It's not really about extractable or available
I misunderstood the question about pH

Soil potassium, from significantly unavailable to highly available

Yesterday I wrote about extractable vs. available elements and why that is not how to interpret soil tests. This is another example.

When I was a graduate student at Cornell, I did some research on the flagged section of L-93 creeping bentgrass in this photo.


I collected soil samples, did lots of different soil tests on those samples, applied different rates of potassium (K) to the plots, then did more soil tests and tissue nutrient analyses.

This green was a calcareous sand, the grass quality was not improved by any addition of K fertilizer, and soil samples were taken to a depth of 4 inches (10 cm).


At the start of the experiment, before making any K applications, samples were tested for K using different testing methods. The data in this chart are all the same soil, taken from the same section of the green on the same day, and the difference in the results is due to the testing procedure -- the extraction method.


The quantity of K extracted by the different methods ranged from 8 ppm in water to more than 10,000 ppm in a complete digestion of the soil. If you want to read about different forms of K in the soil, this is a good document. You'll notice that the terminology from the soil scientists is not the same as the terminology on the abc report I discussed yesterday.

Suffice it to say that different testing procedures applied to the same soil sample will extract different amounts of K from the soil. Here's a closer look at the more common tests, adjusting the scale and omitting the microwave acid digestion, which turns the sand into a liquid and measures the total amount of K in the soil.


I'm not suggesting one should test a single sample in multiple ways. What I do suggest is to pick a single method that works good for one's soil. The Mehlich 3 test is in common use and that is what we've used to develop the MLSN guidelines.

Then, the only thing one needs to do is compare the result of the test to the guideline. If there is enough, that element is not required as fertilizer. If there is not enough, that element should be applied as fertilizer. It is not a matter of availability. It is a matter of enough or not enough.

One could describe these same test results on some type of availability scale. I made this up. It sounds a bit ridiculous, kind of like this that I used as an inspiration. You can imagine that with a bit of thought, though, I could come up with availability descriptions that sound plausible. But that wouldn't make them correct, or meaningful, or applicable! It still comes down to enough or not enough.



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