I received a few questions on potassium applications in almonds. These include points on rate determination, application timing, and other considerations.
Q. How much potassium is needed?
Almonds remove about 72 lbs of elemental potassium (K) for every 1000 kernel pounds. This is equivalent to 92 lbs of potassium oxide (K2O), the unit which potassium fertilizers are measured. This means that for every 1000 pounds of kernels removed, 92 lbs of K2O must be available or applied to prevent future deficiency. This is equivalent to 184 lbs of potassium sulfate (50% K2O) or 368 lbs of potassium thiosulfate (25% K2O).
The amount of potassium applied should take into account leaf tissue levels as well as soil analysis. If the soil analysis reveals K values greater than 150 ppm, the trees are less likely to respond to K applications. In these cases, applications should match removal. If high amounts of K are available in the soil (>250 ppm), K rates could be reduced to help reduce costs (applications should match ~80-85% of demand) until soil analysis begins to show a decrease in K values. If lower than 150 ppm, rates should be increased to help build reserves. please note that some soils-such as sands- can not hold this much potassium.
Leaf tissue levels of K are variable. “Critical values” established at other times of the year have not been verified to production levels of trees through research. Therefore, mid-July leaf sampling is still recommended for determining K status of the tree. If trees have a leaf K percentage greater than 1.7%, minimal gains will be observed with increasing applications. Leaf tissue concentrations below 1.4% require increased applications.
Q. How should potassium be applied?
K fertilizers can be applied by granular applications or through the irrigation system. Ground applications are often made in dormant or early spring. Depending on soil type, the amount applied could vary from 50-100% of the potassium budget. It is assumed that ground applied materials take some time to move into the rootzone at a rate which is dependent on rain or irrigation. Examples include potassium sulfate or potassium chloride.
Fertigated materials can be applied in-season or in dormant. These products are thought to have a faster rate of tree uptake due to being suspended in the soil water. Often these products have a higher price point than ground applied fertilizers, offsetting the convenience. Examples include potassium thiosulfate, potassium nitrate, potassium chloride, potassium carbonate, or high grade potassium sulfate.
Q. Should the potassium application strategy differ by soil type?
Yes. Sandy soils (cation exchange capacity <12 meq/100g) should consider a reduced, later application in the dormant period. The remainder should be applied in-season, from April to August. An example of this type of program would be an application of 50-60% of the potassium budget using potassium sulfate in Late January or February, with the remainder applied in-season using potassium thiosulfate. In these soils potassium should be applied to the largest soil area possible.
Heavier soils (CEC>12 meq/100g) could apply a larger percentage of the potassium budget in the dormant period. This could mean that the whole budget is applied in the dormant. In heavier soils, the product should be banded to help increase movement of K into the rootzone. These soils might also require the application of gypsum or other calcium products to help saturate soil exchange sites and move K into the soil water. This is more common of a practice in heavy clay soils of the Sacramento valley.
Q. How is potassium pulled into the tree?
Unlike nitrogen, the uptake of K appears to be relatively linear. This means that the K should be present and available in the active rootzone through the growing season.
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