ANION TRANSPORT THROUGH COLUMNS OF HIGHLY WEATHERED ACID SOIL - ADSORPTION AND RETARDATION

Subsoils of highly weathered, acid soils usually contain appreciable q uantities of anion-exchange capacity (AEC), which retards the passage of anions through the profile. This AEC varies in magnitude with ambie nt soil solution parameters such as pH and ionic strength. In order to be able to predict NO3- movement in such soils, estimates of the effe ct of subsoil charge characteristics on retardation are required. The effect of subsoil Liming and ionic strength on the movement of NO3- in repacked Kanhapludult soil columns was studied for this purpose. Colu mns comprising topsoil (untreated) over subsoil limed to different lev els to give a range of AEC were used. The soil was first leached with a solution of 10 mmol(c) Ca(NO3)(2) L(-1) followed by a 10 mmol(c) CaC l2 L(-1) solution. Convection-dispersion equation (CDE) parameters for Cl- were determined using CXTFIT. Liming reduced the retardation coef ficient (R) from 2.39 in the unamended subsoil to 1.12 in the treatmen t that received 2.08 g Ca(OH)(2) kg(-1). Both subsoil pH (r(2) = 0.96) and AEC (r(2) = 0.98) were correlated with R for Cl-. To test the eff ect of electrolyte concentration on retardation, columns of untreated subsoil were leached with solutions of Ca(NO3)(2) and CaCl2 at four in put concentrations: 5, 10, 20, and 30 mmol(c) L(-1). Retardation coeff icients for Cl- at these concentrations were 2.81, 2.25, 1.76, and 1.5 1, respectively. Anion retardation needs to be taken into account in m odeling NO3- movement through acid subsoils.