Potassium (K) acts over the enzymatic and metabolic processes at the cellular environment, and its availability to plants depends on its forms and on its behavior in the soil. The extraction of available contents of K is established as a reference for diagnosis and fertilization calculations in Brazil. However, the absence of response of crops to potassium fertilizer has been verified in soils with low contents of this element, which leads to the hypothesis that other less available forms of K), which are not extracted by conventional extractors, can contribute to plant nutrition. There is no critical limit among the soluble, the exchangeable, the non-exchangeable and the structural forms of K in the soil because the change of availability is dynamic. K is required in large quantities by the soybean (Glycine max L.). This study will be conducted with samples from an Oxisol cropped under no tillage since 1983 in an experimental field area in Londrina, state of Paraná, Brazil, to evaluate the effect of levels of potassium fertilizer on soybean yield. Since 2008, K fertilization has been stopped, and the plants have been nourished by the residual contents of K in the soil. We aim to (i) evaluate changes in chemical, physical and mineralogical soil properties as a function of time with restriction of potassium fertilization; (ii) study the mineral reserve of K in the clay fraction of the soil through sequential extractions of minerals as well as determining of the contents of potassium content in the extracts; (iii) to correlate the exchangeable and the non-exchangeable forms of K in the soil with the nutrient levels in the tissues and with the soybean yield.
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