Effect of dispersed clay and soil pore size on the hydraulic conductivity of soils irrigated with saline-sodic water

Effect of dispersed clay and soil pore size on the hydraulic conductivity of soils irrigated with saline-sodic water

A shortage of good quality water has led to the use of low quality, high saline-sodic water that was considered unsuitable for irrigation purposes in the past. Using such water can increase the potential of soil degradation, soil pore blockage and consequently reductions in soil saturated hydraulic conductivity (Ks). This can limit crop growth in the long term through the impacts on water and nutrient availability. Most studies use reductions in soil Ks as an indicator of soil pore blockage when saline-sodic irrigation water is used. However, the use of saline-sodic irrigation water requires improved understanding of the of cation exchange processes, soil structural degradation and the potential for soil pore blockage. In order to address this three laboratory trials were conducted to investigate the effect of soil pore size on soil cation exchange processes, dispersed clay movement and the mechanisms of soil pore blockage in relation to changes in soil Ks.
An initial study was conducted on two soils packed into soil cores at two different bulk densities (ρb) and leached with solutions containing varying concentrations of clay sediments. Measurements of soil Ks showed that increasing soil ρb and the concentration of clay sediment in the applied solution significantly increased the percentage of clay sediment retained within the soil columns. This in turn significantly decreased soil Ks particularly for compacted soils. Soil pore blockage occurred near the soil surface in compacted soils while soil pore blockage occurred at depth in soil packed at low ρb. This confirmed that soil pore size has a significant effect on dispersed clay movement and potential soil pore blockage. The soil pore size distribution does not provide information about the mechanism of soil pore blockage. It was hypothesised that clay sediment migration and soil pore blockage occurs in saline-sodic soils and that the soil response will vary depending on clay mineralogy and the quality of irrigation water applied. These issues were addressed in later experiments that used saline-sodic irrigation water applied to soil with different properties. A resin impregnation method was also used to gain a better understand the mechanism of soil pore blockage.

Solutions with different sodium adsorption ratios (SAR) were used to understand the relationship between ion exchange and changes in soil Ks for three soils. A Red Ferrosol (RF), Grey Vertosol (GV) and Black Vertosol (BV) were packed into soil cores at two ρb (1 and 1.2 g cm-3). The rate of ion exchange and reductions in soil Ks during leaching processes were found to be significantly higher in soils with higher proportions of soil macropores (> 30 μm) compared to those dominated by micropores (< 0.02 μm). Further, the correlation between the rate of ion exchange and changes in soil Ks was stronger for the RF soil compared to the BV soil. This indicated that dispersion was the main mechanism for soil Ks reductions in the RF soil while swelling contributed significantly to reductions in soil Ks for the BV soil. Even though the reductions in soil Ks were greater in lower ρb soils, they maintained significantly higher Ks values compared to compacted soil. The measurement of soil exchangeable cations after leaching with high SAR solutions showed a significant increase in soil ESP at the soil surface. The RF soil also reached chemical equilibrium earlier than the GV and the BV soils.

Applications of rain water to the soil cores post-leaching with saline-sodic water led to leaching of soluble cations and significant reductions in the electrical conductivity of the soil solution and soil Ks. These significant reductions were observed just after the first pore volume (PV) of leaching. However, the RF soil maintained higher Ks
values compared to the GV and BV soil. Soils with lower ρb maintained higher Ks values and produced higher concentrations of dispersed clay in the leachate compared to compacted soils.

Image analyses of resin impregnated RF soil taken at depth from soil cores irrigated with good quality water (GQW) and saline-sodic irrigation water showed significant soil pore blockage after the application of saline-sodic water. This confirmed that dispersed clay migrated with infiltrated water. In compacted RF soil the soil pore blockage took place near the soil surface. For the BV soil, pore blockage occurred at and near the soil surface for ρb = 1 and 1.2 g cm-3, respectively. Soil micropores were completely blocked while incomplete blockage was observed in the soil macropores. After applying rain water to soil cores post-leaching with saline-sodic water significant pore blockage occurred at the soil surface in all treatments except for RF packed at low ρb where soil pore blockage was only observed in deeper soil layers. Soil Ks was positively correlated with increasing the soil porosity and soil macroporesity while a strong negative correlation was found between Ks and soil microporosity. Small changes in soil porosity and soil macropores resulted in significant changes in Ks. This research has shown soil pore size has a significant effect on cation exchange processes, dispersed clay movement and the potential for soil pore blockage. However, this interaction differs between soils and is dependent on the dominant clay mineralogy.