Choosing a target concentration
Fertigation \u2014 feeding a crop through the irrigation system \u2014 works by mixing a small, steady stream of concentrated fertilizer (the stock) into a much larger stream of irrigation water, so the water reaching the plant carries a set concentration of nutrient. That concentration is almost always expressed in parts per million (ppm), which for a water-based solution is the same as milligrams of nutrient per liter of water. The injector's job is simply to add stock fast enough, relative to the system's flow, to land on that ppm.
There's no single correct target: it depends on the crop, its growth stage, and how often it's being fed. Container nursery production is one of the few areas with a clearly published, staged target, which makes a useful reference point.
| Growth phase | Typical N target | Why |
|---|---|---|
| Establishment | 25\u201350 ppm N | Keeps salt levels low around new roots that can't yet handle much. |
| Rapid growth | 75\u2013200 ppm N | Main growing period; the exact rate tracks the desired growth curve. |
Field crops, turf and landscape fertigation are usually set from a soil test or a per-acre nutrient budget rather than a single ppm figure, since the total amount applied over a season matters as much as the concentration on any one day. If that's what you're planning, the fertigation calculator works out a full nutrient budget rather than a single injection rate.
Read your crop or product's own recommendation first. The table above is specific to container nursery stock; using it for a different crop without checking is a common source of over- or under-feeding.
How this fertilizer injection rate calculator works
The calculator treats this as a simple mass balance: whatever nutrient the injector adds per hour has to end up spread through the system's hourly water flow at your target ppm. That only takes two steps.
1. Convert the stock's label percentage to ppm
Stock concentration
Stock ppm = Nutrient % \u00d7 10,000 \u00d7 Specific gravity
Percent is parts per hundred, so multiplying by 10,000 converts it to parts per million on a mass basis. Specific gravity accounts for stock that's denser (or lighter) than water \u2014 skipping it is the single most common source of error in this kind of calculation.
2. Solve the dilution
Injection rate
Injection rate (gal/hr) = Target ppm \u00d7 System flow (GPM) \u00d7 60 \u00f7 Stock ppm
This is the same relationship University of Florida IFAS Extension calls a "dilution factor" approach: the ratio of stock concentration to target concentration tells you how many times more dilute the injected stream needs to become, and that sets the rate. The small addition the injected stock itself makes to total flow is left out, which is standard practice since it's normally well under 1% of the system flow.
In "Delivered ppm" mode, the same relationship is solved the other way \u2014 useful for checking a pump that's already set, or one you measured with a calibration jug test.
Delivered ppm
Delivered ppm = Injection rate (gal/hr) \u00d7 Stock ppm \u00f7 (System flow (GPM) \u00d7 60)
Rounding policy
The gal/hr (or L/hr) result is shown as an estimate, with "about," because it depends on the specific gravity you enter \u2014 exact only if that figure is exact. Per-minute and dilution-ratio figures round to a sensible working precision rather than a false level of accuracy.
When this calculator isn't the right tool
It's built for a continuously adjustable metering pump dosing a liquid stock into the main flow. Use a different approach for:
- Fixed-ratio proportional injectors (Venturi or Dosatron-style, set to something like 1:100 regardless of flow). See the injector ratio calculator.
- Mixing a stock tank by weight, such as ounces of dry, water-soluble fertilizer per gallon for a greenhouse feed program. See the fertilizer ppm calculator.
- Chlorine or acid injection, which follow different chemistry and their own dosing rules. See the chlorine injection calculator or the acid injection calculator.
- A full-season nutrient budget rather than a single rate. See the fertigation calculator.
Injection rate calculation example
A nursery block is fed through a zone flowing 50 GPM. The target is 150 ppm N, using UAN-32 (32% N, specific gravity 1.32) as the stock.
- Stock ppm: 32 \u00d7 10,000 \u00d7 1.32 = 422,400 ppm.
- Injection rate: 150 \u00d7 50 \u00d7 60 \u00f7 422,400 = 1.065, shown as about 1.07 gal/hr.
