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. 
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7 thoughts on “Late Fall, A Time for Potassium Soil Applications

  1. Is this “banded” application a “banded rate” or just putting the whole 500 lbs/ac next to the tree? Do you reduce the rate like you would for a banded herbicide application?

  2. The application suggested above should be made as a 500 pound application of potassium sulfate banded 5 feet from the trunk for every acre. So, a 10 acre orchard would apply 5000 pounds of potassium sulfate. There is no rate reduction due to applying a band.

    Keep in mind that “slugging-on” potassium sulfate is not the best form of application in classified sand, loamy sand, or other low exchange capacity soils. In these soils, smaller, more frequent applications should be made to maintain use efficiency.

  3. Yes, Potassium nitrate is a replacement for potassium sulfate. Potassium nitrate is more expensive, but is water soluble and can be ran through the drip. Be cautious about using it too close to leaf drop/dormancy as the nitrate leaches easily in most soils.

    This product can be used in-season (April-May) to apply nitrogen and potassium to meet the trees needs.

  4. I have applied 500 lbs. of sulfate of potash per acre last year after harvest. July leaf samples for the last 2 years have shown K at lower level than the recommended lowest level. After the application of K it rose only slightly, but still below the lowest level recommended. I’m considering 1,000 lbs per acre this year to try to catch up. I’m sure multiple applications would be better but difficult with smaller acreage such as mine. What’s your opinion on the subject?

    1. Ernie,
      Where are you located?

      If deficient (mid July less than 1.5%) potassium applications greater than removal are needed. Instead of one large “slug”, I would consider split applying some of your potassium. If the orchard is on a sandy soil (Cation exchange capacity <15 meq/l), maybe 50% in dormant, 50% in-season applying some potassium in-season. On heavier soils, consider a 75% dormant, 25% in-season split. In season-applications could be applied as water ran potassium sulfate, potassium nitrate, or potassium thiosulfate.

      In contrast to the above statement - if you are on clay or clay loam soils, the potassium may be binding to soil particles and not available for plant uptake. In these cases, it may be best to apply the potassium as a large application in the fall and apply gypsum over-the-top to help make it available for plant uptake. This kind of strategy is tricky - and I would encourage you to contact your local advisor for help and more specific site suggestions.

      Hope that helps,

    2. Ernie – here is a response from Joe Connell:

      Hi David,
      I hope all is going well for you. In our medium and heavy textured soils with shrink-swell clays the water run approach for potassium applications is only recommended with drip irrigation and very limited coverage micro-sprinklers where the K is sufficiently concentrated with the water. We have virtually no sandy soils as you have in Merced Co.

      Most of the Butte growers have solid set sprinklers for frost protection and irrigation. Spreading the K out over the entire orchard floor with water run full coverage irrigation is ineffective in moving it into the soil for our loam and clay loam soils. K. Uriu calculated that K would only move in less than 1 inch if spread out through solid set irrigation at 500 lbs. per acre.

      The where are you located? question is a very important consideration for K applications as is the irrigation method. Personally I’d only use water run K applications on our soils under drip irrigation.

      Best wishes, Joe

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