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

Abstract

Core Ideas We verified the effect of soil moisture on portable X‐ray fluorescence (pXRF) results with undisturbed soil samples. Heavy oxides (Fe and Ti) are much less affected by soil moisture than light oxides (Si and Al). The power model described the effect of soil moisture on pXRF results for SiO2 and Al2O3 well A practical correction factor for SiO2 and Al2O3 was proposed for accurate in‐field measurements 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 ∼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 – bxc). 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.

Elemental concentration via portable x-ray fluorescence spectrometry: Assessing the impact of water content

“…However, Zr (Z = 40) and Pb (Z = 82) were also influenced by water content (Hürkamp et al, 2009;Shuttleworth et al, 2014;Hangen;Vieten, 2016). In some Brazilian soil samples the elements Si (Z = 14) and Al (Z = 13) were much more influenced by moisture than Ti (Z = 22) and Fe (Z = 26) (Ribeiro et al, 2018). Frequently, non-effect of water content on Fe quantification via pXRF has been reported (Berger et al, 2009;Stockmann et al, 2016).…”

“…The pXRF analysis is fast, non-destructive, environmentally friendly, allowing the determination of several elements simultaneously (e.g., from Mg to U) in-situ or ex-situ within the order of seconds. The Soil Science has been greatly benefited from pXRF analysis (Ribeiro et al, 2017;Ribeiro et al, 2018;Santana et al, 2018;Silva et al, 2018;Mancini et al, 2019, Weindorf et al, 2014Stockmann et al, 2016).…”

“…In general, water content between 5% and 15% does not seem to influence the pXRF results (Laiho;Perämäki, 2005;Piorek, 1997;Peinado et al, 2010). Considering the effect of water content on pXRF results, some correction factors have been proposed (Ge et al, 2005;Bastos et al, 2012;Ribeiro et al, 2018) in order to overcome such constraints. The procedures for pXRF analysis of soils and sediments are summarized in the US EPA 6200 method (US EPA, 2007) and by Weindorf and Chakraborty (2016).…”

Elemental composition and moisture prediction in manure by portable X‐ray fluorescence spectroscopy using random forest regression

“…Development of spectrometers equipped with vacuum or helium atmospheres has greatly improved the measurement of light elements (Mg to Fe) (Brouwer, 2013; Guerra et al., 2014; Towett et al., 2016), which otherwise were difficult to determine (Potts, 2008). In addition, in situ or laboratory analysis of the light elements in moist plant, soil, and manure samples is limited by variations in moisture content (Imanishi, Bando, Komatani, Wada, & Tsuji, 2010; Pearson et al., 2017; Ribeiro et al., 2018; Sahraoui & Hachicha, 2017; Weindorf et al., 2008). Water scatters primary X‐rays and absorbs characteristic X‐rays of the analyte, thus reducing the photon counts from the nonwater portions of the sample (Ge, Lai, & Lin, 2005; Stockmann, Jang, Minasny, & McBratney, 2016).…”

“…Development of spectrometers equipped with vacuum or helium atmospheres has greatly improved the measurement of light elements (Mg to Fe) (Brouwer, 2013;Guerra et al, 2014;Towett et al, 2016), which otherwise were difficult to determine (Potts, 2008). In addition, in situ or laboratory analysis of the light elements in moist plant, soil, and manure samples is limited by variations in moisture content (Imanishi, Bando, Komatani, Wada, & Tsuji, 2010;Pearson et al, 2017;Ribeiro et al, 2018; Core Ideas • Manure sample moisture negatively affects elemental concentration determination by PXRF. • Random forest regression determined moisture content during PXRF analysis of samples.…”