What is in this article?:
- Potassium (K) is required by plants in large amounts. It has a major role in many plant processes, such as promoting root growth, increasing fruit size, and providing key features in metabolism that include the formation of starch, translocation of sugars, stomata regulation, and the formation of xylem vessels.
- The potassium concentration in grapevines can range from 1 percent to 4 percent on a dry weight basis, depending on the tissues and time of sampling.
- Generally, there is no hard or fast rule on potassium application, amount or timing, but too much just before or just after veraison can affect potassium uptake by fruit and pH.
Grapevines tend to show K deficiency when they are heavily cropped and maintenance applications of K have not been made in the vineyard. Deficiency can be more likely to occur under these conditions:
• Soil cut areas
• Areas where the K-rich surface soil was removed
during land leveling
• On sandy soils that have low native K fertility
• On clay soils of certain geologic origin
• Shallow soil areas
• Poorly drained soils
• Where soil pests have caused root problems
• Water stress can also increase this deficiency by reducing vine uptake of K — keep this in mind when using deficit irrigation on red grapes.
Deficiency symptoms can appear in early spring in cool, wet years, into June, but mild deficiencies will not be seen until just before harvest. The first symptom is a fading of green color at the leaf edges and between the main veins, while leaf margins tend to curl upward. The leaves may turn chlorotic and begin to turn brown on the margins, and some leaves may die as the deficiency becomes more severe. Severe K deficiency also reduces vine vigor and crop yield, and can result in defoliation.
Often petioles can remain attached as blades defoliate. Vines also tend to have fewer and smaller clusters that are tight, with unevenly colored, small berries.
Local soil composition and K fixation in soil
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.