EFFECT OF POTASSIUM REMOVAL BY CROPS ON TRANSFORMATION OF ILLITIC CLAY MINERALS

Abstract

We studied the effect of K+ fertilizer application on the transformation of clay minerals in soils from long-term field experiments and exhaustive cropping pot experiments. Soils used were alluvium, gray-brown, and brown soils with illite as the principal clay mineral. X-ray diffraction analysis of soil samples revealed that cropping without K+ fertilizer application had led to a substantial decrease in illite content and to an increase of smectite and interstratified illite/smectite minerals. In the alluvium soil containing carbonate, stabile smectite formed, probably because of the high soil pH, whereas in the other soils with lower pH, degradation of illite resulted in the formation of interstratified illite/smectite minerals. Illite degradation was especially intensive in the <0.06-μm and <0.2-μm particle size fractions. We suggest that the removal of K+ by plants resulted in a depletion of interlayer K+ in illite followed by the degradation of the clay mineral. Pot experiments with clover and ryegrass showed that the K+ uptake of the crop from the nonexchangeable fraction (interlayer K+) was quantitatively related to the increase in the dry K+-fixation capacity of the soil.