Drip irrigation and fertigation for greenhouses and nethouses.
Fertigation means feeding dissolved fertiliser through the drip lines, so each plant gets water and nutrients together. If you grow in a greenhouse or nethouse, the decision is what the fertigation unit must include before you buy it: which parts, how fine a filter, how to keep fertiliser out of your water source, and what to check before the first crop. This guide gives you that list and the setup steps.
Written for greenhouse and nursery growers.
How fertigation works
Every fertigation system has three parts: tanks of concentrated fertiliser, called stock solution; a water source; and a way of mixing the two in the right proportion and carrying the feed to the plants. In most greenhouses the mixing is done by an injector, which draws a set amount of stock solution into the water as it passes.
Each feed follows the same order. Bring the drip lines up to full pressure with clean water, inject the fertiliser, wait for the last of it to reach the farthest dripper, then run clean water to flush the lines, so no fertiliser is left inside to block the drippers.
Four words come up throughout:
- Flow: how much water passes in a set time, in litres per hour (L/h) for a dripper, or cubic metres per hour (m³/h) for a pump or a pipe. One cubic metre is 1,000 litres.
- Pressure: how hard the water pushes inside the pipe, measured in bar.
- EC (electrical conductivity): a meter reading of how much dissolved salt the water carries. Most fertilisers raise it, so it shows the strength of the feed. Urea is the exception: it does not show on an EC meter.
- pH: how acid or alkaline the water is. Fertigation water is often made slightly acid to stop scale forming.
Sources: FAO (2013), Good Agricultural Practices for greenhouse vegetable crops, principles for Mediterranean climate areas (Plant Production and Protection Paper 217): chapter 9, pages 174 and 186, and chapter 10, pages 256 and 257; UF/IFAS Extension (2012, reviewed 2022), Fertigation for Vegetables: A Practical Guide for Small Fields (HS1206), step 7.
The parts of a fertigation unit
The water passes these parts in about this order. Designs differ, for example with a second filter after the injector, so ask for the order to be shown on a drawing.
- Water tank: holds the water for the day's watering
- Pump: pushes the water through the unit and into the house
- Filter: stops sand and grit that would block the drippers
- Non-return valve: lets water flow one way only, so feed cannot run back
- Injector: draws fertiliser from the two stock tanks below it into the water
- Drip lines along the beds, with a dripper beside each plant
What each part does
| Part | What it does | What to ask for |
|---|---|---|
| Tank or water source | Holds or supplies the water for the day's watering. | A source that keeps up with the drippers for the whole watering; see the tank guide. |
| Pump | Pushes the water through the filter, the injector and the drip lines. | A flow and pressure that match the drippers, read from the pump's curve. |
| Filter | Stops sand, silt and other particles that would block the drippers. | The fineness the dripper maker asks for, in microns or mesh. |
| Non-return valve | Lets water flow one way only, so feed is much less likely to run back to the water source. | Fitted between the water supply and the point where the fertiliser goes in. |
| Injector | Draws stock solution into the water at a set ratio, for example 1:100. | A type and size that suit your flow (see the spec sheet section below). |
| Stock tanks | Hold the concentrated fertiliser, with separate tanks for fertilisers that react. | Tanks made of a material that resists acid and salts, such as polypropylene or PVC. |
| Pressure gauges | Show the pressure before and after the injector. | One on each side, so faults show up quickly. |
| Pressure regulator | Lowers the pressure to what the drip lines need. | A setting inside the pressure range on the drip line's specification sheet. |
| Drip lines in the beds | Deliver water and feed beside each plant. | Pressure-compensating drippers where beds are long or the ground slopes. |
Sizing an irrigation storage tank
Sources: FAO (2013), Good Agricultural Practices for greenhouse vegetable crops, principles for Mediterranean climate areas: chapter 8 (filters and drippers), chapter 10, page 257, and chapter 12, page 316; UF/IFAS Extension (2020, reviewed 2024), Regulations and Guidelines for Chemigation (AE542), tip 1; University of Arkansas Cooperative Extension (2026), Troubleshooting a Fertigation System (FSA6176), figure 1.
Keep fertiliser out of the water source
Fertiliser must never run back into a borehole, a well, a storage tank or a town main. It can happen when the pump stops and the water in the pipe drains back, or when the injector keeps running after the irrigation has stopped.
