Summary: The original soil exchangeable potassium (K) concentrations are at or above critical levels in many Brazilian Cerrado (savanna) soils. Hence, many cropped areas have been fertilized with low K rates, below crop requirements, but yields have not decreased as expected. In these areas, topsoil exchangeable K analyses have shown no decrease, or even some increase. The aim of this study was to evaluate exchangeable and non-exchangeable K forms in soils under different uses and managements in the Vale do Araguaia region of Mato Grosso state, Brazil. Soil samples were taken from 91 sites at depths of 0-20 cm and 20-40 cm, in areas under grain crops, pasture and native vegetation (Cerrado or forest). Silt content ranged from 12 to 175 g kg-1 and clay from 90 to 595 g kg-1, and the predominant clays were kaolinite, hematite, goethite and gibbsite. Under pasture, the soils had high levels of exchangeable K in the 0-20 cm layer and high levels of non-exchangeable K from 20 to 40 cm. This can be a result of the absorption of non-exchangeable K by grasses, the main cultivated species, by recycling K to the exchangeable fraction in the topsoil. There was a positive relationship between silt and non-exchangeable K contents. Ratios of exchangeable to non-exchangeable K were over 3 in soils with silt above 70 g kg-1, in which non-exchangeable K was over 100 mg dm-3. Cover crops growing in soils rich in silt take up non-exchangeable K and exchangeable K from deeper layers, which is recycled to the soil as exchangeable K upon plant residue mineralization, which may have been responsible for the maintenance or increase in exchangeable K levels in the 0-20 cm layer in areas where low K rates have been used for grain production.

Publication year: 2017

Types of publication: Paper in annals and proceedings

Unit: Embrapa Soils