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.

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 are:

• 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. Do 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).