Abstract
Four air-dry soil samples from 4 locations in Ehime Prefecture were subjected repeatedly to a series of treatments: water soaking-feeezing at-20°C-thawing-drying at 110°C, and the treated soils were examined for particle-size distribution, ignition loss, plastic and liquid limits, pF-water retention curve, and X-ray diffraction behaviors of clay minerals contained. Three out of the 4 soil samples were from the C horizons (30-40 cm) of upland soils derived from granite, crystalline schist and mixed sandstone and shale, and the rest was from the A horizon (5-20 cm) from a paddy field. The textural class ranged from silty sand (SM) to clay (CL), according to UJSSMFE system.
No change occurred in the particle-size distribution of soils even after a treatment repetition of 70 times, indicating that the treatments caused no disintegration of coarser particles nor aggregation of finer particles. The repeated treatments, however, caused the ignition loss, plastic and liquid limits and water retention at pF's below 3.2 to decrease significantly. The decrease was interpreted in terms of a decline in the reversibility of water adsorption by soil particles, and in terms of a change in the aggregate structure.
It was shown by X-ray diffraction (XRD) analysis of the clay fractions (<2μm) that the hydrazine intercalation ratio (HIR) of halloysite was significantly decreased by the repeated treatments. The HIR herein refers to the ratio of diffraction intensity (peak height) at 10Å to that at 7Åof the hydrazine-treated samples. Its decrease means, therefore, that the layer-to-layer bonding of halloysite has been tightened by the repeated treatments. It was noticed also that the 2: 1 type layer silicates, such as vermiculite and its intergrades to chlorite, collapsed or became liable to be collapsed due to the repeated treatments. Such changes in the XRD behaviors of clay minerals may imply that the repeated treatments have removed the interlayer water and have rendered its re-penetration difficult, and so, were thought to be responsible at least partly for the decline in water adsorption reversibility stated above.
