In the early 1980s, UC Davis professor James A. Cook and co-workers first observed phosphorus (P) deficiency in California North Coast hillside vineyards and the foothills of the Sierra Nevada.⁶ Affected vines were discovered with stunted shoot growth, poor fruit set, and characteristic leaf symptoms. Research and field experience have since increased our understanding of and ability to manage low levels of phosphorus in vineyards.

Phosphorus deficiency in grapevines inhibits the initiation and maintenance of fruit clusters and flowers within developing buds.¹⁷ As a result, fruit yield from P-deficient grapevines is reduced.^15,17 Vine growth, whether measured as leaf area, shoot weight, or weight of dormant season prunings, is also reduced by phosphorus deficiency.^7,15

Late in the growing season, leaves on the lower part of shoots of severely P-deficient vines develop characteristic symptoms. These include yellow patches between the veins on white fruited varieties and red patches between the veins on red fruited varieties.⁶ Initially the patches are most numerous near the edges of the leaves, but increase in number inward with time. All these effects of phosphorus deficiency are due to disrupted biological processes, including the synthesis of molecules, energy transfer, and photosynthesis.¹⁰

Many California North Coast and Sierra foothill soils are more highly weathered than in drier portions of California. Weathering occurs when rainwater moves through the soil and removes the components of soluble minerals, including many plant nutrients. In these higher rainfall areas, greater weathering, in combination with the additions of greater quantities of plant matter to the soil, results in soil acidification (pH less than 6.0).

Acid soils are favorable chemical environments for reactions that fix phosphorus, making it unavailable to plants. Grapevine phosphorus deficiency occurs under these conditions.^1,9,14

Many acidic soils have phosphorus concentrations lower than 8 ppm by the Bray 1 test. Under these conditions phosphorus deficiency in grapevines is likely to develop. Other methods, such as the Olsen bicarbonate test, are used to evaluate the phosphorus concentration of soils, but the Bray 1 test is the most reliable for acidic soils.

Soil series that include soils of low pH and low phosphorus are the Aiken, Manzanita, Musick, Pentz, Redding, and Sobronte. Grape growers developing vineyards in acidic, low phosphorus soils will likely need to apply fertilizers containing phosphorus.

Fumigation of a low phosphorus soil can increase the difficulty of acquiring phosphorus by grapevines. Most soil fumigants are biocides that indiscrimately kill the living organisms they contact, including mycorrhizae.¹¹ Mycorrhizae are a group of naturally occurring fungi that infect roots, increasing the nutrient-absorbing capabilities of the host plant.¹⁸ Inoculation of young grapevines in fumigated soil with selected mycorrhizae may result in infection and increased vine growth, but introduced fungi are generally not as effective as native fungi and recovery from fumigation is usually not complete.¹⁶