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| Some fallacies I see in your post:
1) It requires a soil test of 170 ppm K to insure high alfalfa production
I believe the optimal level of ppm K is based on CEC values. The higher the CEC, the higher the required level of K. There is no "one" magical number.
(My older agronomy guide says the range should be : CEC x 18.8 for the bottom side of "optimum", and CEC x 31 for the upper side of "optimum".)
The other numbers I would look at are how the K is balanced in relation to Mg & Ca.
2) I would think magnesium and calcium would hog all the electron attraction points on the clay surfaces, leaving no room for potassium.
I don't remember the name of the process, but there is a basic principle in chemistry which says that the elements need to be in balance. I think you will never see clay surfaces holding onto calcium and magnesium to the exclusion of potassium. I do agree that calcium and magnesium will be held more tightly, but not "to the exclusion". There needs to be a balance. Also, how molecules are held is related to the size of the molecule. I am thinking smaller molecules are held more tightly - i.e. Mg would be held more tightly than Ca. (Goes back to the atomic weights on the Periodic Chart.)
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Soil test K levels and % availability?
