Technical brief · For growers, technical growers and controlled-environment agriculture engineers · Published: September 30, 2026
Table of Contents
Key Takeaways
- Clean source water typically reads 0–0.6 mS/cm; once fertilizers go in, the fertigation solution lands at 1.5–2.5 mS/cm, which makes EC the concentration dial of the whole system [1]
- Water used to mix fertilizer should sit at pH 5.0–7.0, and the mixed solution is usually trimmed to about 5.5 so the root zone settles near 6.0–6.5 [1]
- Alkalinity is best held at 0–100 ppm CaCO3, with 30–60 ppm the working target for most crops [2]
- Plugs and seedlings are the salt-sensitive stage, and they want the leanest water the site can supply — typically 0.5 mS/cm or under [3]
- Pythium, Phytophthora and several viruses travel easily in recirculation water, so return water gets disinfected before it re-enters the tank [4][5]
- pH matters mostly through nutrient uptake; drift in the root zone shows up first in phosphorus, magnesium and manganese availability [3]
Why Recirculation Changes Everything
A drain-to-waste greenhouse can get away with mediocre water, because every irrigation event pushes yesterday’s problems off the bench. A recirculating house cannot. Every drop of return water — carrying dissolved fertilizer, leached ions, root exudates and whatever microorganisms came in with the last crop — lands back in the mixing tank and goes out again. Recirculation is the right call environmentally and economically. It also turns irrigation water from a consumable into a process stream that has to be measured, corrected and disinfected like any other closed industrial loop.
That reframing has a practical consequence. The greenhouse needs monitoring points, target ranges and alarms, in the same way a cooling loop or a plant factory does. What follows maps the parameters, the control levers and the sensor layout.
EC: The Concentration Dial
Electrical conductivity is the workhorse measurement because it tracks total dissolved ionic load — and in a fertigation system, most of that load is nutrients somebody paid for. Clean water runs 0–0.6 mS/cm; with fertilizer added, solutions typically range from 1.5 to 2.5 mS/cm [1]. High EC means salt is accumulating. Low EC may mean the crop has eaten the nutrients out of the solution [1].
Managing EC in a closed loop is really a dilution-and-dosing problem. Plants take up water and nutrients at different ratios, so return water is rarely balanced: sodium, chloride and bicarbonate build up while nitrogen and potassium fall away. Growers handle this by throwing away a fraction of the return stream and re-dosing the rest. Continuous EC measurement at the mixing tank and on the return line is what separates deliberate fractionation from blind averaging.
Crop stage sets the target. Young plants in small cells are the most salt-sensitive, and extension guidance keeps their water lean — 0.5 mS/cm or below is the usual recommendation for plugs and seedlings, which matters because some regional sources test an order of magnitude higher [3]. Mature crops tolerate and need more. Either way, the EC setpoint is a crop-stage decision, and it drifts with season and growth rate.
pH and Alkalinity: Two Numbers, One Story
The most common analytical mistake in greenhouse water is treating pH and alkalinity as one parameter. They measure different things. pH is the instantaneous hydrogen ion concentration. Alkalinity is the water’s capacity to neutralize acid — the bicarbonates and carbonates in solution, usually expressed as ppm CaCO3 [2].
For fertigation, water used to mix fertilizers should be pH 5.0–7.0, and the solution is commonly adjusted to about 5.5 so the root environment stabilizes around 6.0–6.5 [1]. Step outside that band and you get micronutrient lockout or toxic metal release [1].
Alkalinity is the number that actually moves root-zone pH over time, because bicarbonate behaves like a slow-drip liming agent on the substrate. The desirable range for irrigation water is 0–100 ppm CaCO3, with 30–60 ppm considered optimum for most plants [2]. High-alkalinity water pushes growing-medium pH upward across weeks, and that suppresses phosphorus, magnesium and manganese uptake even when the fertilizer recipe is right [3]. The control levers are acid injection at the mixing tank, plus blending high-alkalinity source water with rainwater or RO permeate.
| Parameter | Typical target range | Control lever |
|---|---|---|
| EC, source water | 0–0.6 mS/cm [1] | Source selection; blending with RO or rainwater |
| EC, fertigation solution | 1.5–2.5 mS/cm [1]; ≤ 0.5 mS/cm for plugs/seedlings [3] | Fertilizer dosing; discard fraction on return water |
| pH, irrigation water | 5.0–7.0 [1][2] | Acid dosing at the mixing tank |
| pH, root zone | ~6.0–6.5 [1] | Alkalinity management; acidic fertilizer formulations |
| Alkalinity | 0–100 ppm CaCO3; optimum 30–60 ppm [2] | Acid injection; water blending |
| Pathogens | Not detectable in return flow | UV, ozone, heat treatment, slow sand filtration [4][5] |
One warning worth repeating straight from the extension literature: sodium-based softening has no place in greenhouse water treatment. A conventional softener swaps calcium and magnesium for sodium, and elevated sodium is a crop problem, not a fix [3]. Where hardness or salinity has to come down, reverse osmosis, rain harvesting or source blending are the horticulturally sound routes.
Pathogens: The Invisible Load
Waterborne root pathogens are the reason recirculation without disinfection eventually fails. Research going back decades shows that most root-infecting pathogens persist in soilless systems, and that Pythium, Phytophthora, cucumber green mottle mosaic virus, tomato mosaic virus and the viral vector Olpidium move readily in recirculation water — hence the requirement to disinfect before reuse [4]. Surface water, recirculated irrigation water and stored runoff are commonly contaminated, and without sanitation the organisms spread quickly through the irrigation system [5].
