Saline Borehole Water and a Sandy Plot
A farmer irrigates a 0.4-hectare vegetable plot from a borehole. Over two seasons, yields of lettuce and beans have declined, and white crusts appear on the soil surface between beds. The soil is sandy and drains quickly. The borehole water has an electrical conductivity of 3.2 dS/m. The farmer applies 40 mm of irrigation three times a week and adds 2 kg of urea per bed each month. There is no drainage ditch, and the plot sits in a slight depression. The farmer wants to grow tomatoes next and asks whether to continue as before.
1. Diagnose the problem and explain the role of each observation.
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The white crust and declining yields are consistent with secondary salinisation. The water at 3.2 dS/m is moderately saline; applied three times a week at 40 mm, that is 120 mm per week, which is a large salt load. Because the plot is in a depression with no drainage, salts are not leached away but accumulate as water evaporates. Sandy soil drains fast, but the depression traps water and the water table may be shallow, so capillary rise can bring salts to the surface. Urea adds nitrogen but not salt in large amounts; the main salt source is the irrigation water. The combination of saline water, high application frequency and poor drainage explains the crust and the yield decline.
2. Recommend changes to irrigation and drainage, with reasoning.
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First, reduce the frequency and apply fewer, deeper irrigations so that water moves below the root zone and carries salts with it, rather than frequent light applications that leave salt near the surface. Second, install a drainage ditch or shallow drains to remove excess water from the depression, which is essential for any leaching to work. Third, where possible, blend the borehole water with a lower-salinity source such as rainwater harvested from roofs. Fourth, monitor soil electrical conductivity periodically. Leaching requires that drainage exists; without it, adding more water worsens waterlogging and capillary rise. The sequence matters: drainage first, then leaching, then adjusted irrigation scheduling.
3. Advise on the tomato crop and on the urea programme.
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Tomatoes are moderately sensitive to salinity, so the farmer should choose a tolerant variety and expect reduced yields unless salinity is reduced. Seedlings are most sensitive, so planting on raised beds and irrigating to leach before planting helps. The urea programme should be reviewed: 2 kg per bed monthly is a large, concentrated dose that can burn roots and is easily lost from sandy soil. Split applications and incorporate them, or use a balanced fertilizer that supplies phosphorus and potassium, which help the plant cope with salinity. Excess nitrogen without potassium can make plants more succulent and susceptible to stress. The farmer should not continue as before.