Where the water comes from a town main, the list below is not enough on its own: the University of Florida asks for special backflow precautions on public supplies, and Florida's rules call for an approved backflow device fitted under the water supplier's supervision. Where the same borehole, tank or pipework also gives drinking water, keep a physical gap between them: feed the fertigation unit from its own tank, filled over an air gap, or fit the approved backflow preventer that a plumber specifies.
For every system, the University of Florida's guidance on chemigation lists these protections:
- A non-return valve, also called a check valve, between the water supply and the injection point.
- A vacuum breaker and a low-pressure drain, placed where the valve or injector maker's drawing shows.
- A shut-off valve on the fertiliser line, so the stock tank cannot empty into the pipe.
- An automatic stop on the injector for when the water stops flowing, and an injection pump with its own power wired so that it runs only while the irrigation pump runs.
If your water comes from a town main, ask the water supplier what protection it requires before an injector is connected.
Sources: UF/IFAS Extension (2020, reviewed 2024), Regulations and Guidelines for Chemigation (AE542): anti-siphon devices, the physical gap and tip 1; UF/IFAS Extension (1990, revised 2022), Causes and Prevention of Emitter Plugging in Microirrigation Systems (AE032), backflow and injection pumps; UF/IFAS Extension (1990, revised 2024), Chemical Injection Methods for Irrigation (WI004), Florida code 62-555; University of Arkansas Cooperative Extension (2026), Troubleshooting a Fertigation System (FSA6176), inlet pressure checks.
Separate tanks for fertilisers that react
Some fertilisers react when they meet at tank strength and settle out as a solid that blocks filters and drippers. The usual answer, set out in FAO's greenhouse manual, is two stock tanks: tank A for the calcium fertilisers and tank B for the phosphate and sulphate fertilisers. A third tank of acid controls the pH and helps keep the drippers clean. Acid is dangerous: wear goggles and chemical-resistant gloves and clothing, always pour acid into water, never water into acid, never let acid meet chlorine, because together they give off chlorine gas, and follow the label.
Buy fertilisers sold as fully water-soluble. Before you mix two fertilisers in one tank, try them in a jar of water first and look for a cloudy, clay-like deposit at the bottom.
| Fertiliser | Never mix it at tank strength with |
|---|---|
| Calcium nitrate | Fertilisers containing phosphate or sulphate |
| Monopotassium phosphate | Fertilisers containing calcium |
| Potassium sulphate | Fertilisers containing calcium |
| Ammonium sulphate | Fertilisers containing calcium |
Sources: FAO (2013), Good Agricultural Practices for greenhouse vegetable crops, principles for Mediterranean climate areas: chapter 12, page 306; University of Arkansas Cooperative Extension (2026), Troubleshooting a Fertigation System (FSA6176), table 1; UF/IFAS Extension (2012, reviewed 2022), Fertigation for Vegetables (HS1206), step 6; UF/IFAS Extension (1990, revised 2022), Causes and Prevention of Emitter Plugging in Microirrigation Systems (AE032), acid injection.
Check EC and pH
FAO's greenhouse manual calls a pH meter and an EC meter indispensable for fertigation. Both are small, portable and easy to use.
Measure your water before any fertiliser goes in, so you know its own EC and pH. Then measure the feed itself during a feed. The EC should rise by about what the feeding programme expects, and the pH should sit where your agronomist or the fertiliser maker sets it. FAO's manual notes that fertigation water is often acidified to keep the growing medium's pH generally between 5.5 and 6.5.
Keep checking through the crop. FAO advises measuring the EC of the feed every week to make sure the injector is still working properly.
How much stock solution a feed uses
FAO's greenhouse manual works through one feed for a small tomato house. The numbers below follow it, to show how the injector's ratio sets the amount of stock solution and the feeding time. FAO's figures are for Mediterranean greenhouses, so check the dripper flow and the feed volume against your own crop and soil.
Sources: FAO (2013), Good Agricultural Practices for greenhouse vegetable crops, principles for Mediterranean climate areas: chapter 10, pages 257 to 260; UF/IFAS Extension (2012, reviewed 2022), Fertigation for Vegetables (HS1206), step 7.
Setup checks before the first crop
- Flush the pipes in order: the main first, then the sub-mains, then each drip line, each until clean water has run from the open end for at least two minutes.
- Check that the arrows on the non-return valve and the injector point the way the water flows.
