Anisotropic shrinkage and swelling of some organic and inorganic soils X. P ENG & R. HORN Institute of Plant Nutrition and Soil Science, Christian-Albrechts-Universita ¨ t, Olshausenstrasse 40, 24118 Kiel, Germany Summary Shrinkage and swelling affect hysteretic changes of soil volume as a function of water content. The pro- cesses complicate the precise description of changes to soil structure during cycles of wetting and drying, particularly the interpretation for pore structure from water retention characteristics. We investigated soil shrinkage and swelling curves and the hysteresis of void ratio as a function of moisture ratio of four soils (Eutric Histosol, Histic Gleysol, Calcic Gleysol and Dystric Gleysol), and illustrated the anisotropy of soil shrinkage and swelling in two dimensions. We used a model we had developed previously to fit and to define the endpoints of different shrinkage or swelling ranges. The two organic-rich soils showed only structural and proportional shrinkages, whereas the two inorganic Gleysols covered all four shrinkage ranges. Of all the shrinkage ranges, proportional shrinkage contributed to 60.2–96.8% of total decrease in volume. A typical soil swelling curve was composed of two distinct parts, which were defined as virgin swelling and residual swelling with separation of the wet-side maximum curvature. The virgin swelling accounted for 81.2–91.9% of the total increase in soil volume. We found pronounced hysteresis during shrinkage and swelling. A single cycle of wetting and drying lost 10–78% of the soil volume. Shrinkage of the organic-rich soils was significantly greater than that of inorganic Gleysols. The geometry factor, depicting the relationship between the vertical and horizontal deformation, increased from 1.0 to 3.2– 4.0 during drying but decreased from 4.6–21.7 to 3.5–5.6 during wetting. The dependence of water content on the geometry indicates anisotropic shrinkage and swelling of the soils. Our results improve the under- standing of the alteration of soil structure during shrinkage and swelling and can be also applied to predict hydraulic properties more accurately. Introduction A quantitative description of the volume changes that occur in soil during wetting and drying is fundamental to the influence of shrinkage and swelling on soil structure, storage of air and fluids, and the fluxes of water, heat and solutes. Volume changes alter the pore-size distribution by rearranging particles and aggre- gates, and modify the three-phase structure. Shrinkage or swell- ing curves or both are usually used to analyse the variation of soil structure with water content. The soil shrinkage curve is described by the void ratio (e) and the moisture ratio (q) (McGarry & Malafant, 1987; Groenevelt & Grant, 2001, 2002). Shrinkage can be plotted as a curve across the whole range of soil water content. A typical shrink- age curve is sigmoid and composed of four characteristic zones from the wet side to the dry side: (i) structural shrinkage, (ii) proportional (basic) shrinkage, (iii) residual shrinkage, and (iv) zero shrinkage (Figure 1). These four regions indicate dif- ferent structural rigidities of the soil as it dries. The segment from saturation to the macropore shrinkage limit is defined as structural shrinkage, where large water-filled pores become fil- led with air with little change in volume as the soil dries. The region ranging from the macropore shrinkage limit to the air entry point (Sposito & Gira´ldez, 1976) is called ‘basic shrink- age’, as discussed by Mitchell (1992). We prefer to call it ‘pro- portional shrinkage’, as proposed by Groenevelt & Grant (2001), because the decrease in soil volume is proportional to the volume of water lost (McGarry & Malafant, 1987). In cases where the soil dries intensely, the residual shrinkage zone may be truncated. Zero shrinkage is defined from the shrink- age limit to the dry endpoint, where the volume of soil remains rigid or nearly constant although water is totally lost from the soil. Numerous models have been used to describe the soil shrinkage curve (Groenevelt & Bolt, 1972; McGarry & Malafant, 1987; Braudeau et al., 1999; Chertkov, 2003). For present pur- poses we have chosen the three-parameter model recently Correspondence: X. Peng. E-mail: xh.peng@soils.uni-kiel.de Received 16 February 2005; revised version accepted 31 January 2006 98 # 2006 British Society of Soil Science European Journal of Soil Science, February 2007, 58, 98–107 doi: 10.1111/j.1365-2389.2006.00808.x