Potassium Relationships of Three Ohio Soils

Three Ohio soils, Hoytville clay, Brookston silty clay loam, and Wooster silt loam located at various branches of the Ohio Agricultural Research and Development Center, were studied. Bulk samples from each horizon were tested for exchangeable basic cations, pH, sulfuric acid extractable potassium (K+), and particle-size distribution. The surface horizons of each soil were characterized by measurement of cation exchange capacity, quantity-intensity adsorption isotherms for K and x-ray diffraction of the clay (<2 /J.) fraction. The Hoytville soil was highest in exchangeable and sulfuric acid extractable K+; Wooster was lowest; and Brookston was intermediate but closer to Wooster than to Hoytville. Hoytville and Brookston had high quantityintensity labile buffer capacities for K+, the values being 2-23̂ > times greater than that for Wooster. Hoytville clay was predominantly illitic while the Brookston and Wooster clays were of a more mixed mineralogical nature. The potassium content and crystallinity of the illite in the Hoytville clay is lower than those of the Brookston and Wooster clays. OHIO J. SCI. 79(3): 114, 1979 Potassium (K+) is considered a macronutrient for plants and, although Ohio soils contain 1-2% K+ by weight, much of this K+ exists in unavailable forms in the soil. Since Ohioans spend millions of dollars annually on K fertilizer, studies on soil K+ and its availability to plants are of economic significance as well as academic interest. Plant roots absorb K+ from the soil solution. The forms of K+ in the soil can be grouped into 3 categories: readily available K+, that dissolved in the soil solution or existing as an exchangeable cation adsorbed on soil colloids; intermediate, i.e., slowly available K, and unavailable mineral K found in minerals such as K-feldspars and micas and released only as the minerals are chemically weathered in the soil (Tisdale and Nelson 1975). Most soil testing laboratories in the North Central United States measure available K+ by extracting air dried soil with 1 N ammonium acetate at a pH of 7.0 (Carson 1975). Hunter and Pratt (1957) developed a strong H2SO4 acid extraction K+ procedure which removes both the readily available and slowly Manuscript received 27 June 1978 and in revised form 16 November 1978 (#78-39). available fractions. The correlation between H2SO4-extractable K+ and K + uptake was very good (r = 0.946 for corn on 16 soils and 0.990 for alfalfa on 9 soils). HoSO-i-extractable K was found to predict K+ uptake better than exchangeable K alone. Woodruff (1955) presented the concept that availability of K+ to plants is related to the energy of exchange of K+ for Ca on soil colloids or plant root surfaces. In mid western soils with a neutral pH, Ca is usually the predominant exchangeable cation. The energy of exchange (AG) is calculated as: