# Soil Potassium (K)



## vhaby (Dec 30, 2009)

This Originally Was Posted by hay wilson in a Soil Sampling Thread...

"The potash thing is confusing. Took me a good while to realize that 1% K alfalfa plant analysis is deficient even if the soil test came back 350 ppm K. I have difficulty getting the hay analysis above 2% K. The growers up north are worried about winter die off, you and I worry about summer drought die off. 
A number of soils Labs have charts that give higher levels of K needed to meet the critical needs of our crops. 
To add to our joy high shrink clay is built in layers, and when moist, the clay swells and traps the potassium. The processing of the soil for analysis has the dirt dried and ground to a powder. There is no place to hide and the unavailable K gets counted as available. 
So yes my soils are labled as having excessive levels of potassium, even though the crops are telling us the K is deficient. 
The first Ph D who put me on the right track did research in California, and is now in Nebraska. He was kind enough to share his knowledge. 
Our Vincent vhaby did a lot work in high shrink soils in Montana and he has been a great mentor. 
When you finally learn what to look for, there is a good bit of information out there. 
I find it interesting that the Black Lands not far from Waco may look the same but it is closer to neutral and has a reasonable clay soil." 
It is truly interesting.
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Here is a followup explanation of what likely is happening:

Commun. in Soil Sci. Plant Anal., 19(1), 91-106 (1988)

Effect of Sample Pretreatment on Extractable Soil Potassium1

Key Words: Soil Testing, K release, Soil rehydration, Soil freezing.

V. A. Haby2, J. R. Sims3, E. O. Skogley3, and R. E. Lund3
2Texas Agric. Exp. Stn., Texas A&M Univ. Agric. Research and Extension Center, Drawer E, Overton, TX 75684

Abstract: Effect of soil sample treatment on potassium (K) release characteristics of Northern Great Plains soils was studied. Samples of agricultural soils were extracted with neutral (pH 7) 1.0 normal ammonium acetate for K after selected pretreatment. Field-moist sub-soil samples with approximately 100 mg/kg (ppm) extractable K released nearly double this amount after they were air or oven-dried prior to analysis. The percentage increase in extractable K in dried soils decreased as the K level in the field-moist soils increased. At 420 and 500 mg/kg, the extractable K content of air-dried and oven-dried soils, respectively, equaled the extractable K content of field-moist samples. Above these concentrations, drying decreased extractable K. Oven-drying (60 C) (140 F) affected extractable K more severely than did air-drying the soils. Drying and grinding the 15- to 30-cm (6- to 12-inch) depth samples significantly increased exchangeable K compared to field-moist samples. Rehydration of oven -dried soils to 40% water did not affect extractable K. However, results...

1Published with approval of the Director, Montana Agric. Exp. Stn., Journal series number 1070. Research conducted with partial support from the Montana Wheat Research and Marketing Committee.

3Montana Agric. Experiment Station, Montana State University, Bozeman, MT 59717._
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What this is saying is that when laboratories dry soil samples prior to grinding and extracting them, the drying process causes the shrink/swell clay particles that are tightly bound together at the silica and alumina layers when wet, to peel back at the edges and this releases soil trapped K that the plants roots may not have access to. If the original soil is relatively low in natural soil K, drying this soil nearly doubles that amount of K determined by soil testing. As the natural soil K that is available to plants increases toward very high levels, drying the soil for analysis causes the release of less and less additional K. At the high level of natural soil K, drying the soil has little or no effect on changing the extractable level of soil K. Above these high levels of naturally available soil K, drying actually began to tie-up extractable soil K.

So, why do your plants respond to applied potassium when the soil test indicates that it may have adequate soil K? Perhaps this is a partial answer.

One way to overcome this drying effect on extractable soil K is to analyze field moist soil samples for K. If my memory serves me correctly, this idea was pushed early on by a Texas native transplanted to Iowa, Dr. John Pesek. See below:

Wallace Chair: Pesek Colloquium_


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## hay wilson in TX (Jan 28, 2009)

Thank you good Sir!

You helped me make one more small step away from ignorance. It is all part of the Big Picture. Put another way, a part of the Universal Truth.


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## Nitram (Apr 2, 2011)

So at the end of the plants cycle and the ground is worked up will this action along with rain fall increase the amount of available K for the following crop? Thank you for all your info. Martin


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## vhaby (Dec 30, 2009)

Your question is not that easy to answer because of the many factors affecting plant available soil K. Potassium moves to plant roots through diffusion in soil solution. The previous crop has removed much of the soil solution K. Tilling the soil and increasing moisture from rainfall will allow the soil to replenish the soil solution somewhat. However, the total amount of soil K is finite, and continued removal by plant uptake removes some of that total K. A much better explanation of soil K availability has been written by a friend and his colleague in Minnesota. This may help provide a better understanding of the availability of soil K for plant growth. 
Potassium for Crop Production


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## hay wilson in TX (Jan 28, 2009)

*That is an interesting link. Makes a good addition to my general soil fertility notebook. Thank you vhaby. *

Nitram from, Penn State, about 25 years ago I found a chart with the desired range for potassium and magnesium based on the soil's CEC. 
In round numbers we want the potassium level to be roughly 10 times the CEC in ppm K. I say roughly because it was not a linier scale. 
Take the CEC times 10 for ppm K & Mg. 
CEC times 160 for ppm Ca. 
Penn State has moved on, but the idea applies here.

My alfalfa ground has a *40 CEC *and my bermudagrass ground has a *50 CEC*. Soil analysis is all well and good, but not complete. The proofer is in the plant analysis. The plant analysis chart from Minnesota is as optimistic as any I have seen. 
In the field it makes no difference what the soil report is it is what the crop is finding. Yield is ultimately due to a long list of interacting factors.


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## Texasmark (Dec 20, 2011)

vhaby said:


> This Originally Was Posted by hay wilson in a Soil Sampling Thread...
> 
> "The potash thing is confusing. Took me a good while to realize that 1% K alfalfa plant analysis is deficient even if the soil test came back 350 ppm K. I have difficulty getting the hay analysis above 2% K. The growers up north are worried about winter die off, you and I worry about summer drought die off.
> A number of soils Labs have charts that give higher levels of K needed to meet the critical needs of our crops.
> ...


_

I don't know what they do at TAMU, but they were pretty explicit about not heating your sample to dry it out prior to packaging and shipping. They said to let it air dry spread out in a pan overnight. This may have been the reason why.

HTH,

Mark_


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## Nitram (Apr 2, 2011)

Mr Haby and Mr Wilson thank you for the great info. I love learning and this will definitely give me a better understanding of the plants growth process. It is really interesting if we focus on only one aspect (soil analysis) we may miss the true cause of the plants stress. Martin


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