Assessment of the Potassium Buffering Capacity of Selected Arkansas Soils.

Potassium (K) is used in large quantities by nearly all agronomic crops. Although total soil K generally exceeds crop uptake, in most soils only a small fraction of the total K exists in plant available forms. 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 not only on the concentration of K in the available phases (Intensity) but also on the amount of K in the non-exchangeable phase (Quantity) which can become available during the growing season. The objectives of this study were: i) to characterize the Quantity/Intensity (Q/I) relationships (buffer power) of Arkansas soils, and ii) to understand the ability of such soils to release or fix K. A total of six soils collected in the Mississippi delta region of Arkansas were used in this study. Nutrients extractions were conducted by using Mehlich-3, Ammonium Acetate (NH₄OAC) and Water. Non-exchangeable K was also determined by using the method proposed by Cox et. al. (1999) and Quantity / Intensity relations were determined by using the procedure of Wang et. al. (2004). The levels of plant available K (Mehlich-3 extractable) were in the “very low” to “low” range for the silt loam soils and from “medium” to “above optimum” for the clay soils, according to University of Arkansas’ guidelines. 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. For this comparison, the NaBPh₄ fraction was obtained after incubating the soils for 10 minutes so it could be comparable to the Mehlich-3 extraction. When the incubation time was extended higher amounts of K were extracted for all the soils. The Q/I relationships showed clear differences among silt loam soils and clay soils, with DK ranging from -0.13 to 0.36 cmol kg^-1 for silt loam soils and from -0.64 to 2.17 cmol kg^-1 for clay soils. These results confirm that the clay soils have a greater capacity to fix K than the silt loam soils. The potential buffering capacity (PBC) of the soils ranged from 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 is an indication of the greater ability of clay soils to provide plant available K. The quantity of DK due to non-exchangeable phase was less than that due to exchangeable phase as the CR was increased for all soils. Results showed that non-exchangeable K contributed between 25 to 40% of the K buffering capacity in the silt loam soils, and from 15 to 19% for clay soils. The relation between DNEK and the Ô was linear, and 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 finer-textured soils. This indicates that from 2% to 3% of added K is converted to the non-exchangeable form (fixation) in silt loam soils and from 3% to 9% in clay soils. Results indicate that the clay soils are much more highly buffered than the silt loam soils. Clay soils can sustain K supply for longer periods than silt loam soils because of their higher buffering capacity. The majority of this buffering was attributable to the exchangeable K phase. 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.