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Abstract: Phosphate (P) sorption and desorption characteristics have been investigated for sixty-two representative samples of the major Thai upland agricultural soils which are Oxisols and Ultisols formed on diverse parent materials and under various climatic conditions. P sorption characteristics are well described by Langmuir and Freundlich equations. Values of Langmuir P sorption maximum (X m) and b coefficient range from 47 to 1250µgg−1 soil and 0.5 to 30mLµg−1, respectively and the Freundlich k varies between 12 and 1694µgg−1 and B coefficient 0.09 and 0.67. Soils derived from basalt have larger values of P sorption maximum than soils on other parent materials. Both P sorption maximum and Freundlich k are closely related to pH (NaF), SSA, total titanium, aluminum and iron, dithionite and oxalate extractable Fe and Al, and other soil properties that reflect specific surface area. Soil pH in NaF is highly effective in predicting the P sorption capacity of Thai upland soils explaining 79% of the variation in P sorption by these soils. P desorption curves for adsorbed P are well described by the Freundlich equation. The Freundlich k d coefficient for P desorption is highly, linearly related to the Freundlich k coefficient of P sorption indicating that the above soil factors that contribute to P sorption similarly affect P desorption. [Copyright &y& Elsevier]

Silicon (Si) is an abundant element in terrestrial ecosystems and makes essential contributions to agricultural crops, environments, and industries; however, current knowledge of Si speciation in soil systems is based primarily on crystalline mineral types with limited information about the effect of Si-adsorbed species on the most chemically reactive clay fraction. Herein, we investigated Si speciation in the clay-sized fraction of tropical soils. Silicon K-edge X-ray absorption near edge structure (XANES) spectra of clay samples, together with a wide range of reference compounds varying in clay and Fe/Al oxide mineralogy, showed the predominance of Si in a beidellite structure and of Si adsorbed to montmorillonite and to kaolinite. Unique features, including a pre-white-line feature, white-line position and intensity, and post-edge numbers and positions of Si reference spectra, could differentiate most Si reference standard spectra. Silicon adsorbed to goethite, Si-ferrihydrite-natural organic matter complexes, and Si in the structure of biotite, illite-smectite, chlorite, quartz, and Si nanoparticle, variously occurred in different samples. The CaCl 2 extraction data revealed that extracted Si was below the limits of Si deficiency in one-half of the total soil samples and accounted for only 0.02% of total soil Si. Soil extractable Si had a positive correlation with pH, organic matter, and clay. Our results present a novel application of Si K-edge XANES as a tool for identifying Si speciation in natural clays that are the most chemically reactive phase of Si retention and release in terrestrial and aquatic systems. [Display omitted] • Reference spectra of silicon sorbed to clay mineral and Fe/Al oxides are presented. • Si-ferrihydrite-natural organic matter complexes have been synthesized and proposed. • White-line and post-edge features of Si reference spectra can identify most Si standards. • Structural Si in beidellite and Si sorbed to montmorillonite/kaolinite dominated Si species. • Si nanoparticles could form in clay-sized fractions of soil systems. [ABSTRACT FROM AUTHOR]