Potassium Buffering Capacities of Selected Arkansas Soils

Although total soil potassium (K) exceeds crop uptake, in most soils only a small fraction of the total K is available. Potassium is found in four phases: solution, exchangeable, non-exchangeable and mineral. It is known that the ability of a soil to supply K for plant uptake depends on the concentration of K in the available phases and on the amount of K in the non-exchangeable phase. The objectives were: i) to characterize the Quantity/Intensity relationships of Arkansas soils, and ii) to understand the ability of such soils to release or fix K. Six soils were used in this study. Nutrients extractions were conducted by using Mehlich-3, NH₄OAC and Water. Non-exchangeable K was determined by using NaBPh₄ and Quantity / Intensity relations were determined by using the procedure of Wang et. al. (2004). The levels of available K were in the very low to low range for the silt loam soils and from medium to above optimum for the clay soils. Mehlich-3 and NH₄OAC extracted similar amounts of K, while water extracted the least and NaBPh₄ extracted the most. The NaBPh₄ extraction represents plant available and some non-exchangeable K. The Q/I relationships showed clear differences among silt loam and clay soils, with DK ranging from -0.13 to 0.36 cmol kg^-1 and from -0.64 to 2.17 cmol kg^-1 respectively. These results confirm that clay soils have a greater capacity to fix K. The potential buffering capacity of the soils was 9.4 to 10.9 cmol kg^-1/(cmol L^-1)^1/2 for the silt loam soils and from 53.9 to 127.9 cmol kg^-1/(cmol L^-1)^1/2 for clay soils, which indicates the greater ability of clay soils to provide available K. The quantity of DK due to non-exchangeable phase was less than that due to exchangeable phase. Non-exchangeable K contributed between 25 to 40% of the K buffering capacity in the silt loam soils, and from 15 to 19% in clay soils. Soil ability for K release and fixation (β) varied from 0.02 to 0.03 for silt loam soils and from 0.03 to 0.09 for clay soils. This indicates that from 2% to 3% of added K is converted to the non-exchangeable form in silt loam soils and from 3% to 9% in clay soils which indicates that clay soils are much more highly buffered than the silt loam soils and can sustain K supply for longer periods. When K is added, clay soils also have higher potential to convert K to the non-exchangeable form. This information reinforces the idea that soil buffering capacity should be considered when formulating K fertilizer recommendations.