MODELING OF SORPTION EXPERIMENTS AND SEEPAGE DATA OF AN AMAZONIAN ULTISOL SUBSOIL UNDER CROPPING FALLOW

The results of physico-chemical investigations of an Ultisol subsoil u nder a a-year old fallow in eastern Amazonia are presented. Subsoil ch emistry was studied using 4 different approaches: i) concentrations of H, Na, K, Ca, Mg, Mn, Al, and Fe in seepage water were measured under field conditions, ii) the equilibrium soil chemistry was studied in s equential batch experiments where the soil was treated with different solutions, iii) results of batch experiments were simulated with a che mical equilibrium model, and iv) the seepage data were calculated usin g selectivity coefficients obtained by modelling the batch experiments . The model included multiple cation exchange, precipitation/dissoluti on of Al(OH)(3) and inorganic complexation. Cation selectivity coeffic ients were p(X/Ca)(sel): X = Na: 0.3, K: 0.8, Mg: -0.1, and Al: 0.4. T he amount of cations sorbed ranged from -0.2 to 2.0 (K), -0.7 to 2.3 ( Mg), -1.6 to 1.8 (Ca), -4.8 to 3.6 (Al) and 0.0 to 8.5(Na) mmol(c) kg( -1). The model predictions were good with values lying within 0.3 pH u nits (for the pH range 3.7 to 7.2), and 3% of CEC for individual catio ns. The most important proton buffer reaction seemed to be the dissolu tion of gibbsite and a large release of Al into the soil solution. Whe n selectivity coefficients obtained by the modelling procedure were us ed to predict the field data for cation concentrations in the seepage water, they decreased in the following order: Na > K > Ca > Mg > Al. T hese calculated values were similar to the measured order: Na > Ca > K approximate to Mg > Al. Thus the options for managing these soils sho uld be carefully chosen to avoid soil acidification which may result f rom inappropriate use of fertilizer during the cropping period.