Having a major role in many plant processes, potassium promotes root growth, increases kernel/fruit size, and provides key metabolic features that include the formation of starch, translocation of sugars, stomata regulation, and the formation of xylem vessels. In general, plants deficient in potassium tend to have slow growth, with small, pale leaves. Trees that are severely deficient may have necrotic tips and margins. In many cases, the leaf tip curls upwards in a common symptom that is named the “Vikings Prow” (Figure 1).
Since Potassium plays a large role in tree health, it is important to maintain proper levels of the nutrient within the tree. A critical leaf value of 1.4% has been established by the University of California and current research has suggested that levels above this value do not increase yields. Recent field studies by Roger Duncan (UCCE Stanislaus) have demonstrated that leaf potassium levels in excess of the 1.4-1.6% range did not increase yield. Through the study, leaf levels between 1.4-1.6% gave the best yield results, with yield decreasing when potassium levels were below this level. Leaf potassium levels higher than this range did not increase yield, and may actually reduce yields if applied in excess.
Potassium usage by the almond crop is high. Upon harvesting the hulls and kernals, potassium is removed from the orchard. Studies by Dr. Patrick Brown (UC Davis), have shown that 76 pounds of potassium (92 lbs of K2O) are removed from the orchard for every 1000 pounds of kernals harvested. From nutrient analysis of the fruit parts, 70-80% of the potassium removed by the harvest is within the hull, while the rest is within the shell and kernel.
Even though a large amount of potassium is used by the almond crop, it doesn’t always mean that large applications of potassium are needed to maintain critical levels. Some soils may have naturally high levels of potassium, depending on the parent material, percent of parent material degradation, soil texture, and the irrigation system. A soil analysis should be made to determine the amount of potassium within the soil and coupled with leaf tissue analysis to see if the potassium within the soil is available to the trees.
Once the potassium level within the soil drops, it will take several years of large applications to bring the levels back to normal. Potassium fertilizer products including sulfate of potash, muriate of potash, potassium nitrate, potassium thiosulfate, and a few others. Organic applications of potassium can be made through manure composts, green manures, guano, and wood ash. It is important to note that some potassium fertilizers may have unwanted chemicals/traits – chloride, sodium, alkalinity, and food safety concerns – which may have a negative impact on the orchard when applied in excess. Applications should be made to maintain the leaf critical level of 1.4%.
With the recent increases in costs of potassium fertilizers, it is important to keep in mind the potential damage caused by potassium deficiency. Inadequate levels will decrease the number of fruiting spurs on the tree, reduce the flowering of the surviving spurs, and reduce shoot and spur growth. In other words, deficient trees will yield poorly. Keep in mind that by the time symptoms of potassium deficiency are visible, yield has already been lost. Therefore, annual leaf samples and proper applications of potassium are recommended to maintain tree health and yield.