Relationship Between Soil Potassium Forms and Mineralogy

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Abstract
The relationship between soil K forms (water soluble, exchangeable, HNO3‐extractable, and mineral) as a function of clay mineralogy and taxonomy, was determined using 102 soils from the continental USA and Puerto Rico, representing 10 soil orders. These relationships may have use in evaluating potential soil K fertility and prediction of K cycling and plant uptake. The soils were divided into three groups (kaolinitic, mixed, and smectitic) based on dominant mineralogy of the clay fraction. For each group of soils, water soluble K was related to exchangeable K content (r2 of 0.86 to 0.96), with exchangeable K solubility increasing from smectitic, mixed, to kaolinitic soils. For each group, exchangeable K was also related to HNO3‐extractable K (r2 of 0.81 to 0.83), thus, the capacity to supply K under continuous cropping, is greater for smectitic than kaolinitic soils of similar exchangeable K contents. Mineral K was a function of clay content for each soil group (r2 of 0.66 to 0.90). Using these relationships, water soluble (2–37 mg kg−1), HNO3‐extractable (21–2691 mg kg−1), and mineral K contents (1.59–16.93 mg kg−1) of 60 independent soils, were closely predicted (r2 of 0.78 to 0.96) from exchangeable K, clay content, and clay mineralogy. These results indicate that determination of both exchangeable and HNO3‐extractable K could give a better indication of the potential K supplying power of a soil. Further, the relationships obtained will allow determination of initial soil K pool sizes from readily available soil taxonomic, clay mineralogy, and chemical properties, for use in modeling the cycling and plant uptake of soil K.