Salt leaching in fine-grained, macroporous soil: Negative effects of excessive matrix saturation

Irrigation and tile drainage are commonly used to reclaim salt-affected soils, but salt leaching can be hindered in low permeability soils when infiltrating water preferentially flows along vertically connected macropores and bypasses the saline soil matrix. To identify the important controls on salt leaching effectiveness in fine-grained, macroporous soil, a two-year irrigation and drainage field experiment was undertaken to return a salt-affected soil to agricultural production. Salt leaching effectiveness in a 20 × 20-m irrigated test plot was evaluated using time-lapse electrical resistivity tomography, soil sampling, drainage monitoring and subsurface water sampling. Results were compared to an adjacent unirrigated control plot. During the first year of the irrigation experiment, effective leaching resulted in a 23-40% decrease in salt mass over a depth of 0-2.4 m. In the second year, similar amounts of applied water (irrigation + precipitation) resulted in negligible salt leaching due to a rise in the regional water table and wetter soil conditions that limited macropore-matrix interaction. Chloride mass recovery in tile drainage water, which was greater in Year 2 due to upward seepage, was not a reliable indicator of leaching from the root zone. Proper monitoring and control of irrigation and drainage to avoid overly wet soil matrix conditions can increase salt leaching efficiency, giving shorter reclamation timelines and reduced volume of applied water.