Rehabilitating a Saline-Sodic Soil Under Irrigation
A 12-hectare paddock has been irrigated with groundwater for eight years. Soil tests from the top 30 cm show electrical conductivity of the saturated extract (ECe) of 9.5 dS/m, sodium adsorption ratio (SAR) of 22, and pH of 8.6. The soil is a medium clay with an infiltration rate that has fallen from 12 mm/h to 1.5 mm/h. The irrigation water has EC of 1.1 dS/m and SAR of 9. A nearby source of gypsum is available cheaply. The grower wants to return the paddock to a salt-sensitive crop within two seasons. Rainfall averages 550 mm/year, mostly in winter, and the paddock has no subsurface drainage.
1. Diagnose the soil condition and explain why the low infiltration rate is a problem for any remediation strategy.
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Diagnosis. ECe of 9.5 dS/m exceeds the threshold for salt-sensitive crops (commonly around 2 dS/m for sensitive species), so the soil is saline. SAR of 22 with pH 8.6 indicates sodicity: excess sodium has displaced calcium and magnesium on clay exchange sites, causing dispersion of clay aggregates. The combination makes the soil saline-sodic. The collapse in infiltration from 12 to 1.5 mm/h is the physical signature of dispersion blocking soil pores.
Why infiltration matters. Leaching salts requires water to move through the profile and out of the root zone. With infiltration at 1.5 mm/h and no subsurface drainage, added water ponds or runs off rather than flushing salts downward. Applying gypsum alone will not fix this: gypsum supplies calcium to displace sodium, but the displaced sodium and the salts must be carried away in drainage water. Without drainage, leaching simply moves salts to the base of the profile and they can return by capillary rise. Remediation therefore requires both a chemical amendment and a drainage pathway.
2. Outline a remediation sequence the grower could follow, and explain why the order of steps matters.
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Sequence.
- Install subsurface drainage (or at minimum verify an outlet) before leaching. This provides the pathway for salt and sodium removal.
- Apply gypsum at a rate based on the exchangeable sodium percentage, incorporated into the topsoil. Calcium from gypsum displaces sodium on exchange sites, and the sodium leaves with drainage water.
- Leach with good-quality water, applying enough to push salts below the root zone. The leaching fraction must account for the irrigation water EC of 1.1 dS/m, which is itself marginal for salt-sensitive crops.
- Re-vegetate promptly to protect the surface from crusting and erosion.
- Monitor ECe, SAR and infiltration rate each season.
Why order matters. Gypsum applied without drainage cannot lower salinity and may raise it, because the added calcium sulfate contributes salts. Leaching before amendment risks further dispersion: sodic clay with low electrolyte concentration disperses more readily, so flushing with low-salinity water first would worsen the infiltration problem. Applying gypsum first raises the electrolyte concentration and supplies calcium, which stabilises aggregates so that leaching water can actually pass through. The two steps are interdependent, which is why a single-season fix is unrealistic and the two-season target is tight but achievable if drainage is installed early.