By Erica Lundquist, Ph.D.
Former LCWC Viticulturist

Phosphorous (P) deficiency in grapevines was first recognized in the early 1980s following expansion of vineyards to shallower, more acid hillside soils . Many Lake County vineyards are on these kinds of soils, and some have the added challenge of being on volcanic soils. The following review gives background on the forms of P in soil and under what soil conditions it is likely to be unavailable. It describes how to recognize and correct P deficiency in grapevines, and it describes the role of mycorrhizal fungi in P nutrition.

Phosphorous stands out among the plant macronutrients (nutrients required by plants in large amounts) as being very tightly held by soil solids. That means that the P dissolved in the soil solution, the form that plants use, is usually at very low concentrations. Under natural conditions, almost all plants have a beneficial fungus, mycorrhizae, associated with their roots, which helps them to acquire P.

P fixation (the process by which P is tied up in unavailable forms) is especially severe in very alkaline or acid soils, and in soils with high amounts of iron and aluminum oxides. Under alkaline soil conditions, P forms highly insoluble compounds with calcium. In acid soils P forms very insoluble compounds with iron and aluminum, and it binds strongly to the surface of iron and aluminum oxides. Volcanic soils often contain large amounts of allophane, a term for amorphous aluminosilicate gels. These amorphous aluminosilicate gels are tiny mineral particles with the same size as clay particles, but without being organized in distinct layers as clays are. Allophane contains iron and aluminum oxides and frequently has a high reactive surface area that binds P regardless of soil pH.

Soil P is most available in soils with a pH range of 6.0-6.5, and in coarse textured (sandy) soils.

Soil organic matter is another important solid form of soil P and can make up 25-60% of a soil's total phosphorous. It becomes available to vines through the process of decomposition.

In the soil solution P is present as phosphate in two forms; H2PO4- predominates at pH below 7.2, and H2PO42- above pH 7.2. These are the forms that plants take up through their roots.

Phosphorous' Role in Plants Phosphorous is part of important biological molecules such as DNA and RNA, the molecules that contain and transfer genetic information, ATP, an important molecule in energy transfer within cells, and phospholipids, molecules that form a major part of cell membranes. It is not surprising that plants deficient in P are stunted and less fruitful than plants with adequate P.

The most effective way to determine whether vines are P deficient is to take petiole samples at bloom. Adequate P levels at this time are 0.15%, questionable levels are between 0.1 and 0.15%, and deficient levels are below 0.1%. Guidelines for petiole sampling can be found in the note 'Assessing Vine Nutrient Needs' in the Vineyard Notes Archives of this website.

Visible signs of P deficiency may not occur until vines are severely P deficient. When they do occur they are fairly distinct . P deficiency will slow shoot growth and reduce internode lengths. Shot berries and poor fruit set can occur, and the area with poor fruit set may occur in the mid-portion of clusters. Leaf symptoms of P deficiency are distinctive with scattered red dots on basal leaves later coalescing to form red bars at right angles to secondary veins, and finally to forming red leaves with only the veins remaining green.

Because P is rapidly fixed by soil, it is best applied in a concentrated band or in the drip zone. Liquid forms of P can be applied through the drip system, and dry soluble forms can be placed under drip emitters. When placed under drip emitters it is preferable to build small basins so that the water and fertilizer will remain in a small concentrated area rather than running over the soil surface. Phosphorous can also be applied in a narrow band 1-2 feet from the vine row. Phosphorous should be applied in the spring so that it is available to support new vegetative and fruit growth. (If it is applied as a surface band, this must be done in time for rain or frost protection water to move it into the soil.) If vine growth is weak, making sure that the vine has adequate nitrogen can enhance root growth and P uptake. For P applied through the drip system or under drip emitters, try a fertilizer that gives the equivalent of 50 lbs P2O5 per acre, and for banded P start with 100 lbs P2O5 per acre.

Organic fertilizers such as manure or compost can supply the soil with P. These materials vary substantially in their nutrient content and in the rate at which they decompose to release nutrients. They have the advantage of supplying a complete range of nutrients to vines and of improving soil tilth. However it is more difficult to predict P availability from organic fertilizers, and it may be difficult to apply adequate amounts to severely P deficient vines.

The importance of mycorrhizal fungi to grapevine growth is most clearly seen when they are absent. Vineyards planted following soil fumigation sometimes exhibit weak growth, and subsequent investigations have shown that mycorrhizae are at very low levels in these vineyards.

Mycorrhizal fungi grow on and into plant roots. The fungus and plant have a symbiotic relationship in which the fungus receives carbohydrates, and the plant receives nutrients such as P, zinc and copper. Of these the most consistently demonstrated benefit of mycorrhizae to plant growth has been improved P nutrition. The group of mycorrhizae associated with grapevine roots are called arbuscular mycorrhizae, or AM.

Mycorrhizae are found on almost all plants in nature, and they are abundant in soil and as windborne spores. In most cases, it is not necessary to add mycorrhizae to grapevine roots, as they are already present. Only in situations where very few mycorrhizae are present at the time of grapevine planting, such as following soil fumigation, or on terraced soils where topsoil has been removed, is inoculating with AM likely to be beneficial.

Once AM are present in a vineyard, they do not need to be reestablished. It may be possible to increase AM colonization of grapevine roots by planting cover crops or leaving resident vegetation growing in proximity to vine roots. One group of plants that will not encourage AM colonization are the Brassicas, including mustards and radishes, which do not form mycorrhizal associations.

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1. 'Phosphorous Deficiency Identified in California Vineyards', Pat Summers, California Grape Grower, May 1983.
2. 'Phosphorous Deficiency in California Vineyards', James A. Cook, William R. Ward, and Alan S. Wicks, California Agriculture, May-June 1983.