Knowledge about soil shrinkage improves the understanding and prediction of unsaturated hydraulic properties in nonrigid soils. Until now, there is no general model available, which is widely accepted and applied to quantify soil shrinkage curves. The objectives in this present paper are (i) to propose a new and simple model and to test it with a wide range of soil types; (ii) to mathematically distinguish the shrinkage zones; and (iii) to evaluate probable physical meaning of its parameters. The results show that the modified van Genuchten water retention curve model fits the data obtained from Reeve and Hall's six soils, Talsma's three soils and three Typic Chromexert' soils well and the correlation coefficients in the tested 12 soils are always higher than 0.995. The different shrinkage zones defined by the mathematical method agree well with actual soil shrinkage curve. The proportional shrinkage zone accounts for 30.9 to 79.9% of the total water loss and 63.5 to 93.9% of the total volume decrease, while the zero shrinkage zone only accounts for <2.7% of the total volume decrease. The α, n, and m parameters of the modified van Genuchten model control soil shrinkage curve.

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°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.

Volcanic soils in southern Chile cover approximately 60% of the arable land in the country. These soils are under a wide range of land uses from pristine systems to intensively used ones. The objective of this study was to determine the effect of the land use change on: i) the structural stability after external and internal forces, ii) the hydraulic conductivity and iii) its functional resilience. Disturbed and undisturbed soil samples were collected at 5, 20 and 40 cm depths in an Andisol (Typic Hapludand) under native forest (NF), 50 year old pasture (P50) and 1 year old pasture (P1). The water retention, shrinkage and consolidation curves, hydraulic conductivity (Ks), air permeability (Ka), organic carbon content (CO), soil texture and allophane contents were determined. The unsaturated hydraulic conductivity (Ku) was estimated according to van Genuchten (1980). Soil deformation indexes as a consequence of mechanical (COLEm) and hydraulic (COLE h ) stresses were calculated. The studied soil is subject to continuous changes in their structure as a consequence of mechanical and hydraulic stresses affecting the pore size distribution and its functionality. The magnitude of these changes decreased with the increasing intensity of the land use. The great shrinkage capacity of the soil may have consequences on its hydraulic behaviour depending on the drying intensity through the formation of cracks and preferential path flows. Finally, the soil pores are able to recover its functional integrity after compaction, which is strongly related to the presence of organic matter contents.Keywords: Andisol, structural properties, pore functions RESUMENLos suelos volcánicos del sur de Chile constituyen cerca del 60% del suelo arable del país. Estos suelos se encuentran bajo un amplio rango de usos desde sistemas prístinos hasta otros con un uso intensivo. El objetivo de este trabajo fue determinar el efecto del cambio de uso de suelo sobre: i) la estabilidad del suelo frente a presiones externas e internas; ii) la conductividad hidráulica y iii) su resiliencia funcional. Muestras disturbadas y no disturbadas de suelo fueron recolectadas a 5, 20, 40 cm de profundidad 190 Rev. Cienc. Suelo Nutr. / J. Soil. Sci. Plant Nutr. 9(3): 190-209 (2009)

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