Water characteristic curve and physical quality of soil as influenced by water salinity and sodicity
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This study was done to explore the effect of water salinity and sodicity on soil water characteristic curve (SWCC) of two non-saline and non-sodic agricultural soils: a clay (with clay 41.4%, silt 20.0% and sand 38.6%) and a sandy clay loam (with clay 27.1%, silt 19.3% and sand 53.4%) from Hamadan province, western Iran. All combinations of water EC values of 0.5, 2, 4 and 8 dS m-1 and SAR values of 1, 5, 13 and 18 were applied (in total 16 solutions). Pure NaCl and CaCl2 salts were used to prepare the solutions. Distilled water was also used as control. The soils were poured into cylinders to have an initial similar void ratio of 1.2. In total, 17 solutions × 2 soils in 2 replicates (total 68 soil cores) were prepared. The soil cores were saturated from the bottom with the corresponding solutions for 24 hrs to complete the ion exchanges and were drained for 5 hrs on a ceramic plate under vacuum suction of 100 cm. The wetting and drying processes were repeated 5 times. After that, soil cores were again saturated with the corresponding solutions. The SWCC of the soil cores was measured at matric suctions of 2, 5, 10, 20, 50, 70 and 100 cm on a sandbox and at matric suctions of 300, 500, 1000, 2000, 5000, 10000 and 15000 cm in a pressure plate apparatus. The van Genuchten-Mualem (VG-M) model was fitted to the SWCC data with the RETC program. Dexter's index of soil physical quality (S) was calculated using the fitted parameters of VG-M model. Water quality could affect the shape and slope of SWCC. Saturated water content and water retention at similar matric suctions were increased with increments of EC and SAR. The effect of water quality on water retention was noticeable at both low and high matric suctions. As water EC increased, the soil particles became flocculated and some new pores were created in soil which finally resulted in higher soil water holding capacity. In contrast, increases of SAR dispersed soil microaggregates and altered some macropores and mesopores to micropores which consequently increased water retention especially at higher matric suctions. The soil physical quality as quantified by S decreased when using low- quality waters (high SAR values). This might be related to soil dispersion and swelling which damaged soil structure. The effect of sodicity on S was not significant for high saline waters (i.e. EC of 4 S m -1 ) because of the improving effect of salinity. The same trend was observed for waters with low salinity (i.e. 0.5 dS m -1 ) due to two opposite processes (soil pores and structural collapse, and creation of new macro- and mesopores or microcracks due to soil shrinkage/separation of soil particles) which simultaneously occur when using non-saline waters. This trend does not imply that soil microstructure was not affected by SAR but means that the S index alone may not be sufficient to assess the effects of quality of low-saline waters on soil physical quality.