A common question received from growers after they see their leaf sampling results is “How come my potassium levels dropped significantly from last year?” The short answer is that it was removed with last year’s harvest, but there are many complicating factors that should be taken in consideration.
Potassium Removal from the Orchard System. Studies by UC Davis have shown that 76 pounds of potassium are removed from the orchard for every 1000 pounds of kernels 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.
Potassium loss from the orchard can also occur through leaching. Leaching of potassium is reduced in soils with high exchange capacities, which includes loams, clays, and silts. Sands and loamy sands have a relatively low exchange capacity, lower amounts will bind to the soil particles. Furthermore, this bond is not as strong within acidic soils which can lead to leaching in areas that are over irrigated or received excessive rainfall.
Since potassium and sodium have the same charge strength, strategies used to move sodium out of the rooting zone will also move potassium as well. These include applications of gypsum or other strongly charged cations to “flush” the system. Excessive applications of water applied as a leaching coefficient may also leach potassium.
Proper Leaf Levels of Potassium. 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 excessively 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 1.6-1.8% range did not increase yield. Through the study, leaf levels between 1.5-1.7% 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. Keep almonds within the 1.6-1.8% range.
Sources of potassium. Potassium fertilizer products including potassium sulfate (K2SO4), potassium chloride(KCl), potassium nitrate (KNO3), potassium carbonate (K2CO3), potassium thiosulfate, and a few others can be used. 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. The best bargain is KCl, followed by K2SO4, the water soluble products which include potassium thiosulfate, KNO3, and K2CO3, and then the foliar sprays. Organic sources vary in price and potassium concentration, and thus are hard to compare to mineralized sources.
Strategies of potassium applications. In soils with lower exchange capacities, applications should match current strategies of nitrogen fertilization – multiple smaller applications made throughout the year. Potassium chloride, potassium sulfate, and compost/organic forms should be applied in the dormant period. In season applications of potassium thiosulfate, potassium nitrate, and potassium carbonate can be fertigated in-season. Growers on heavier soils can apply large “slugs” of potassium chloride or potassium sulfate in the dormant period and rely on in-season products to fine tune their fertilization program. Keep in mind the added benefits and risks with in-season applications. Potassium nitrate is about 13% nitrogen, and can be applied as a foliar spray or fertigated. Potassium carbonate can be used to help buffer acidic soils. Potassium thiosulfate acidifies the soil, but commonly causes phyo-toxicity when applied incorrectly.