Studies on the kinetics of K and Mg release improve our understanding of nutrient dynamics in soils and can provide information to adjust fertilization recommendations and increase plant yields. The selected soils represented different parent materials (sandstone - Bauru Group, Granite - Araxá Group, basalt - São Bento Group and sediments - Detritic-Lateritic-Tertiary Cover) and weathering degrees. The total K content in soil and its fractions were determined by HF, HNO3 and H2SO4 digestion. To estimate the potential of clay minerals to release K and Mg from non-exchangeable and structural forms to plants, nine sequential extractions were used with 0.1 mol L-1 citric acid solution, with the following contact times: 2, 12, 24, 48, 96, 144, 192, 288, and 576 h. The total contact time was 1.382 h. Reserve K and Mg release were determined by parent materials and weathering degree. The clay fraction of Ultisols developed from sandstone (Uberaba Formation) and granite (Araxa Group) generally presented higher total K and Mg concentrations and amounts extracted by 0.1 mol L-1 citric acid during the the entire kinetics study. Nevertheless, even in Oxisols (low total contents) a relatively high K and Mg release was observed (accumulated contents after 1.382 h) compared to the exchangeable contents, evidencing the ability to supply plants' with non-exchangeable and structural K and Mg throughout the cultivation period. The parabolic diffusion equation showed that K and Mg release occurred at different rates and in two stages: for the majority of the samples, the release rate was higher for K and lower for Mg in the first stage. This suggests that K mainly occurs in non-exchangeable forms in medium/high energy retention sites, while Mg occurs in the structure of ferromagnesian mineral structures, mainly biotite, retarding the nutrient release by citric acid weathering reactions.
K reserve; Mg reserve; citric acid; parabolic diffusion equation
