Irrigation Salinity in a Drip-Fed Tomato Block
A 6-hectare tomato crop is drip-irrigated from a bore with an electrical conductivity of 2.8 dS/m. The grower applies 5 ML/ha over the season and the soil is a sandy loam with a leaching fraction estimated at only 8%. Mid-season, plants in the lower, poorly drained corner show stunted growth, dark green leaves with marginal scorch, and a white crust on the bed shoulders. A soil test of that corner returns ECe 6.1 dS/m in the top 20 cm, while the well-drained upper block returns ECe 2.2 dS/m. The grower wants to keep the corner in production for the remaining eight weeks rather than abandon it.
1. Diagnose why the two blocks differ despite receiving the same water, and explain the role of the leaching fraction.
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Both blocks receive identical water quality (2.8 dS/m), so the difference is hydrological, not a difference in source salinity. Salt accumulates in the root zone whenever evapotranspiration removes water but leaves salts behind; the only removal pathway is water draining below the root zone. With a leaching fraction of about 8%, roughly 92% of applied water is transpired or evaporated, so salts concentrate by a factor of about 1/0.08 ≈ 12 relative to the drainage water. The poorly drained corner cannot pass water below the root zone, so its effective leaching fraction approaches zero and salts accumulate to ECe 6.1 dS/m. The upper block drains freely and sits at 2.2 dS/m. The white crust confirms capillary rise and surface evaporation concentrating salts at the bed shoulder. The diagnosis is secondary salinisation driven by restricted drainage, not a toxic ion specific to the bore water.
2. Recommend a short-term management plan for the remaining eight weeks and justify each action against the mechanism you identified.
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First, restore downward flux in the corner: apply a controlled leaching irrigation (a pulse of extra water sized to push salts below 20 cm, roughly 100–150 mm in two applications) and check that drainage actually occurs, since adding water without an outlet will raise the water table and worsen the problem. Second, reduce the salt load reaching the plant by shifting to more frequent, smaller drip cycles so the root zone stays moist and concentration at the wetting front is diluted; avoid long dry intervals that let salts concentrate. Third, do not try to 'flush' with the same 2.8 dS/m water indefinitely - blending with lower-EC water or using gypsum where sodic conditions accompany salinity would help infiltration, but gypsum will not remove chloride. Fourth, accept some yield loss in the corner and prioritise harvest quality on the upper block. Monitor ECe weekly to confirm the trend is downward before investing further.
3. The grower proposes installing subsurface drainage across the whole block. Evaluate this proposal.
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Subsurface drainage directly addresses the cause: it provides the below-root-zone outlet that allows a real leaching fraction, so applied water can carry salts away rather than concentrating them. On the poorly drained corner this is the durable fix and would likely bring ECe back toward the 2.2 dS/m seen uphill. However, whole-block installation is questionable because the upper block already leaches adequately at 2.2 dS/m; capital cost and the risk of exporting saline drainage water to a waterway must be weighed. A staged approach is better: install drains in the corner first, monitor ECe and water table depth for a season, and extend only if the upper block shows rising salinity. The proposal also needs an environmental plan for disposal of saline effluent, since draining salt off-site simply relocates the problem.