How to evaluate greenhouse irrigation and fertigation suppliers: water quality, crop needs and operating cost

Decision tree: what kind of system does your water and crop need?

• Start with the water analysis. Penn State Extension explains how to read irrigation water tests for pH, alkalinity, electrical conductivity (EC) and specific ions (Penn State Extension). High alkalinity points to acid injection. High salinity or problem ions point to treatment, blending or a different source.
• Then the growing system. Soil crops, open (drain-to-waste) substrate and recirculating systems need different filtration, dosing control and drain handling.
• Then the scale and crop mix. One crop with one recipe can use a simpler dosing set-up than several crops or zones with different recipes.
• Then the peak demand. Pumps, filters and the dosing unit must deliver the peak-day volume. Crop water needs are calculated with the FAO-56 method (FAO-56).
• Only then compare suppliers. Each proposal should show how it answers these four points. For the full bid-comparison method, use our supplier RFQ scorecard.

What buyers need to give suppliers

• A recent laboratory analysis of the actual water source, with the sampling date. If you have more than one source, include each one.
• Crop, cultivar type, growing system, planted area per zone and crop calendar.
• Peak-day water demand and planned drain fraction, with assumptions, as in our planning checklist.
• Number of irrigation zones and recipes, and whether drain water will be recirculated.
• Available power, water pressure and space for the fertigation room, and any local backflow or discharge rules.
• The data you want: EC, pH, flow and volume logs, alarms and export format.

Dosing accuracy and calibration: what to check

An injector ratio is not a concentration. UMass Extension explains that a 1:100 setting means one part of stock solution makes 100 parts of final solution, not 100 ppm of nitrogen. Fertiliser for stock solutions should be measured by weight, not volume, and solution colour is not a reliable guide (UMass Extension).

UNH Extension recommends checking injector calibration at least once before each growing season. It describes two methods: the flow method and the EC method. With the flow method, the measured ratio should be within 1–2% of the setting. Its worked example: 18,925 ml of diluted solution ÷ 94 ml of stock = a ratio of 201 (UNH Extension). UGA Extension notes that injectors can also deliver acids, wetting agents and other materials, and that a malfunctioning injector can damage or ruin a crop (UGA Extension).

Ask each supplier how its system measures EC and pH, where the sensors sit, how often they need calibration, and what happens on a sensor fault: alarm, stop, or continuing to dose.

Worked example (illustrative): operating cost differences buyers often miss

Assumptions, for illustration only: 1 ha of substrate crop; annual crop water use of 6,000 m³; annual fertiliser spend of USD 20,000. Your figures will differ. Drain targets must be set by your agronomist and depend on your water quality.

Drainage volume: at a 30% drain fraction, applied water = 6,000 ÷ 0.70 ≈ 8,571 m³. At 20%, applied water = 6,000 ÷ 0.80 = 7,500 m³. The difference is about 1,071 m³ a year of water, plus the fertiliser and pumping energy it carries, before any recirculation.

Dosing error: if an uncalibrated injector over-doses by 10%, fertiliser spend rises by about USD 2,000 a year (10% of USD 20,000). This excludes any crop damage, which can cost far more.

What this tells the buyer: two proposals with similar prices can lead to different running costs. Ask each supplier to state water, fertiliser, energy, filter and sensor-calibration costs under the same assumptions.

What to do and what to avoid

• Do: send every supplier the same water analysis, crop plan and peak-demand figures.
• Do: ask for the dosing method, sensor positions, calibration interval and fail-safe behaviour in writing.
• Do: ask for an annual list of consumables (filters, sensor electrodes, calibration fluids, acid) with quantities.
• Avoid: accepting a fertigation unit sized only from the greenhouse area.
• Avoid: comparing prices when one offer includes water treatment and another assumes clean water.
• Avoid: leaving backflow protection and drain-water discharge to be 'decided on site'.

Common costly mistakes

• Designing from an old or borrowed water analysis that does not represent the actual source.
• Ignoring alkalinity, which can lead to pH drift and nutrient problems.
• Skipping calibration after installation, so the system runs off-target unnoticed.
• Choosing recirculation without budgeting for disinfection, monitoring and the skills to run it.
• Buying a controller whose logs cannot be exported, which makes it hard to trace dosing errors.

Questions to ask suppliers

• Which parameters from our water analysis drove your design, and what treatment do you propose?
• What peak flow and daily volume is the system designed for, and with what assumptions?
• How are EC and pH measured and controlled, and what is the stated dosing accuracy and how is it verified?
• What calibration routine, intervals and spare parts are included, and who trains our staff?
• What water, fertiliser, energy and consumable use do you expect each year under our assumptions?
• How are backflow protection and drain handling addressed under local rules?

Summary

• Water analysis first, crop and growing system second, peak demand third, and suppliers last.
• Compare accuracy, calibration and running cost, not only purchase price.
• Use the irrigation planner and the water and flow calculator to frame demand, then send a structured project request. SeedMatchGroup reviews each request and organises comparable proposals from suitable independent suppliers.

Supplier evaluation matrix

Score each proposal on the same criteria. A missing answer counts as an open risk, not as a pass.

Fertigation proposal checklist

• ☐Recent laboratory analysis of each actual water source attached
• ☐Crop, zones, recipes and growing system defined
• ☐Peak-day demand and drain fraction stated with assumptions
• ☐Treatment scope (filtration, acid, disinfection) defined
• ☐EC/pH measurement, accuracy and fail-safe described
• ☐Calibration routine, interval and training included
• ☐Annual operating-cost sheet under shared assumptions received
• ☐Backflow protection and drain discharge addressed

Frequently asked questions

Related SeedMatch tools

• Greenhouse irrigation planner
• Irrigation water and flow calculator
• Greenhouse operating cost calculator
• Greenhouse bid comparison tool

• Commercial greenhouse planning checklist: crop, climate, structure, utilities and automation requirements
• Greenhouse climate-control systems compared: ventilation, cooling, heating and energy trade-offs
• Low greenhouse marketable yield: diagnose before buying upgrades
• Commercial greenhouse system selection: climate, water and CAPEX
• Greenhouse and irrigation bid comparison: a supplier-neutral RFQ scorecard
• Plan a greenhouse project around crop goals: use free calculators and project tools before sourcing suppliers

When the outcome and constraints are defined, our team reviews your project request (free for buyers, typically USD 250,000+ B2B projects) before approaching suitable independent suppliers and organising comparable proposals. Any financing is arranged only through independent third parties.

Sources

• Penn State Extension — Interpreting Irrigation Water Tests
• FAO Irrigation and Drainage Paper 56 — Crop evapotranspiration (Allen et al., 1998)
• UNH Extension — Fertilizer Injection Calibration (fact sheet)
• UMass Amherst Extension — Fertilizer Injectors for Greenhouses
• UGA Extension Bulletin 1237 — Fertilizer Injectors: Selection, Maintenance and Calibration (2024)
• FAO (2013) — Good Agricultural Practices for greenhouse vegetable crops: principles for Mediterranean climate areas, Plant Production and Protection Paper 217

Extension guidance is general. Crop-specific targets, local regulations and qualified local advisers take precedence.