Mineralogical controls over soil K reserves: A case study in Southeastern Brazil (Rio de Janeiro State)

Assessing soil potassium (K) reserves is essential to help developing strategies focused on reducing the dependence of tropical countries for K-fertilizers. Samples from six soils located at Southeastern Brazil (Rio de Janeiro State), were investigated to better understand the mineralogical control over their K reserves. Mineral assemblage was characterized through X-ray diffraction, using Rietveld Refinement to determine minerals contents in the coarse fractions and NEWMOD to describe and quantify interstratifications in clay fraction. Total (K_t) and non-exchangeable K (K_ne) were determined in the samples. The highest K_t and K_ne contents were observed in moderately-weathered Cambisol samples, with high primary mica contents in coarse fractions, as well as illite-rich interstratified phases in the clay fraction. Highly-weathered soils (Lixisols and a Histosol) rich in kaolinite, poorly-weathered soils (Phaeozem and Chernosol) rich in weatherable primary minerals (K-feldspars, plagioclases and amphiboles), and interstratified clays 2:1 clays presented low-to-moderate K_t and K_ne contents. There was a positive and significant correlation of K_t and K_ne contents with illite layers, mainly allocated at kaolinite-illite interstratifications. Multivariate statistical approach enabled assigning K_t, K_ne, primary micas and illite layers as combined variables accounting for differentiating Cambisol samples from other samples, which presented lower K reservoirs. The absence of correlations between muscovite and K_ne may be related to an underestimation of K_ne content by standard method (Na-tetraphenyl borate), likely due to its incapacity of extract K_ne efficiently from coarse particles. These findings emphasize illite-rich interstratified clays as major mineralogical controls over K reserves in the investigated tropical soils.