Water Quality Effect on Plant-Available Water and Pore Size Distribution of Two Texturally-Different Calcareous Soils

This study was conducted to investigate the effect of water salinity and sodicity on pore size distribution and plant-available water of two clay and sandy clay loam calcareous soils. 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 (in total 16 solutions) were used to wet and dry the soil samples for five cycles. Then، water retention of the soil cores was measured at matric suctions of 0 (θs)، 10 (θ10) 100 or 300 cm (θFC) and 15000 cm (θPWP). The following quantities were calculated: the difference between θ100 or θ300 and θ15000 considered as available water contrent، the θs and θ10 as macrorosity، the θ10 and θ100 as mesoporosity، and the θ100 as microrosity. The initial porosity of both soils was similar، but the greater values of pore indices and θFC، θPWP and AWC were measured in the clay soil due to clay swelling. As water EC increased، mesopores were destructed and altered to macropores and micropores. Salinity altered the mesopores into macropores due to contraction of diffuse double layer and particle’s flocculation and consequently decreased the θFC، and created new micropores which were responsible for the higher value of θPWP. These trends ultimately diminished the AWC. As water SAR increased، mesopores were destructed and altered to micropores but it did not significantly affect the macropores. With increment of SAR، both θFC and θPWP increased due to structural distruption clay swelling and dispersion resulting in increased adsorptive and interlayer surfaces. The increasing effect of SAR on θPWP was greater and more distinct so that AWC was reduced. As a result، high values of SAR of irrigation water decreased the soil available water to plants besides its toxicity and hazardous effect on plants. With increment of irrigation water salinity، the destructive impacts of SAR diminished. The influence of water quality on water retention was pronounced for the clay soil. ‎