Heat, Dry Air and Salt Build-Up in a Gerbera Polyhouse
A 1,000 m2 naturally ventilated polyhouse near Talegaon, Pune, with a 200-micron UV-stabilised film roof and 50% shade net on top, grows gerbera on raised beds at 6 plants/m2 (6,000 plants) in a soil-coco peat mix. Last season output was 25 stems per plant with 55 cm stems and 9 cm flowers, sold at Rs 8 per stem to a Pune wholesaler. This April the inside temperature reaches 41 C at 13:00 against 36 C outside, and relative humidity falls to 22%. Stems are now 32-35 cm, flower diameter 6.5 cm, 8-10% of flowers show bent necks, and yield has dropped to about 17 stems per plant. Fertigation is running at EC 2.8 dS/m and pH 7.4; leachate EC is 4.6. The borewell used for irrigation has EC 1.6 dS/m and sodium 9.8 meq/L. The owner has Rs 5 lakh and is comparing foggers, a fan-and-pad system, and simply adding more shade net.
1. Rank the causes of the drop in stem length, flower size and bent necks, and evaluate the three options the owner is considering.
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Ranking of causes
- Vapour pressure deficit, not temperature alone. At 41 C and 22% RH the air is demanding roughly 6 kPa of vapour - about three times a comfortable gerbera VPD. Stomata close at midday, photosynthesis collapses, cell expansion in the stem and petal tissue is reduced, and calcium (which moves with the transpiration stream) is not delivered to young tissue. Calcium shortage in the flower neck is the direct cause of bent necks.
- Root-zone salinity. Leachate EC 4.6 against an input of 2.8 with water at 1.6 and sodium at 9.8 meq/L means salt is accumulating from insufficient leaching. High root-zone EC reduces water uptake exactly when the plant most needs it, compounding cause 1.
- Nutrition imbalance: pH 7.4 in the drip reduces availability of Fe, Mn and B, and the calcium supply is clearly inadequate.
Options assessed
- Extra shade net: reject. Light is already reduced by 50% shading; more shade lowers photosynthesis and yield further without changing VPD.
- Foggers: best value. They cut temperature by 4-6 C and lift RH into the 60-70% range, directly attacking VPD, at perhaps Rs 2.0-2.5 lakh installed.
- Fan-and-pad: technically superior but heavy. Rs 4.5-5 lakh plus recurring power of Rs 50,000-70,000 a year and a reliable water supply; only justified if the fogger trial proves insufficient.
2. Design the water and fertigation correction for a source with EC 1.6 dS/m and sodium 9.8 meq/L. Show the leaching calculation and the changes you would make to the nutrient programme.
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Water quality first
Sodium at 9.8 meq/L (about 225 mg/L) with EC 1.6 dS/m is a high-sodium source for a salt-sensitive crop like gerbera. Leaching alone cannot solve it; the sodium load keeps rising.
- Blend or dilute. Blend with rainwater harvested from the polyhouse roof or with low-EC canal water to bring irrigation EC to about 0.8-1.0 dS/m before use. This is the decisive step; without it, every corrective measure is fighting a rising baseline.
Leaching fraction calculation
Using LF = ECw / (2 x ECe - ECw) with a target root-zone ECe of 2.0 dS/m for gerbera:
- Unblended water (ECw 1.6): LF = 1.6 / (4.0 - 1.6) = 0.67, i.e. 67% extra water - impractical and would waterlog the beds.
- Blended water (ECw 0.9): LF = 0.9 / (4.0 - 0.9) = 0.29, i.e. about 30% extra water over crop demand. That is workable.
So: apply roughly 1.3 times the crop water requirement, monitor leachate EC and hold it at 2.5-3.0, and never let it climb toward 4.6 again.
Nutrient changes
- Acidify to pH 5.8-6.2 with nitric or phosphoric acid, checking pH weekly.
- Use calcium nitrate and potassium nitrate; replace muriate of potash with sulphate of potash to stop adding chloride and sodium.
- Target calcium around 150 ppm and magnesium around 50 ppm, with boron at 0.5 ppm, since bent neck is a calcium-delivery problem.
- Split feeds into small, frequent doses rather than one strong solution, and flush the drip lines after each cycle.
3. Using the numbers given, work out the payback on the Rs 2.3 lakh fogger installation plus Rs 20,000 a year in running costs, and state the conditions under which you would still recommend the fan-and-pad system.
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The arithmetic
- Current output: 17 stems x 6,000 plants = 1,02,000 stems a year.
- Recoverable output if VPD is corrected: 25 stems x 6,000 = 1,50,000 stems, a gain of 48,000 stems.
- At Rs 8 per stem that is Rs 3.84 lakh additional gross income, before grade improvement. Correcting VPD also raises the share of 55 cm stems, which often fetch a premium, so this figure is conservative.
- Net of Rs 20,000 running cost: about Rs 3.64 lakh a year.
- Payback on Rs 2.3 lakh: about 0.6-0.7 of a year, roughly 7-8 months. Even at half the yield recovery the payback is under 18 months.
Decision: install foggers and fix the vents and leaching at the same time. Foggers without the water correction would push salts around without solving the salinity, and the water correction without humidity control leaves the calcium problem in place. Check eligibility for the protected-cultivation subsidy under MIDH/NHB, which can halve the capital cost.
When fan-and-pad would still be right
- If summer temperatures routinely exceed 42 C and fogging cannot hold VPD, or the site has low wind and poor natural ventilation.
- If a reliable low-EC water source and three-phase power are available, since fan-and-pad consumes much more water and electricity.
- If the crop is changed to a higher-value flower where a two-degree temperature gain justifies the running cost. For this 1,000 m2 unit with Rs 5 lakh in hand, foggers plus water correction leave working capital for the salinity fix; fan-and-pad would consume the entire budget.