Pore shrinkage dependency of inorganic and organic soils on wetting and drying cycles

Cycles of wetting and drying (WD) occur naturally in soils and affect the pore structure through altered hydraulic stresses. Two organic-rich soils, a Eutric Histosol and a Histic Gleysol, and two inorganic soils, a Calcic Gleysol and a Dystric Gleysol, ranging in texture and microstructure, were investigated. Undisturbed soil samples were predried to either -100 kPa water potential by using a ceramic plate or to 30 C by using an oven and then resaturated for one or three WD cycles. In addition, different combinations defined by the intensity, frequency, and sequence of WD cycles were analyzed. Soil structure was altered significantly if the intensity of drying was severe at 30 degrees C, while drying to -100 kPa had only a small effect. The frequency and sequence of WD cycles did not alter the structure and shrinkage behavior significantly. Compared with the initial pore volume, intense WD cycles decreased it by 23.6 to 60.1% in the two organic-rich soils, whereas it increased by 1.5 to 4.8% in the silty Calcic Gleysol and by 3.6 to 15.1% in the clayey Dystric Gleysol. Both organic-rich soils showed more shrinkage but less swelling than did the two inorganic soils. Intense WD cycles altered the water potential vs. void ratio curves of the two organic-rich soils more gradually, while steeper patterns were observed for the two inorganic soils. This study shows that the changes in soil structure and pore shrinkage depend mostly on the maximum intensity of previous WD cycles.