The Influence of Soil Moisture on Oxide Determination in Tropical Soils via Portable X-ray Fluorescence

Portable X-ray fluorescence (pXRF) has been used in many countries to assess the total elemental composition of soils and has agronomic, pedological, and environmental applications. This technique is still incipient in tropical developing countries like Brazil but has potential to improve and decrease the costs of research. Soil moisture is one of the most important factors influencing pXRF results. This pilot study evaluated how soil moisture affects elemental concentrations of oxides common in Brazilian soils (SiO2, Al2O3, Fe2O3, TIO2, and P2O5) as assessed by pXRF. Undisturbed soil samples were collected from the surface and subsurface horizons of six representative Brazilian soils (representing similar to 40% of the country) with variable texture, weathering, or leaching status. Soil moisture ranged from complete saturation to oven-dry. During drying, six pXRF measurements were made and oxide contents were recorded. In all soils, the P2O5 content was below the pXRF detection limit, and SiO2 and Al2O3 were strongly influenced by soil water content. As soil moisture increased, SiO2 and Al2O3 contents decreased (described by a power model, y = a - bx(c)). The TiO2 content was much less affected by soil moisture, whereas Fe2O3 was not significantly affected by soil moisture. Soil texture and weathering status are closely related to variation in the SiO2 and Al2O3 contents determined by the pXRF with soil moisture. These findings are critical to the appropriate application of pXRF in tropical countries featuring oxide-laden soils. For in-field measurement of oxides affected by soil moisture (Si and Al), a practical correction factor was determined.