Inject Right & Protect the Water
Sizes the injection rate
Get the required injection rate from your flow, target concentration and stock strength, check the venturi suction, and confirm the backflow device your chemical and water source legally require — the part that, when missing, contaminates wells.
Fertigation recipe & safety
Next: do not inject yet — install a reduced-pressure-zone (rpz) assembly on the mainline upstream of the injection point. Backflow into a drinking-water source is the contamination route regulators care most about.
Sizing uses a mass balance (target ppm × system flow ÷ stock strength). Backflow requirements follow ASABE EP409.1 + chemigation safety standards; venturi suction needs a downstream:upstream pressure ratio ≤ 0.7. Always confirm against your local chemigation regulations.
Fertigation safety — key facts
- Injection rate
- target ppm × flow ÷ stock mg/L
- g/L → mg/L
- × 1,000
- Venturi suction
- downstream:upstream ≤ 0.7
- Min chemigation set
- check valve + LP drain + vacuum relief
- Potable / acid / pesticide
- RPZ or air gap
- Standard
- ASABE EP409.1
- Unsafe verdict
- backflow device inadequate
- Privacy
- Runs in your browser; nothing uploaded
Sizing the dose is the easy half — protecting the water is the half that matters
Fertigation means putting fertilizer, acid or pesticide into the irrigation water, and it lives or dies on two things: metering the right amount, and making sure none of it can travel backwards to the source. The dose is a simple mass balance — target concentration times system flow, divided by the strength of your stock tank. The safety half is where growers get into trouble: when a pump trips or a line breaks, the pressure can reverse and pull concentrated chemical out of the pipe and into a well or a public main. That is the contamination route regulators write rules about.
This tool computes the injection rate in L/h and L/min, checks whether a venturi has enough pressure differential to actually draw chemical, and looks up the backflow device your specific chemical and water source require under ASABE EP409.1 and chemigation-safety guidance. It then rolls everything into a single safe, caution or unsafe verdict with the next concrete step. Pair it with the Total Dynamic Head and Hazen-Williams friction tools to design the whole delivery system.
Backflow device requirement by chemical and water source
| Chemical | Water source | Minimum backflow device |
|---|---|---|
| Fertilizer | Dedicated irrigation line | Check valve + LP drain + vacuum relief (EP409.1 set) |
| Fertilizer | Surface / canal water | Check valve + LP drain + vacuum relief |
| Fertilizer | Potable well | RPZ assembly |
| Fertilizer | Municipal (public) supply | Air gap or RPZ assembly |
| Acid | Dedicated irrigation line | RPZ assembly |
| Acid | Surface / canal water | RPZ assembly |
| Acid | Potable well | RPZ assembly |
| Acid | Municipal (public) supply | Air gap or RPZ assembly |
| Pesticide (chemigation) | Dedicated irrigation line | Check valve + LP drain + vacuum relief + interlock |
| Pesticide (chemigation) | Surface / canal water | RPZ assembly |
| Pesticide (chemigation) | Potable well | RPZ assembly |
| Pesticide (chemigation) | Municipal (public) supply | Air gap or RPZ assembly |
Source: ASABE EP409.1 “Safety Devices for Chemigation”; UF/IFAS & Texas A&M AgriLife chemigation-safety guidance; EPA cross-connection rules. Confirm with your local regulator.
Injector types and accurate rate bands
| Injector | Accurate band | Driven by | Note |
|---|---|---|---|
| Venturi (Mazzei) | 5 – 2,000 L/h | Pressure differential | Needs ≥~30% pressure drop; cheapest, no moving parts. |
| Positive-displacement | 1 – 4,000 L/h | Powered (independent) | Most accurate; meters fixed volume per stroke. |
| Suction / by-pass tank | 10 – 1,500 L/h | By-pass flow | Concentration drifts as tank dilutes; least precise. |
How to use it
- 1. Enter the recipe. System flow (L/min), the target concentration in the line (mg/L), and your stock-tank strength (g/L).
- 2. Set the hydraulics. Pick the injector and enter the upstream and downstream pressures so the venturi suction can be checked.
- 3. Declare the chemical and source. Fertilizer, acid or pesticide; dedicated line, canal, well or municipal supply.
- 4. Select the installed device. Tell the tool which backflow device is currently fitted.
- 5. Act on the verdict. Read the injection rate, the suction check and the safe/caution/unsafe verdict, then do the next step it names.
