Potassium, K, and Magnesium, Mg, Relationships
Potassium and Magnesium ions are the same size, compete with each other for uptake and can be antagonist to each other.

The Mississippi River flood plain that runs from West Tennessee through Mississippi and Arkansas down through Louisiana have areas that are high to extremely high in Magnesium.  When Mg saturation is greater than 25% and K saturation is 2 to 3%, a K deficiency can occur, especially with cotton.  Our recommendation in OptiGro is that when Mg saturation is greater than 25%, increase potash rates by 50% for cotton and 20 to 25% for corn and soybeans.
 
To compute cation saturation, from the soil test report, divide pounds of the cation per acre, K, Mg or sodium, Na,  by the atomic weight of the ion times 20.  Divide this number by the CEC and multiply by 100.  This gives ion base saturation.  The atomic weight of K is 39, Mg is 12 and Na is 23.  Therefore, the conversion factor for K is 780, Mg is 240 and Na is 460. 
 
For example, Soil test K is 300 lbs/a, Mg is 800 lbs/a, Na is 300 lbs/a, and the CEC is 10 millequivalents,me, per 100 grams of soil.  To compute K saturation, divide 300 by 780, this gives 0.3974 me K/100 grams of soil.  Divided 0.3974 by 10 and multiply by 100 to get 3.97% K saturation. Divide 800 by 240, then divide by 10 and multiply by 100 to get a Mg saturation of 33.3%.  Divide 300 by 460 then divide by 10 and multiply by 100 to get a Na saturation of 6.52%.
 
So the recommendation is to increase potash recommendation by 50% for cotton and 20 to 25% for corn or soybeans.