Soil mineralogy and K reserves in soils from the Araguaia River valley, Brazil

In humid tropical regions, intense weathering extensively depletes soil nutrients. However, soils in the Araguaia River Valley, Brazil, are not responsive to potassium (K) fertilization. To explore the reasons for this lack of responsiveness, an extensive exploratory soil sampling campaign was performed in areas of the valley with different land uses and soil management practices. Soil was sampled at depths of 0–20 cm and 20–40 cm, and exchangeable K (Ke) and non-exchangeable K (Kne) were assessed. The average soil Ke content in the region was 66.3 mg kg−1, above the minimum required value for crop growth. The soil Kne content ranged from one to five times the soil Ke content and was greater in the 20- to 40-cm layer than in the 0- to 20-cm layer. The linear correlation between Ke and Kne was significant only in the silt fraction. Mineralogical analysis of the clay and silt fractions of selected samples showed that the dominant minerals in the clay fraction were kaolinite, vermiculite with or without interlayer hydroxy-Al (HIV), illite, goethite, and gibbsite. Mica and feldspar were found in the silt fraction of several samples. In the silt and clay fractions, the main source of Kne was 2:1 phyllosilicates, which may be interstratifications of illite/vermiculite-hydroxy and illite/vermiculite. The results indicated that Urochloa spp. functions as a K scavenger in deep layers and promotes K biocycling in the system. An equation for predicting Kne extracted with nitric acid on the basis of soil Ke and silt contents was developed. •Tropical soils, with primary minerals, can be a source of K for plants, and use can change the structural theses minerals.•Soils with high silt contents show high non-exchangeable K.•The land use with high K-demanding species and low K replacement will deplete soil K.•Non-exchangeable K is an effective means of increasing the K use efficiency of a cropping system.