- Per minute: 1.07 \u00f7 60 \u00d7 128 oz/gal \u2248 2.27 fl oz/min, a practical number to check against a calibration jug.
- Dilution ratio: 422,400 \u00f7 150 \u2248 1 : 2,816 \u2014 one gallon of stock spread through roughly 2,816 gallons of total flow.
Running that same 1.07 gal/hr rate back through "Delivered ppm" mode, with the same 50 GPM flow and UAN-32 stock, returns about 150 ppm \u2014 confirming the two modes agree, within normal rounding.
The same calculation in metric
A greenhouse zone flows 200 L/min. The target is 80 ppm N, using a generic 20% N liquid stock with a specific gravity of 1.20:
- Stock ppm: 20 \u00d7 10,000 \u00d7 1.20 = 240,000 ppm.
- Injection rate: 80 \u00d7 200 \u00d7 60 \u00f7 240,000 = exactly 4.0 L/hr.
- Per minute: 4.0 \u00f7 60 \u00d7 1,000 mL/L \u2248 66.7 mL/min.
- Dilution ratio: 240,000 \u00f7 80 = 1 : 3,000.
Measuring your inputs and applying the result
The calculator is only as good as the flow rate and fertilizer figures you put into it, and both are worth measuring rather than assuming.
System flow
Use the flow at the actual point of injection, not a pump's rated capacity or a nameplate figure \u2014 valves, elevation changes and a partly open zone can all change what's really flowing. A bucket test (timing how long it takes to fill a container of known volume) or a clamp-on flow meter both give a real number you can trust.
Fertilizer percentage and specific gravity
The nutrient percentage comes straight from the guaranteed analysis on the product label \u2014 use the number for the specific nutrient you're targeting, since a blend like 10-34-0 has different figures for nitrogen and phosphate. Specific gravity (or density in pounds per gallon) is usually on the label or safety data sheet; if it isn't, the typical values below cover the most common liquid nitrogen grades.
Setting and checking the pump
Set the metering pump to the calculated rate, then confirm it rather than trusting the dial: run a calibration jug test with plain water, time how long the pump takes to draw a measured volume, and compare that against the calculated gal/hr. An EC meter reading at an emitter, compared with the water source's own EC, is a good final check that fertilizer is actually reaching the target concentration.
Common mistakes
- Confusing an injection rate (volume per hour) with an injector ratio (a fixed mixing ratio like 1:100) \u2014 they answer different questions and aren't interchangeable.
- Leaving specific gravity at 1.00 for a fertilizer that's actually denser, which understates how much stock volume is needed.
- Using the whole N-P-K label number when only one nutrient in a blend is actually the target.
- Trusting a pump's dial setting without a calibration check, especially on older or partly clogged equipment.
What changes the result
| Factor | Effect | What to adjust |
|---|---|---|
| System flow entered wrong | The injection rate scales directly with flow, so an inaccurate GPM gives a proportionally wrong rate. | Measure actual flow at the injection point rather than assuming a rated capacity. |
| Specific gravity left at default | A denser fertilizer needs less injected volume for the same ppm; assuming 1.00 for a dense product understates the volume required. | Use the specific gravity from the product label or spec sheet whenever it's available. |
| Wrong nutrient percentage used | A multi-nutrient blend has a different percentage for each nutrient. | Use the guaranteed-analysis number for the nutrient you're actually targeting. |
| Pump not drawing its dial setting | Metering pumps drift and clog over time, so the dial number can diverge from real output. | Confirm with a calibration jug test before relying on the calculated rate. |
| Multiple stocks injected together | Combining nutrients or products changes the effective concentration of each. | Calculate each nutrient and stock combination separately. |
| Backflow or check-valve failure | Fertilizer can siphon back into the water supply if protection fails. | Use and maintain a certified backflow preventer, per local code. |
Typical density and specific gravity by liquid fertilizer
Real values vary by manufacturer and temperature \u2014 use the product's own label or safety data sheet when you have it.
| Product | Density | Specific gravity |
|---|---|---|
| UAN-28 (28-0-0) | 10.65 lb/gal | 1.28 |
| UAN-30 (30-0-0) | 10.80 lb/gal | 1.30 |
| UAN-32 (32-0-0) | 11.06 lb/gal | 1.32 |
| Water, for comparison | 8.34 lb/gal | 1.00 |
Frequently asked questions
How do I calculate my fertilizer injection rate?
