Greenhouse Tomato Salinity and Fertigation Decision
A 0.8 ha greenhouse tomato crop is drip-irrigated with water from a borehole. The system has 2 L/h emitters at 30 cm spacing and runs four 45-minute cycles per day. The water analysis shows EC 1.9 dS/m, sodium 210 mg/L, chloride 180 mg/L, bicarbonate 140 mg/L and pH 7.8. The grower uses a two-part fertigation recipe with potassium chloride and ammonium sulphate, targeting EC 2.8 dS/m in the drip. Over six weeks, fruit set has fallen from 82% to 61%, leaf margins show necrosis, and root-zone EC from suction cups has risen to 5.2 dS/m. Drainage fraction is only 8%. The grower wants to increase fertilizer concentration to improve fruit size. Night temperature is 16 C and daytime RH is 78%. The crop is at second truss. The water source cannot be changed, but a 30% reverse-osmosis blend is available.
1. Diagnose the cause of the declining fruit set and leaf necrosis. Compare osmotic stress, specific-ion toxicity, and a micronutrient problem, and state what additional measurements would confirm the diagnosis.
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Several observations point to salt accumulation rather than a simple fertilizer shortage. The irrigation water already carries EC 1.9 dS/m, about 9.1 mmol/L Na and 5.1 mmol/L Cl. With only 8% drainage, the root zone has concentrated to EC 5.2 dS/m, so the crop is exposed to both a high osmotic potential and elevated Na/Cl. Tomato fruit set is sensitive to osmotic stress; the fall from 82% to 61% fits that pattern. Marginal necrosis on older leaves is more consistent with Na/Cl accumulation than with Ca deficiency, which typically affects young leaves and fruit. Increasing fertilizer would raise EC further and worsen the problem. Confirm with: (i) saturated paste or 1:5 extract from the root zone; (ii) leaf tissue Na and Cl compared with local norms; (iii) drainage EC and volume per emitter; (iv) emitter flow uniformity; and (v) pH and bicarbonate in the drip. A micronutrient check is secondary; no evidence points to Fe or Mn deficiency.
2. Calculate an approximate leaching fraction to move the root-zone EC toward 3.0 dS/m. State assumptions. Then estimate the blended water EC with 30% RO permeate (assume 0.1 dS/m) and explain whether that alone is sufficient.
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Use a conservative steady-state leaching equation: LF = ECw / (5 ECe - ECw). With ECw = 1.9 and target ECe = 3.0, LF = 1.9 / (15.0 - 1.9) = 1.9/13.1 = 0.145, i.e. about 15% drainage. The 5 factor is an approximation; for drip localized leaching a higher value, 20-25%, is safer. Current drainage is 8%, so salt is accumulating. Blending 30% RO permeate: ECblend = 0.7 x 1.9 + 0.3 x 0.1 = 1.36 dS/m. Recalculating LF = 1.36/(15.0 - 1.36) = 0.10, about 10%. That is lower, but still above the present 8% and does not address the Na/Cl load or the fertilizer salts. Also, RO blend reduces but does not eliminate Na and Cl. It alone is not sufficient; the drainage fraction must be increased and the fertilizer recipe changed.
3. Recommend a revised irrigation and fertigation plan for the next two weeks. Include recipe changes, acidification, and monitoring, and justify each change.
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First, stop increasing fertilizer. Extend or add one irrigation cycle to achieve 20-25% drainage; measure drainage volume from catch cups under representative emitters. Switch the K source from KCl to potassium nitrate or potassium sulfate, and the N source from ammonium sulfate to calcium nitrate, to cut Cl and avoid ammonium competition with Ca. Keep target drip EC near 2.5-2.8 dS/m initially, not higher, and check that the Ca:Na ratio is adequate. Acidify to pH 5.5-6.0 to convert bicarbonate; at 140 mg/L HCO3, acid injection is needed to prevent precipitates and improve Ca availability. Use the 30% RO blend if it can be supplied continuously; do not alternate abruptly. Monitor daily: irrigation and drainage EC/pH, drainage fraction, and emitter uniformity. Weekly: leaf tissue Na/Cl and root-zone extract. If Na remains high, consider gypsum only with a soil-structure test, because gypsum adds salinity. Reassess fruit set after two weeks.