Exchangeable K - Why Is It so Darn Complicated?.

One answer to the question posed by the title of this presentation is that we are asking too much of a single parameter. Exchangeable potassium is, of course, a surrogate for what we really want to know, which is plant-available potassium over the course of a growing season. When we measure the amount of soil potassium that exchanges with ammonium (or some other cation) in a soil-test extraction, we are assessing only a fraction of the potassium might be taken up by a plant. Under some conditions, that fraction can be a reasonable predictor of seasonal plant uptake, but the prediction is not as reliable an index as crop producers need today.

     The factors that limit the reliability of exchangeable potassium as an index of availability are well known, but their impacts are highly variable. The absolute mass of potassium on the exchange sites of a soil sample varies with cation exchange capacity, which is itself a function of clay concentration, layer-silicate mineralogy, organic matter concentration, Eh, and pH. Exchangeable potassium at any point in time also depends on recent fertilization, the capacity of soil minerals to supply potassium to exchangeable positions, the capacity of soil minerals to fix potassium in nonexchangeable forms, and the rates at which potassium moves between exchangeable and nonexchangeable forms.

     The amount of potassium that is in soil solution and available for plant uptake is dynamic and is regulated by the amount of water in soil pores as well as solution-phase concentrations of other nutrients. Finally, uptake also depends on the growing plant’s responses to the soil environment, including changes in rooting volume and development of a mucilage to mediate root - soil contact. These factors interact with one another in temporally and spatially variable processes that that are challenging to measure or model.

     In some respects, it is amazing that the snapshot-like assessment of exchangeable potassium by soil-test methods correlates with crop response as well as it does.