The established options each trade cost, footprint and residue:
UV irradiation. Non-chemical, works against oomycetes and viruses, but only in clear water. Suspended solids shield organisms from the dose, so pre-filtration and lamp maintenance are part of the system, not accessories.
Ozone. Strong oxidation delivered inline with no crop-active residue left behind. It suits closed-loop fertigation because it treats irrigation and drain water without adding chemicals [5].
Heat treatment. Pasteurizing return water kills essentially everything, viruses included. The trade is energy, which is why it fits best where waste heat is recoverable.
Slow sand filtration. Biological treatment at almost no operating cost, paid for in footprint and low flow rates.
Whichever route a site picks, the design rule doesn’t change: disinfection must see the entire return stream before it re-enters the mixing tank, and its performance has to be verified by measurement rather than assumed.
Where to Put Sensors and How to Alarm
A recirculating greenhouse lives or dies by four measurement points.
1. Source water. EC, pH, alkalinity and hardness, either continuously or per batch. Source quality shifts with season and with the treatment plant’s own practice, and every downstream setpoint inherits those shifts.
2. The mixing tank. EC and pH are the two non-negotiables here, because this is the last point where the grower controls what the crop receives. Alkalinity correction and fertilizer dosing both act here.
3. The return stream. Return-line EC is the crop’s report card. A steady gap between supply and return EC says uptake is working; a converging pair says salts are piling up faster than the crop can use them. This is also the sampling point that feeds the disinfection step.
4. Post-disinfection water. Whatever the treatment method, a verification reading — UV dose or intensity, residual oxidant for chemical routes — closes the safety loop before water goes back into circulation.
Threshold alarms turn those readings into protection. A practical alarm layer holds EC inside the crop-stage band, holds pH within ±0.3–0.4 units of setpoint, and treats any unexpected shift in return-flow parameters as an early warning of root-zone stress or a fault in the irrigation logic. On the data side, the sensor layer should speak the greenhouse controller’s language: a digital output into the climate or fertigation computer, not an isolated display standing in the corner. Weeks of correlated EC, pH and climate data are what let a grower close the loop between what the crop took up and what gets dosed tomorrow.
On the instrumentation itself, integration quietly pays. A multi-parameter probe that covers pH, conductivity, ORP and temperature in one body replaces four fittings, four calibration routines and four failure points, and over five years that usually outweighs the price difference on any single sensor.
A Practical Commissioning Checklist
- Test source water fully — EC, pH, alkalinity, hardness, sodium, chloride — before setting any fertigation recipe.
- Set EC and pH alarms by crop stage, and tighten them for plug and seedling production [1][3].
- Measure return-flow EC continuously, and manage the discard fraction deliberately rather than by feel.
- Neutralize alkalinity with acid at the mixing tank, targeting the 30–60 ppm CaCO3 optimum [2].
- Never reuse return water without disinfection, and verify the disinfection step with its own measurement [4][5].
- Keep sodium out of the water loop — no conventional softening on irrigation water [3].
Recirculating irrigation is a closed economy: what the water carries comes back, every time. Measuring EC, pH and pathogen load at the right points, then acting on those numbers with dosing, blending and disinfection, is what turns that closed loop from a disease and salt trap into the most water-efficient way to grow.
References
- Filtox — Greenhouse Water Management: typical EC ranges for clean water (0–0.6 mS/cm) and fertigation solutions (1.5–2.5 mS/cm), pH targets for fertilizer mixing (5.0–7.0) and root zones (~6.0–6.5), plus TDS guidance for seedlings and mature crops. http://www.filtox.com/applications/greenhouse-water-management
- UMass Amherst Extension — Water Quality for Crop Production: alkalinity definition, desirable irrigation-water range of 0–100 ppm CaCO3 and optimum of 30–60 ppm, and the requirement to test both pH and alkalinity. https://www.umass.edu/agriculture-food-environment/greenhouse-floriculture/greenhouse-best-management-practices-bmp-manual/water-quality-for-crop-production
- Government of Saskatchewan — Water Quality in Greenhouses: EC guidance for plugs and seedlings, alkalinity as the main driver of growing-media pH, and the effect of pH shifts on phosphorus, magnesium and manganese uptake. https://www.saskatchewan.ca/business/agriculture-natural-resources-and-industry/agribusiness-farmers-and-ranchers/crops-and-irrigation/horticultural-crops/greenhouses/water-quality-in-greenhouses
- Acta Horticulturae — A Review of Possibilities for Disinfection of Recirculation Water from Soilless Cultures: transmission of Pythium, Phytophthora, viruses and Olpidium via recirculation water, and the requirement to disinfect before reuse. https://www.actahort.org/books/382/382_25.htm
- Master Plant-Prod — Water Sanitation Methods in Greenhouse Production: contamination of surface water and recirculated irrigation water by waterborne pathogens such as Pythium and Phytophthora, and sanitation options including ozone. https://www.plantprod.com/?p=3971
- Shanghai ChiMay — Company website: online water quality analyzers, multi-parameter pH/conductivity/ORP/temperature sensors and controllers for irrigation and water-treatment applications. https://www.chimaytech.com/
About the author: Written by the Shanghai ChiMay Technical Editorial Team — the group behind Shanghai ChiMay’s multi-parameter water quality analyzers, writing from daily work on irrigation, fertigation and water-reuse monitoring in greenhouses and controlled-environment agriculture.
