The Concept of ‘Minimal’ Exchangeable Potassium

The Concept of ‘Minimal’ Exchangeable Potassium

When the quantity of potassium (K) applied is less than the quantity removed by crop harvest, soil fertility declines, writes Dr. T. Scott Murrell, Director, North American Program, International Plant Nutrition Institute. The exchangeable K concentration also declines, but it drops only so far. It eventually evens out at a lower level. A new equilibrium is created, which keeps the exchangeable K concentration fairly constant. This lower equilibrium is termed “minimal” exchangeable K. It serves as one measure of the K supplying power of a soil.


In 1969, Drs. Tabatabai and Hanway, both working at Iowa State University at the time, introduced the minimal exchangeable K concept. Scientists still publish research using their concept, confirming its lasting significance.

Their work was conducted in the greenhouse. They grew ryegrass in a measured amount of soil in pots. They repeatedly cut the ryegrass, each time allowing it to regrow, only to be cut again. They analyzed the stems and leaves at each cutting and calculated how much K had been removed from the soil. Their calculations showed fertility was definitely declining, but was the exchangeable K test a sensitive measure of that decline?