B. Ludwig et al., MODELING OF SORPTION EXPERIMENTS AND SEEPAGE DATA OF AN AMAZONIAN ULTISOL SUBSOIL UNDER CROPPING FALLOW, Zeitschrift fur Pflanzenernahrung und Bodenkunde, 160(5), 1997, pp. 447-454
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.