Potassium Webinar Q&A

At the end of February, David Doll and I presented a webinar on potassium nutrition of almond orchards hosted by Western Farm Press with support from Compass Minerals.  During the webinar, listeners asked questions and we tried to answer them at the end of the webinar.  We could only get through a few.  In this post, we try to answer at least a few of those questions asked (another link to the webinar…click here!).

Q.  Does soil pH influence the equilibrium of soluble K? If yes, would banding SOP with an acid or base material enhance availability? 

A.  I don’t know if pH directly affects the soil solution: cation exchange equilibrium.  Low soil pH can decrease K availability by decreasing the cation exchange capacity held as base cations.  Acid cations (particularly Al⁺³) increase in acid soils – in the soil solution and on the exchange sites on the soil.  The more acid cations on the exchange phase the fewer sites available for the base cations – Ca⁺², Mg⁺², and K⁺ — to be held on the soil, so the soil holds less K⁺.  I have not seen any research showing that increased soil pH reduces or improves soil K availability over neutral soil pH.

So, would banding acid or base material enhance plant available soil K?  This could only help if your soil has low pH.  In that case you should be working to improve overall soil pH (not just in the banded zone of fertilizer K to improve overall mineral nutrition (see graph below).

Q.  Have critical potassium values been established for April leaf samples?

A.  No, not for almonds, at least not yet.  Several years ago, Dr. Steve Southwick then Extension Specialist at UC Davis, determined that, for prunes, an April leaf level of 2.0% K was sufficient to avoid dropping leaf levels below “adequate” = 1.4% K in July leaf samples.

Q.  What K does the Ammonium Acetate test represent?

A.  The Ammonium Acetate test is a standardized test where a set amount of dry soil is extracted with a solution of ammonium acetate.  This test measures exchangeable and soil solution potassium, although soil solution K is a very small percentage of that total.  Does this test measure all plant available K for all crops?  No, but it is a good starting place, effectively showing a grower or CCA a relative value for K levels in a soil.  It does a good job identifying very low or very high soil K levels, but is often not a good indicator of crop response to added K when the test results fall into the middle ground between high and low.  Leaf K levels are still the best way to evaluate the K fertility of an orchard soil.

Q.  Can using a gypsum machine cause a potassium deficiency?

A.  Potentially, if the soil has a sandy texture (low CEC) and excessive irrigation water is added, then adding gypsum can cause a potassium deficiency by pushing some potassium off the exchange phase (CEC) and thus making it vulnerable to loss from the root zone by leaching.  On finer textured (clay loam, etc. with high CEC) adding gypsum may improve potassium availability by moving  potassium deeper in the root zone, thus enlarging the portion of the root system exposed to fertilizer potassium.  I have seen no field studies that document an increase in almond tree yield or health from using gypsum to move potassium in the soil profile, although I have heard that this is done commercially.

Q. Foliar application of what type of K and how much could be done with Mite and Now sprays?

A.  I have heard of PCAs adding 10 lbs KNO₃/acre to foliar pesticide sprays, but until recently, I hadn’t heard of any tests that study any benefit in pest control.  ]Ten pounds per acre KNO3 delivers about 5 lbs K₂O/acre if all sprayed fertilizer is delivered into the tree.  That is less than 5% of the K contained in a 1000 lbs kernel crop.

In the last two years. David Haviland, UC Entomology Advisor in Kern County, has tested 10 lbs KNO₃/acre with different miticides to determine if the added K provides any improvement in mite control.  He has not seen a consistent improvement in mite control when KNO₃ is added to miticides.

In prunes, foliar applications of 20-25 lbs of KNO₃/acre using 100 gpa are common.  Research has shown that multiple applications (4-6 sprays) totaling at least 100 lbs KNO₃/acre/season maintains sufficient leaf K to avoid deficiency and deliver results comparable to 400 lbs of soil applied potassium fertilizer banded on the soil surface in the fall.  That amount of foliar KNO₃ doesn’t supply the entire crop’s need, but it provides enough K to supplement soil K uptake so that the orchard doesn’t become deficient.

Q.  How much k do newly planted trees need?

A. Very little.  The crop potassium requirement drives the high potassium needs of many tree crops.  Potassium deficiency symptoms appear when young orchards growing under low K soil conditions start carrying heavy crops.  Young orchards need very little potassium.

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