Blossom-end rot in a hot shade-house tomato crop
Ms T. manages a 0.15 ha shade-house tomato crop near Rundu. There are 2,400 plants in sandy soil, irrigated by drip with 2 L/h emitters, one per plant. The timer runs two 20-minute cycles per day. The fertigation solution has EC 3.0 dS/m and pH 6.4; soil calcium is 150 mg/kg, above the critical level. During a hot spell (38°C, low humidity), many fruits on the first truss develop a dark, leathery patch at the blossom end. The patch is not soft or foul-smelling. Leaves look healthy, but the topsoil is dry to 10 cm and roots are concentrated near the emitters. The farmer proposes spraying calcium nitrate foliar twice a week and reducing irrigation to stop the fruit rot.
1. Diagnose the disorder. Calculate the current water application per plant per day and compare it with likely peak demand for tomatoes in hot weather.
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Diagnosis: blossom-end rot (BER) caused by poor calcium transport to the fruit, not by lack of soil calcium. Current application = 2 L/h × (20/60 h) × 2 cycles = 1.33 L/plant/day. That is very low for a tomato crop at 38°C; peak demand is commonly 2.5–4 L/plant/day. The dry topsoil and roots near the emitter support under-irrigation. Calcium moves in the xylem with water, so when transpiration is high and water supply is limiting, young fruit at the blossom end receive little calcium even when soil Ca is 150 mg/kg. The dark leathery patch with no soft rot is typical. The problem is irrigation/water relations, not a soil Ca shortage.
2. Explain why calcium spray and reduced irrigation are unlikely to solve the problem, and outline a corrected irrigation/fertigation plan.
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Foliar calcium nitrate will not reliably reach the fruit because calcium is largely phloem-immobile and the spray covers leaves, not fruit. Reducing irrigation would make the deficit worse. A better plan: increase cycle frequency and duration gradually, e.g. from two 20-min cycles to four to six shorter cycles, and measure output with a catch can to confirm 2.5–3.5 L/plant/day at peak. Check emitter uniformity and pressure; one blocked emitter can cause local BER. Keep the root zone moist but not waterlogged, mulch to reduce evaporation, and avoid pushing EC above about 2.5 dS/m in the root zone. Maintain a balanced Ca:K ratio and monitor new trusses. Foliar Ca can be a minor supplement, not the main fix.
3. The farmer wants to add more calcium to the fertigation. Using the soil test, evaluate whether this is justified.
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Adding more calcium to the fertigation is not justified by the soil test. Soil Ca is 150 mg/kg, above the usual critical level, and the fruit symptom is caused by transport, not supply. Extra Ca may raise EC and compete with K or Mg without solving the problem. First check water quality and root-zone EC, because a high EC or fluctuating moisture restricts water uptake. If a substrate test shows low Ca or a very high K:Ca ratio, adjust the fertigation recipe; otherwise invest in irrigation scheduling and emitter checks. Re-test after correcting moisture before buying more calcium.