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REACTION OF FERTILIZER POTASSIUM IN SOILS OF DIFFERING MINERALOGY

The distribution of added potassium (K) in soil K forms (water-soluble, exchangeable, and nonexchangeable) as a function of clay mineralogy and taxonomy was determined using 102 soils from the continental United States and Puerto Rico, representing 10 soil orders. The soils were divided into three groups (kaolinitic, mixed, and smectitic) based on dominant clay mineralogy. A linear increase in water-soluble, exchangeable, and nonexchangeable K content of each soil was obtained after a 25-wk incubation with K (0 to 250 mg K kg-1). Slopes of these relationships represent water solubility (S), exchangeability (E), and fixation (F) indexes, with a significant decrease in S and increase in E and F observed from kaolinitic to mixed to smectitic soils. The indexes were closely related (r2 = 0.75 to 0.96) to clay content for each group of soils. The K buffer capacity (KBC) was significantly greater for smectitic than mixed and kaolinitic soils and was closely related to clay, CEC, and K saturation for each group of soils. Using the relationship between clay content and S, E, F, and KBC, the distribution of fertilizer K in an independent set of 60 kaolinitic, mixed, and smectitic soils and published data was closely estimated. Differences in these relationships between each group of soils were consistent with K reaction mechanisms of the dominant clay minerals. Use of these relationships may allow prediction of fertilizer K disposition in soils of differing taxonomic and mineralogical properties and, thus, aid fertilizer K recommendations for certain soils.