In 2004, I asked "Does potassium fertilizer really increase roots?" in TurfNet Monthly. The answer, pretty clearly, is No, except in the case where the added potassium eliminates a deficiency. I was asked a question about potassium at a conference, was intrigued, and went to the library to find and read a number of papers that were cited as showing that higher soil potassium levels yield increased root development and branching. After reading the papers, I came away with a completely different understanding:

An increase in roots was obtained with the first increment of potassium fertilizer that was added, but more potassium than the initial increment had either no effect or actually decreased root mass. In all of these studies, just one thing stands out: potassium deficiency inhibits root growth. One can readily deduce that a positive root response to potassium fertilizer can only be expected when initial soil potassium levels are extremely low. Application of more potassium, above and beyond that amount required to eliminate the deficiency, can actually reduce roots.

Read the full article here. I recently read a number of papers about potassium and stress tolerance, with a similar interpretation, and have developed this calculator to determine how much potassium fertilizer is required to keep the soil levels above those dangerous low levels.

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