Soil composition plays a large role in which areas of a vineyard are K deficient. K ions are strongly adsorbed on clay; without this adsorbing ability, the soluble K in sandy soils is easily leached from surface soil. Therefore, sandy soils or sand streaks often have less plant available K.

Soils high in clay or silt content may also need added K because of their K fixing capacity. In K fixation, clay minerals remove K from solution by trapping it on sorption sites within the mineral layers. Many soils in the San Joaquin Valley have high K fixing capacity and can tie up 50 percent or more of added K fertilizer. This K is not lost, but rather stored between layers of clay and slowly released in soil solution as exchangeable K.

However, most will not become available fast enough during times of high K demand, especially following veraison. The actual K available for plant uptake represents a very small fraction of the total K in soils – it is found in the soil solution and on the cation exchange sites of both clay particles and humus. This is why soil K levels have generally not been reliable criteria for indicating the actual K status of grapevines.

The major clay minerals responsible for K fixation are illite, weathered mica, smectite, and vermiculite. Soils high in vermiculite are found on the east side of the Central Valley of California including in the Lodi wine grape district, especially on landscapes with soils deriving from granitic parent material and that are weakly to moderately weathered.

Vermiculite is a clay mineral, but actually it can be found in the silt and fine sand size fractions, which explains why coarse-textured soils are often found to fix K.

In recent years, graduate students and staff in the UC Davis laboratories of Drs. Randy Southard, Toby O’Geen, and Stu Pettygrove have examined the K fixing capacities of soils in Sacramento and San Joaquin counties and have developed a map of five general regions with similar “soilscape” characteristics. These regions help predict the likelihood of soil's K fixing capacity. Soils with a high K fixation potential may need greater K applications to reverse any deficiencies. We are currently comparing fertilizer applications in vineyards having high and low soil K fixation in both Sacramento and San Joaquin counties.

Potassium fertilization

Response to a K fertilizer strategy may be influenced by several factors, including soil type and depth, variety, rootstock, cropping pattern, time of year and irrigation system or general growing conditions, especially if in a drought or a heavy rainfall year.

The particular form of K fertilizer chosen offers no inherent advantage as grapevines don’t care how their needed K becomes available, only that it is available. However, particular formulations of K do offer advantages to growers, such as cost and convenience of application vs. possible soil effects (e.g., acidification of soil, salinity, etc.). Foliar sprays for K are not without benefit, but at best they reduce foliar symptoms, at great cost.

K can be applied in the fall as long as field access is possible. Early spring applications in bulk or through drip systems can be as effective, depending on formulation and timing. For a comparison of some general formulations of K fertilizer see the accompanying table.

Generally, K use by grape vines can be as much or more than annual N demand, but the need for K applications is often not as critical as N. This is because K doesn’t leach from the soil profile as readily as N and certain soil types provide enough K for fairly long periods under moderate crop demands.

Because of the strong fixing capacity of some soils and the relatively slower movement of K, a single heavy application (a “slug”) of fertilizer is needed to quickly mitigate K deficiency and show a vine response.

The method of application and formulation of K will be determined by how fast a response is needed and how long it has been since any K was applied, or whether a more maintenance type strategy is being used.

Talk with your PCA or fertilizer company about what you want to accomplish and how fast, and try to keep your winery in “the loop”. Some important considerations:

• What is the soil type?

• Are there some soil concerns, such as pH, salinity, or drainage?

• How long has it been since K was applied, if ever?

• Are symptoms present in just the leaves, in the fruit, or in certain sections of the vineyard?

• What have the yields been in recent years?

• What is the comparative cost per unit of K2O?

• How can the formulation of choice be applied, and at what cost?

• When do you want to apply the fertilizer?

Whether you choose a dry formulation in the fall, or dry in early spring, or liquid in late spring/summer, or a combination depends more on your operation and schedule than on critical periods of the vines’ ability to use K. Talk with your winery about your strategy as some wineries have concerns about amount or timings of K.

Generally, there is no hard or fast rule on K application, amount or timing, but too much K just before or just after veraison can affect K uptake by fruit and pH. However, the interaction of available nutrients, soil type, crop load, irrigation management, variety and rootstock make it complicated to predict. All the more reason to stay in communication with your winery or grape buyer.

For more information on K composition of local soils, see “Soil-landscape model helps predict potassium supply in vineyards”, Calif. Agriculture, vol. 62, no. 4, p. 195-201 (Oct.-Dec. 2008).