Late Fall, A Time for Potassium Soil Applications

Maintaining adequate potassium (K) nutrition is especially critical for almond trees and fall is an excellent time to address K deficiency through soil potassium applications.   

Potassium is found in one of three forms in the soil:  fixed K, exchangeable K, and K in solution.  Fixed K is tightly held within soil particles or is part of potassium-bearing minerals and may only be very slowly released through weathering.  Exchangeable K is attached by electrostatic charges to soil particles and is in flux with potassium ions in the soil solution.  Soluble K consists of ions moving freely within the soil solution constituting a readily available form of K.  At any given time, a soil will contain a unique balance of fixed, exchangeable, and soluble potassium characteristic of that soil type. Potassium is thus in equilibrium and moves back and forth between these states as the supply of K⁺ and other cations varies.

Potassium ions (K⁺) have a one plus charge and are readily adsorbed by negatively charged soil clay particles becoming unavailable to the tree.  Avoid any type of application that broadcasts potassium over a large soil area because more of the K becomes fixed. UC research showed that four years of broadcast applications only moved K 6 inches down into the soil while banded treatments penetrated 2 feet.  Banded treatments have worked well under non-tillage but if you cultivate, shank the band in to get the material closer to the root zone.  Applying a gypsum (calcium sulfate) band overtop of previous potassium bands can help free up more potassium.  The calcium ions (Ca ⁺⁺) in gypsum have a plus two charge and will displace potassium ions on the clay particles thus freeing up more potassium to remain in the soil solution while moving it deeper into the root zone.  Gypsum banded at a rate of 1000 to 4000 pounds per acre in the same location as previous potassium bands will improve K availability.  

Massive doses of 2000 lbs potassium sulfate per acre applied in bands overwhelms the soils ability to fix all the K in the enriched zone and has corrected a deficiency for about 4 years.   Rather than waiting to apply an expensive massive dose, UC research later demonstrated that annual Fall “maintenance” applications of potassium sulfate at 500 lbs/acre banded annually in the same location 4-5 feet out from the tree trunk on both sides of the tree row would maintain K levels before a deficiency became apparent.   Injecting K through in-season drip irrigation is also an efficient potassium delivery system that is effective because the amount of K is very high in the wetted area thus penetrating well enough to be picked up by the tree.  

Soil applications of potassium sulfate (54%K₂0) or potassium chloride (63%K₂0) are most commonly applied in November after leaf drop begins.  Potassium chloride can cause chloride toxicity if chloride is taken up or remains in the root zone.  To avoid any chloride uptake and improve safety, apply potassium chloride later if active leaves are still on the tree.  Potassium chloride should not be used on weak trees, young trees, or in orchards with water tables, hardpan, stratified soils or any restriction which would prevent chloride from moving out of the root zone.  Chloride should be applied early enough to provide for adequate leaching (approximately 10 inches of rainfall).  If rainfall is insufficient then winter irrigation is recommended.  If in doubt, use potassium sulfate.