Impacts of type and concentration of salt cations on soil water retention and desiccation cracking

Abstract: The soil water characteristic curve (SWCC) defines the relationship between soil suction and volumetric water content, and is commonly used to evaluate the size and distribution of soil pores and the soil water availability and holding capacity for investigating the functions of unsaturated soil with various properties and for modeling the transport of soil water and solutes. Besides, the SWCC is usually effectively used to evaluate soil water retention, which is always considered as a basic factor for determining reasonable irrigation schemes. The SWCC can be obtained via centrifuge method; however, soil samples often shrink and crack during the SWCC measurement. Of various influencing factors on water retention, salt ions with different types and concentrations have various effects on water-holding capacity. The present study therefore aims to explore the differences in the characteristics of the soil water retention and the shrinkage and cracking during drying process resulting from 4 types of salt cations with different concentrations. In order to achieve the objectives, silty clay loam from Shaanxi Province was selected as experimental soil. Four replicates of the samples were air dried, sieved through a 2-mm mesh, and then compacted into cutting rings at a bulk density of 1.40 g/cm3. Four powder reagents, namely potassium chloride (KCl), sodium chloride (NaCl), calcium chloride (CaCl2) and magnesium chloride (MgCl2), which included 4 kinds of salt cations (i.e., K+, Na+, Ca2+ and Mg2+), were dissolved in distilled water at the concentration of 5, 30 and 100 g/L respectively. Distilled water was used as the control (CK). And all soil samples were first saturated in the solutions for 48 h before the experiment. The SWCCs were then constructed using a centrifugal method and used to quantitatively assess soil water holding capacity. At the same time, the deposit height of soil samples for each tested soil suction was measured using a vernier caliper, which was used to calculate linear shrinkage ratio and axial shrinkage strain for quantitatively evaluating soil shrinkage during the drying process. Cracks gradually occurred with soil water decreasing and were obtained using digital image processing technique in this paper, which were used to quantitatively evaluate the desiccation cracking based on crack area density and length density. The experimental results showed that first of all, salt cations had no effects on the SWCC shape, and the van Genuchten model was suitable for fitting SWCC saturated by saline solution with the R2 of high than 0.99. Furthermore, the 4 kinds of salt cations could weaken soil water retention, and water-holding capacity was correlated negatively with the concentration of salt cation. According to the fitting results by the van Genuchten model, the 4 kinds of salt cations could also weaken saturated water content, residual water content and air suction, and water-holding capacity was correlated negatively with saturated water content. Besides, K+, Na+ with high concentration and Mg2+ with low concentration were helpful to decrease soil axial shrinkage. And the axial shrinkage strain was correlated negatively with the concentration of K+ (P<0.01), Na+ (P<0.05) and Ca2+ and positively with Mg2+ concentration. Shrink of soils during the drying process was accompanied with the increasing of soil bulk density, and the effect of salt cations ranked in the order of Mg2+>Ca2+>Na+>K+. Finally, the 4 kinds of salt cations were helpful to weaken cracking based on the quantitative analysis of crack area density and length density. And these 2 crack indicators were significantly negatively correlated with the concentration of K+, Na+ and Ca2+ and positively with Mg2+ concentration. These findings are helpful to provide the theoretical and practical guidance for the evaluation of water retention and the determination of irrigation scheme, and to provide some references for developing the engineering practice on saline soils. The further study should add focuses on the effects of salt anions and smaller concentration gradient on soil water retention and desiccation cracking.