Saline–sodic borehole water on a Hardap tomato plot
Mr K. farms 1.5 ha of tomatoes on a sandy loam plot in the Hardap irrigation scheme. He irrigates by furrow from a borehole. The water analysis shows EC 310 mS/m (3.1 dS/m) and SAR 16. A soil sample from the worst patch gives pH 8.8, ECe 5.6 dS/m, ESP 14% and organic matter 0.5%. Tomato plants at flowering are uneven: patches wilt at midday, leaves have scorched margins, and a white crust forms along the furrows. Water stands in the furrows for several hours and the surface seals after drying. The farmer has applied 300 kg/ha of ammonium sulphate and plans to spread 2 t/ha of gypsum, then leach with an extra 30% water. He asks whether this will solve the problem before the next irrigation.
1. Interpret the water and soil results. Is the main problem salinity, sodicity or both? Explain how each contributes to the symptoms.
Read the worked answer
Diagnosis: both salinity and sodicity, with salinity currently the bigger yield threat. Tomato starts losing yield above an ECe of about 2.5 dS/m. At ECe 5.6 dS/m the loss is roughly (5.6 − 2.5) × 10% ≈ 31%, which fits the patchy wilting and scorched margins. The white crust is salt left at the surface. The water is also slightly saline at 3.1 dS/m. Sodicity is shown by SAR 16 and ESP 14%: sodium disperses clay, seals the surface and causes the long ponding and poor infiltration. Poor infiltration then keeps salts in the root zone. The two problems reinforce each other, so adding water alone will not cure the plot.
2. Calculate the leaching fraction needed to bring the root-zone ECe to the tomato threshold of 2.5 dS/m using the water EC of 3.1 dS/m and the formula LR = ECw/(5ECe − ECw). Evaluate whether the farmer can apply it with furrow irrigation.
Read the worked answer
LR = ECw/(5ECe − ECw) = 3.1/(5×2.5 − 3.1) = 3.1/9.4 = 0.33. He needs about 33% more water than the crop uses, i.e. 33 mm extra per 100 mm of crop water, to hold ECe near 2.5 dS/m. His planned 30% is close but slightly low. More importantly, furrow irrigation on a sealed soil cannot apply a uniform leaching fraction: water ponds, evaporates and leaves salt, while deeper drainage may raise the water table. Leaching only works if there is free drainage. He should first improve infiltration (gypsum, organic matter, avoid over-cultivation when wet), install or test drainage, and then leach in split applications. If drainage cannot be improved, he should reduce the area of tomatoes or switch to a more salt-tolerant crop.
3. Evaluate the gypsum and ammonium sulphate plan. State the order of operations and one indicator to monitor.
Read the worked answer
Gypsum is appropriate for sodicity because it supplies Ca²⁺ to displace Na⁺ from the exchange complex, but it must be incorporated into the topsoil and followed by water; surface application without infiltration will not work. The 2 t/ha rate should be checked against the soil test and CEC rather than guessed. Ammonium sulphate is not a substitute: it acidifies and adds salinity, but supplies no calcium to displace sodium. Applying it before leaching adds salt to an already saline soil. Order: test/confirm gypsum rate, incorporate gypsum and organic matter, improve drainage, leach in stages, then re-test. Monitor ECe, ESP/SAR and infiltration rate, not just visual crust. Do not expect a single season to fix ESP 14%.