THE PLASTIC LIMIT OF SILTY, SURFACE SOILS IN RELATION TO THEIR CONTENT OF POLYSACCHARIDE GEL

By using values reported in the literature, the plastic limits of mainly silty surface soils have been found to increase by 6-7 g H2O per g of C present. To explain this, it is suggested that at the plastic limit, particles of coarse silt are being rolled over each other with compacted portions of fine silt/clay matrix in between. The plastic limit is increased because of water sorbed by bonding polysaccharide gels in additional 1-3 mu m sized pores between the compacted portions. The presence of such pores should have little effect on the matrix suction since this is determined by the size of the largest water-filled pores present. Water sorbed will then be proportional to C content as found, provided the composition of the polysaccharides present is constant and polysaccharide C forms a constant percentage of total C. This allows changes in the plastic limit of a soil to be used to indicate relative amounts of gel present. For example, several years after cropping was changed from grass to arable, more gel was present than in the same soils under continuous arable. Mild peroxidation of one soil did not alter the proportion of the total C present in gel form.