DISTRIBUTION OF CATION-EXCHANGE CAPACITY BETWEEN ORGANIC-MATTER AND MINERAL FRACTIONS IN ACID FOREST SOILS (VOSGES MOUNTAINS, FRANCE)

The cation exchange capacity (CEC) of 18 samples (<2 mm) of five acid soils developed on granite or gneiss was measured before and after H2O 2 treatment from pH-unbuffered extraction solution (1 N KCl). The pH i n water of the soils ranged from 3.4-4.7. The samples were separated i nto seven fractions (<2 mu m, 2-20 mu m, 20-50 mu m, 0.05-0.2 mm, 0.2- 0.5 mm, 0.5-1 mm and 1-2 mm) and the CEC of each measured. Thus the co ntribution of each size fraction and of organic matter to the CEC was obtained. From CEC measurements on the different fractions and particl e size distribution dam, a balance calculation was carried out to veri fy the methodology. According to the hypotheses used and soil characte ristics, the CEC of organic matter was found to vary from 35 to 165 cm ol(c) kg(-1) and so represented from 10 to 85% of the total soil CEC i n the tipper soil horizons. In these sandy soils developed on granite or gneiss which were frequently affected by hydrothermal alteration, t he CEC of silt and sand fractions was large. It can represent from 35 to 80% of the total soil mineral CEC. The specific contribution of the 0.2-2 mm fraction can reach 50% of the total soil mineral CEC with va lues ranging from 0.5-5.2 cmol(c) kg(-1). The mineralogical descriptio n showed that hydrothermal white micas and neoformed smectites (precip itated during weathering) were present in all the fractions even in th e coarsest ones. In some subsoils, albite grains containing smectite h ave a CEC as large as 21 cmol(c) kg(-1). This study shows that the CEC of acid soils was not always located in the organic matter and in the clay fraction. The sand fraction can contribute substantially to the soil CEC.