Physical properties of soil surface crusts affected by saline-sodic water

Irrigation with saline water has become a widely diffused practice in the whole Mediterranean area, due to the increase of water demand and reduction in water quality. One major effect is the modification of the soil physical properties and particularly the soil structure, porosity and hydraulic characteristics of the top layer. Objective of this study was to investigate changes in soil physical properties such as pore size distribution by image analysis, the index of aggregate stability in water (IASW) and the swelling/shrinkage effects occurring at the soil surface (crust) resulting from saline-sodic watering after a long time.The experiment with saline-sodic water at different concentrations was carried out over a seven year period on a clay-loam soil in the Volturno Valley at Vitulazio (Caserta province, South of Italy), the treatments were always the same throughout the 7 year period in order to assess the salt accumulation effects on the soil. The long time irrigation has determined the formation of a soil surface crust for 3 irrigation treatments (TA, TB and TC, respectively irrigated with fresh water, moderate salinity and very saline water). The chemical properties in terms of ECe and ESP for each treatment were respectively: 0.86 dSm-1 and 2.33% (TA); 1.22 dSm-1 and 9.72% (TB) and 4.07 dSm-1 and 22.04% (TC). Observations show a clear reduction of the estimated porosity for the TC treatment than for the TA one. Such finding is documented by the higher reduction in pore size on TC than on TA, while an intermediate reduction was observed for the TB treatment. Moreover on TA the higher porosity is due the wide range of pore size classes. Instead the TC crusts show dominant small diameter classes. Again the TB treatment has a intermediate behaviour.Such change on the crust physical properties was also observed on the aggregate stability. In fact for the TC treatment the IASW is lower than for the TA treatment. Moreover, if the quantity of soil dispersed in water after a certain time of shaking is estimated, the TC saline crust has larger dispersed material than the TA treatment and the time in which that occurs is lower than for the no saline crust.Finally, the effects of the salt on the soil crusts are further documented by the shrinkage/swelling characteristics, expressed in terms of void ratio/moisture ratio curves.