- Bring the system up to full pressure with clean water and read the gauges before and after the injector. Most venturi injectors need the outlet pressure to be at least 20 percent below the inlet pressure before they start to draw.
- Calibrate the injector. Put its suction line in a container marked with volumes, run it for a timed period at your setting, and work out how many litres it draws per hour. Repeat for each setting you will use.
- Run a first feed and time how long it takes to reach the farthest dripper, for example by checking the EC of the drops there.
- Measure the EC and pH of the feed at a dripper and compare them with your targets.
- After the feed, flush the lines with clean water and run clean water through the injector.
- After a watering, dig a small cross-section beside a plant to see where the water went, and shorten the run time if it went past the roots.
- Write down the pressures, the injector setting and the travel time, so later readings have something to be compared with.
Sources: UF/IFAS Extension (1990, revised 2022), Causes and Prevention of Emitter Plugging in Microirrigation Systems (AE032), flushing; UF/IFAS Extension (1990, revised 2024), Chemical Injection Methods for Irrigation (WI004), venturi injector; UF/IFAS Extension (2020, reviewed 2024), Regulations and Guidelines for Chemigation (AE542), tip 3 and flushing; UF/IFAS Extension (2012, reviewed 2022), Fertigation for Vegetables (HS1206), step 7; University of Arkansas Cooperative Extension (2026), Troubleshooting a Fertigation System (FSA6176).
What to measure
- Your water: have it tested for salts (EC), pH, hardness, iron and sediment. Iron as low as 0.2 parts per million can start to form deposits in drippers, and hard water with a pH above 7 can form scale.
- What the source gives: the flow it can keep up for a whole watering, and the pressure at the pump.
- The house: its length and width, the number and length of the beds, and any slope.
- The drippers: how many there are and the flow of each, which together give the flow the pump, filter and injector must carry.
- The pipes: the size and length of the main and sub-mains, which set how long a feed takes to reach the last dripper.
Sources: UF/IFAS Extension (1990, revised 2022), Causes and Prevention of Emitter Plugging in Microirrigation Systems (AE032), chemical plugging and iron; FAO (2013), Good Agricultural Practices for greenhouse vegetable crops, principles for Mediterranean climate areas: chapter 10, page 257.
What to compare on the spec sheet
- Injector type. A venturi injector needs no power and has no moving parts, but it takes pressure from the line and draws a fixed ratio. A dosing pump has an adjustable ratio; one driven by the water flow keeps the feed in step with the flow, while an electric one injects at a steady rate whatever the flow.
- Injector range: the flows and pressures it works between, and the ratio or range of ratios it gives, checked against your system's flow.
- Filter fineness, in microns or mesh, against what the dripper maker asks for. Without a maker's figure, UF/IFAS advises removing particles larger than one tenth of the dripper's smallest opening. FAO's manual gives 80 microns, or 200 mesh, as typical for soilless growing systems.
- Drip line: the flow of each dripper, the spacing, and the pressure range over which a pressure-compensating dripper keeps the same flow.
- Stock tanks: made of an acid and salt resistant material such as polypropylene or PVC, with lids.
- Backflow parts: each one listed on the quote as an item, not left out.
Pressure-compensating drip lineSmall disc filter for a drip zoneHand valve to shut off a lineChoosing a drip irrigation filter
Sources: UF/IFAS Extension (1990, revised 2024), Chemical Injection Methods for Irrigation (WI004); University of Arkansas Cooperative Extension (2026), Troubleshooting a Fertigation System (FSA6176), calibration; UF/IFAS Extension (1990, revised 2022), Causes and Prevention of Emitter Plugging in Microirrigation Systems (AE032), filters; FAO (2013), Good Agricultural Practices for greenhouse vegetable crops, principles for Mediterranean climate areas: chapter 12, page 316.
When to ask for site-specific sizing
A small house with clean water and one crop can often use a standard set of parts. Ask for the injector, filter, pump and stock tanks to be sized together for your site when any of these apply:
- More than one house, or several houses fed from one fertigation unit in turn.
- A water test that shows hard water, iron or high salts.
- A borehole or well whose yield you have not measured.
- Crops that need different feeds at the same time.
- Long beds, or ground that slopes along the beds.
- Feeds you want a controller to start on its own.
Choosing a pump for your water sourceChoosing pipes, valves and regulators
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