Modeling of liming in acid agricultural soils of Libreville (Gabon)

Abstract

Soil acidification is an ongoing natural process which can be enhanced by human activities or can be controlled by appropriate soil management practices. Two highly acidified urban garden soils, CHA and NTO, were subjected to single-extraction procedure by water to determine the extractable concentrations of Ca, Mg, K, Na, Al, Cu, Fe, Mn, Pb, Zn and . A modeling of effects of liming on soil solution was carried out from initial pH to pH 8.0 using the PHREEQC-2 code coupled with the PHREEQC.dat thermodynamic database. In NTO soil solution, liming increased the concentration of all cations, exception of Al when in CHA soil solution concentration of Ca, Cu, Fe and Pb increased. The chemical speciation of cations was evaluated in the soil solution. Basic cations Ca, Mg, K and Na were found as free forms (Ca²⁺, Mg²⁺, K⁺, Na⁺) and sometimes complexed by sulfate (,). The concentration of the toxic free ion Al³⁺ decreased quickly and was replaced by less toxic hydroxide complexes. For the other metals, relative to the total soluble metal concentrations the amounts of free Cu²⁺ (1%-99%) and Zn²⁺ (74%-97%) were not different in soil solutions when the amounts of free Pb²⁺ (10%-93%) and Fe²⁺ (65%-98%) in NTO soil solution were generally higher than those of CHA soil solution (8%-34% and 43%-84%, respectively). Statistical correlation analysis used to predict the change occurred in the concentration of the free ion in the soil studied showed significant influence of pH,  and  on Al³⁺, Fe²⁺, Pb²⁺ and Zn²⁺.

Key-words: tropical acid soils, urban garden, liming, modeling, PHREEQC-2