Mineralogia e formas de potássio em dois latossolos do Estado do Paraná e suas relações com a disponibilidade para as plantas

It has been assumed that the occurrence of potassium-bearing minerals and its contribution to plant nutrition is non-existent or at best, unimportant in Brazilian Oxisols. The objective of this research was to verify the hypothesis that the exchangeable form is not the only source of potassium in these soils. Samples of two Oxisols of Parana State, Brazil (LRa and LRd), cropped with soybean and wheat, from 1983 to 1993 with different responses to K fertilization, were collected in treatments with and without Kfertilization. The total K of both soils was low, approximately 800 mg K kg -1 of soil. The silt fraction had the highest content of total K in absolute values. XRD analysis revealed the occurrence of weakly peaks of mica in subfractions ofsilt and clay, and 2:1 hydroxy-interlayered minerals (2:1HI) in the clay fraction. In the LRa soil, with initial exchangeable K of 64 mg kg -1 and no K application, total K dropped 172 mg kg -1 , indicating a significant contribution of the non-exchangeable K to plants. The reduction of total K at the same treatment in the LRd soil was similar to the reduction of exchangeable K (263 mg kg -1 ) indicating that the lack of crop response to Kfertilization was due to the depletion ofthe exchangeable potassium. Sources of non-exchangeable potassium in these soils are probably micas, and can also be associated with the 2:1HI minerals.