Models for potassium release kinetics of four Humic Gleysols high in clay by electro-ultrafiltration

The rate of K release from the soil solid phase to its solution, especially from its rapidly and slowly exchangeable forms, can affect plant K uptake. The electro-ultrafiltration (EUF) technique was used to kinetically charac terize the K release from four soils (Humic Gleysols) from Abitibi-Temiscam ing, Quebec, Canada. Potassium was desorbed from soils that received over 1 66 kg K ha(-1) yr(-1) from 1994 to 1996. The desorption was carried out for 58 min (4, 4, and every 5 min thereafter for a total of 12 desorptions) at 400 V and 80 degreesC. The amount of K desorbed was between 199 and 342 mg kg(-1). Cumulative K desorption was described by the following six equatio ns: power function, simplified Elovich, extended Elovich parabolic diffusio n, zero-order, and first-order. An incremental first-order equation was als o tested. The studied soils have large K supplying capacities as suggested by forage K-uptake and total soil K desorption by EUF. In general, all test ed equations adequately described the K release by EUF from the four soils. The incremental first-order equation, used for the first time, described w ell the soil K release data. The k rate constant from this model was signif icantly related to forage K uptake (R-2 = 0.58). Results from this study in dicate that the investigated soils have large K reserves and that the incre mental first-order equation and most of the cumulative ones are suitable fo r describing the kinetics of the large K release from these fine-textured G leysols.