Soil capacity to adsorb selenium (Se) influence leaching and Se availability to plants. The application of mechanistic models has shown that the behavior of selenite is consistent with an initial adsorption reaction followed by diffusive penetration like phosphate (Barrow, 1992). Barrow et al. (2005) modified a multicomponent Freundlich type equation to describe competitive sorption between selenite and phosphate by including aspects derived from a mechanistic model. The aim of this study was to validate the multicomponent Freundlich type equation proposed by Barrow et al. (2005) to describe mutual interactions of selenite and phosphate on Andisols of Southern Chile. Surface soil samples of Andisols were collected from Pemehue, Corte Alto and Piedras Negras Series of Southern Chile. Sorption experiments were carried out in batch systems for 24 hours at 25ºC and the background electrolyte was 0.1 M KCl. The original selenite concentrations varied between 0 and 5 mM and each of these concentrations was tested with 0, 0.5, 1.0 and 2.0 mM of phosphate. The data were modeled by the multicomponent Freundlich type equation proposed by Barrow et al. (2005):

S_Se = a_Sec_Se /(c_Se+ k_Se,Pc_P)^1-b_Se and S_P = a_Pc_P /(c_P+ k_P,Sec_Se)^1-b_P,

where a_Se and a_P represent anion sorption when concentration (c_Se or c_P) in solution is 1, b_P is the index term of the Freundlich equation, b_Se = b₁c_Se ^b₂ is the Sibbesen term in which the index term of the Freundlich equation is a function of concentration, a_Se = a_Se,0 – m_Se,P S_P, m_Se,P represents the decrease of a_Se with increasing levels of sorption of phosphate (S_P) by effects on the surface charge, a_Se,0 is the value of a_Se at zero sorption of phosphate, and k_Se,P and k_P,Se are competition coefficients of phosphate on selenite and selenite on phosphate, respectively. The ability for sorption of selenite (a_Se) was lower than those of phosphate (a_P) in the Andisols evaluated (Table 1). The addition of equimolar concentrations of selenite and phosphate decreased the amount of selenite adsorbed between 5 and 28%, while selenite decreased the sorption of phosphate in lesser extent. For every soil, competition coefficients (K_i,j) were not symmetrical, which means that selenite and phosphate not only competed for sorption sites, being phosphate a stronger competitor than selenite as shown by Barrow et al. (2005). Furthermore, there was evidence of feed back of phosphate on the sorption of selenite (m_Se,P), which was attributed to a decrease of the surface electric potential as a consequence of diffusive penetration of phosphate as described by mechanistic models (Barrow, 1992). Acknowledgements: FONDECYT-1020934 and MECESUP FRO-0309 References: (1) Barrow, N. J. 1992. The effect of time on the competition between anions for sorption. J. Soil Sci. 43: 421–428. (2) Barrow, N. J., Cartes, P. and Mora, M. L. 2005. Modifications to the Freundlich equation to describe anion sorption over a large range and to describe competition between pairs of ions. Eur. J. Soil Sci. 56: 601–606.