Studies on the effect of clay and organic matter contents on the determination of cation exchange properties in clay soils by the ammonium acetate and methylene-blue methods

The correlation between CEC-values obtained by ammonium acetate and methylene-blue adsorption methods is relatively good (r = 0.86***). The latter method gives, however, about 30 per cent lower values for exchangeable cations. The extraction ratio used in the dye adsorption method has a clear effect on the level of the CEC-values. More complete adsorption was obtained with wider ratios. With increasing clay and humus contents the adsorption becomes more dependent on the soil-dye ratio. The effect of humus is more pronounced than that of clay. The CEC-values obtained by the ammonium acetate method were in better correlation to the clay content of soils than the values obtained by the dye-adsorption method, while the latter values were better correlated to the organic matter content of the soil. The percentages of exchangeable potassium, sodium and, especially, of magnesium, decrease when the clay content decreases, while that of calcium increases slightly. The ratio between exchangeable calcium and magnesium depends on the clay content of the soil ( r = 0.56***) so that the value of the ratio Ca:Mg increases when the clay content decreases (Table 1, q)