At high soil water content, loss of porosity leads to reduced aeration, and roots are not active without adequate oxygen. When soils are dry root growth is limited by increased penetration resistance, or soil hardness. Because degradation of soil physical structure reduces root growth and activity, it will reduce uptake by the mechanisms discussed above.

Rob Stevens, Primary Industries and Resources, South Australia presented this topic. Because, it is generally not a problem in the California North Coast region, I will not summarize the talk here.

Paul Anamosa, Crop Care Associates, Inc. Yountville, CA
Common high magnesium (Mg) soils on the North Coast are Henneke, Maxwell, and Montara. (Like other North Coast Counties, Lake County has areas with these and other high Mg soils.)

High Mg soils tend to be poorly drained, and they have low available water holding capacity. Their structure is usually massive, meaning that the soil is almost solid with no aggregates and pores, or prismatic, meaning that soil structural units are in the shape of angular pillars. The latter structure occurs in high shrink-swell clays. Massive soils have poor drainage and aeration, and roots tend to be confined to cracks in the cracking clays.

The most common chemical imbalance in these soils is to have high Mg levels. In addition the soils are frequently low in calcium, potassium and phosphorous. Because of poor drainage, high salt or boron levels occasionally occur. Toxic levels of nickel are also found occasionally.

Paul rated soils by their percentage of Mg on the cation exchange capacity as follows:

Percentage of Mg on the Cation Exchange Capacity   0-20%       low   20-40%     adequate   40-60%     elevated   60-80%     high   80-100%   very high

At elevated levels, some degradation of soil structure occurs. High levels can be managed with intensive management.

General recommendations for high magnesium soils are as follows:

• Avoid soils with more than 80% Mg on the cation exchange capacity.
• Before planting adjust calcium to 60% on the cation exchange capacity to improve and maintain soil structure. Drainage is usually required to remove the Mg displaced by calcium.
• Except for nitrogen, meet all nutritional needs before deficiency occurs.

Paul Anamosa gave an example of pre-and post-plant management of a Maxwell clay. At this location 45 ton/ha gypsum were applied in the fall and the ground was cross-ripped to 1.2 m. In the spring 45 ton/ha gypsum, 900 kg/ha triple super phosphate, and 560 kg/ha potassium sulfate were applied. Because the gypsum replaces Mg on the cation exchange capacity with calcium, drainage was necessary to remove the displaced Mg. Post plant, 12-26-26, 2 oz per vine, was applied twice, and gypsum was applied at 13 tons per hectare for two years.

For rootstocks on high Mg soils, 4453 is not recommended as it does not take up much phosphorous and is generally not preferred by winemakers. Preferred rootstocks are 101-14 and 3309, which tolerate wet conditions and assimilate potassium.