Almond Potassium Fertilization: Where did My Potassium Go?

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.

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9 thoughts on “Almond Potassium Fertilization: Where did My Potassium Go?

  1. Thanks a lot for this very good information! Just one question: How about the potassium in my well water: I had an Analysis done and it shows 12ppm of Potassium. I would assume that this is highly available potassium for the trees and should be included in the calculation!? How many pounds of potassium do I get if I irrigate e.g. 40 inches a year with my well?

  2. Ralf – Thanks for the comments. Calculating pounds of potassium applied form irrigation water can be done as follows: (PPM K) x (0.23) x inches of water applied. So, for example, if you have 10 ppm of potassium and applied 12 inches of water, an application of 27.6 pounds of potassium was made to the field.

    FYI – This is identical to the equation used to calculate ppm of nitrate-nitrogen applied form ground water.

  3. The general amount of nitrogen fertilizer used for trees seems to be 1 cup per 1″ of tree trunk. Would this same ratio apply for the use of wood ash to be used as potassium fertilzer in orchards?

    I would assume that, since wood ash is organic and slower-releasing than some of the conventional/chemical sources of potassium, that it would best be applied during the dormant season. Is this assumption correct?

  4. @Michael – This all depends on the analysis. Since wood ash can vary in the amount of potassium, it is very important to have the material analyzed for nutrient content. Most likely it will be high in potassium, but it also may contain highly concentrated metals that could negatively effect tree growth.

    If all is clear, and it is applied, my guess is that the potassium will move into the soil stores of potassium readily. Dr. Tim Hartz at UC Davis has shown that potassium leaches from compost piles much more rapidly than previously thought…so my guess is that it will move out of the ash at a rate faster than described above. I think you could still get away applying it in the dormant season, but assuming that it will be there two to three years later may not be wise.

  5. Pingback: Almonds and Potassium | Agro-Culture Liquid Fertilizers

  6. I use sulfate of potash on 60+acres irrigated with a hose pull sprinkler system. I use KTS on 38+ acres irrigated on a dripline. Since K2SO4 is 50% potassium and KTS is 25% what does it mean when I read 1000 lbs of almonds removes 96 lbs of potassium. Is the reference to actual potassium or the compound?

    1. Sam, thanks for the comment. K removal is for every 1000 lbs is 76 lbs of elemental K, or 92 lbs of K2O (potassium oxide). So, the 96 lbs is related to the potassium oxide compound.

      Elemental K is converted to K2O by multiple lbs of K by 1.2. Fertilizers, including K2SO4 and potassium thiosulfate, are reported in K2O values. So, 500 lbs of potassium sulfate is equal to 250 lbs of K2O (or 208.3 lbs of elemental K).

      K2SO4 and potassium thiosulfate do not contain 50% and 25% of elemental K, but rather 50% and 25% of K2O, respectively.

      I hope that helps,

  7. If I apply 20# 36% zinc sulfate post harvest, would adding 1# solubor/100gal. be a waste? Do the buds, or perhaps the wood absorb B?

    1. Hey Bob,
      Adding boron to that spray would not be a waste. As you state, the buds and wood will absorb the B – even if the leaves are “burned” off of the tree. This spray could also be made all the way up to pink bud.

      I would “up” the rate to between 2-4 lbs of solubor unless you plan to apply more in your dormant spray.

      Hope that helps,

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