167-3 Exchangeable K - Why Is It so Darn Complicated?.
See more from this Division: SSSA Division: Soil MineralogySee more from this Session: Symposium--Soil Potassium Tests and Their Relationship to Plant Availability and Native Mineralogy: I
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.
See more from this Session: Symposium--Soil Potassium Tests and Their Relationship to Plant Availability and Native Mineralogy: I