Frequently Asked Questions
How do I calculate the fertigation injection rate?+
The injection rate comes from a mass balance: required mass per minute = target concentration (mg/L) × system flow (L/min); divide by the stock-tank strength in mg/L (g/L × 1000) to get the injection rate in L/min. For example, 150 mg/L in a 600 L/min system from a 100 g/L stock needs 150 × 600 ÷ 100,000 = 0.9 L/min, which is 54 L/h. The tool does this live as you type.
What backflow device do I need for fertigation?+
It depends on the chemical and the water source. ASABE EP409.1 sets a check valve plus a low-pressure drain and vacuum relief as the minimum for chemical injection on a dedicated line; injecting into a potable well or a public supply, or injecting acid or pesticide, raises the requirement to a reduced-pressure-zone (RPZ) assembly or an air gap. The calculator looks up your chemical × source combination and names the required device.
Why is backflow prevention the most important part?+
Without it, a pump shutdown, pipe break or pressure drop can back-siphon concentrated fertilizer, acid or pesticide out of the irrigation line and into the water source — the classic way wells and public supplies get contaminated. A correctly installed and tested backflow device is what stops that reverse flow. That is why the tool treats an inadequate backflow device as an outright unsafe verdict.
How does a venturi injector create suction?+
A venturi (Mazzei-type) injector narrows the flow to speed it up, which drops the pressure at the throat below the chemical-line pressure and draws the chemical in. It only works if there is a real pressure drop across it — as a rule of thumb the downstream pressure must be no more than about 70% of the upstream pressure (a 30%+ loss). If the downstream:upstream ratio is too high the venturi loses suction and injects nothing.
Why does my venturi inject nothing at high pressure?+
Because the downstream pressure is too close to the upstream pressure, so the throat cannot drop below the chemical line and no suction develops. The fix is to create more differential: open a downstream valve, fit the venturi across a pressure-sustaining loop, or add a small booster pump on the chemical line. The calculator flags this when the downstream:upstream ratio exceeds 0.7.
What is a reduced-pressure-zone (RPZ) assembly?+
An RPZ is a high-hazard backflow preventer with two independent check valves and a relief valve between them that dumps to atmosphere if either check leaks, breaking any reverse-flow path. It is the device required when a fertigation or chemigation line connects to a drinking-water source, or when injecting corrosive acids or pesticides. It must be tested by a certified tester, typically annually.
Is a single check valve enough for fertigation?+
Only in the lowest-hazard case — and usually not even then. A spring-loaded check valve alone does not meet the ASABE EP409.1 chemigation set (which also needs a low-pressure drain and vacuum relief) and is never sufficient for a potable-water connection, acid, or pesticide. The tool will return an unsafe verdict if a single check valve is selected where the rules require more.
What injection rate band can each injector hold?+
A venturi holds roughly 5–2,000 L/h but needs the pressure differential; a positive-displacement (piston or diaphragm) injector meters 1–4,000 L/h accurately regardless of line pressure; a suction or by-pass tank covers about 10–1,500 L/h but its concentration drifts as the tank dilutes. If the required rate falls outside your injector's accurate band the tool flags it so you can size a different injector.
How do I convert g/L stock to mg/L for the calculation?+
Multiply grams per litre by 1,000. A 100 g/L stock solution is 100,000 mg/L. The injection rate is then the nutrient mass needed per minute divided by that figure. Keeping units consistent (mg/L for both the target and the stock) is what makes the dilution ratio come out right.
Does this replace local chemigation regulations?+
No. The device requirements here follow ASABE EP409.1 and widely used extension chemigation-safety guidance, but many states, provinces and water utilities have their own legally binding rules, permits and inspection requirements. Use the tool to size the injection and understand the protection class, then confirm the exact device and permit with your local regulator before injecting.
What does 'caution' mean versus 'unsafe' in the result?+
Unsafe means the backflow protection is inadequate for your chemical and water source — the contamination-critical failure, so do not inject. Caution means the backflow device is acceptable but a hydraulic issue exists: either the venturi has lost suction (downstream:upstream ratio over 0.7) or the required injection rate falls outside your injector's accurate band. Caution items are tuneable; the unsafe item must be fixed by upgrading the device.
Can I inject acid and fertilizer through the same point?+
Through the same injection port you can, but never pre-mixed in one tank if they react, and the backflow protection must satisfy the more hazardous of the two — acid raises the requirement to an RPZ on most sources. Run the calculator once per chemical to confirm the device class, and inject them in separate passes or through separate metered lines to control the concentration of each.