Enter your target nutrient concentration in ppm, your system's flow rate, and your fertilizer's nutrient percentage and specific gravity. The calculator multiplies the target ppm by your flow rate, then divides by the fertilizer's concentration in ppm to get the gallons (or liters) per hour to inject. If you don't know your fertilizer's specific gravity, a value of 1.00 gives a reasonable estimate for most liquid blends, though the true rate may differ slightly.
What ppm should I target when fertigating?
There's no single number \u2014 the right target depends on the crop, growth stage and how often you're feeding. Container nursery stock, for example, is typically started at 25\u201350 ppm N during establishment and raised to 75\u2013200 ppm N during rapid growth. Field crops and turf are usually set from a soil test or a per-acre nutrient budget rather than a single ppm figure, so check your crop's specific recommendation or use the fertigation calculator for a full nutrient budget.
How do I find my system's flow rate (GPM)?
The most reliable way is to measure it directly: time how long it takes to fill a container of known volume, or read a flow meter on the mainline if you have one. The bucket test calculator and irrigation system GPM calculator can both help you work out an accurate flow rate if you don't already know it.
Does the specific gravity of my fertilizer matter?
Yes. Liquid fertilizers are almost always denser than water \u2014 UAN-32, for example, weighs about 11.06 pounds per gallon versus 8.34 pounds for water, a specific gravity of 1.32. Using 1.00 when your fertilizer is actually denser will overstate the injection rate needed, since it doesn't account for the extra nutrient packed into each gallon of stock. Check the product label or safety data sheet for the exact figure when you can.
What's the difference between an injection rate and an injector ratio?
An injection rate is a volume per hour, like 1.07 gallons per hour, which is what a continuous metering pump is set to draw. An injector ratio, like 1:100, describes how a proportional (Venturi or Dosatron-style) injector mixes stock solution into the passing water, regardless of the total flow rate. If your system uses a fixed-ratio injector rather than an adjustable metering pump, the injector ratio calculator is the right tool instead.
Can I use this for phosphorus, potassium or another nutrient instead of nitrogen?
Yes. The calculation only needs the target ppm of whatever nutrient you're injecting and that nutrient's percentage and specific gravity in the stock \u2014 it isn't specific to nitrogen. Just make sure the percentage you enter matches the nutrient you're targeting; a blend like 10-34-0 has a different percentage for its nitrogen than for its phosphate, so use the right number from the guaranteed analysis.
What safety precautions apply to fertilizer injection?
Always follow the product label for handling and storage, and use a certified backflow preventer between the injection point and your water source so fertilizer can never siphon back into a well or public supply \u2014 check local regulations, since backflow prevention is often required by code. Wear the gloves and eye protection listed on the safety data sheet when handling concentrated fertilizer, and rinse any spills promptly.
How do I check that my injector pump is actually delivering the calculated rate?
Run a calibration jug test: with the pump drawing from a container of water instead of fertilizer, time how long it takes to draw a measured volume, then convert that to gallons or liters per hour and compare it with the calculated rate. You can also switch this calculator to "Delivered ppm" mode and enter your pump's actual measured rate to see what concentration it's really producing, then check that against an EC meter reading at the emitter.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- Formula, unit conversions and all five test cases verified by hand, including a reverse-mode cross-check.
- Fertilizer density and percentage figures checked against manufacturer and extension data below.
- Method follows the dilution-factor approach used by University of Florida IFAS Extension for fertigation injection.
Sources
- University of Florida IFAS Extension. How to Calculate Fertigation Injection Rates for Commercial Blueberry Production (HS1197).
- University of Georgia Cooperative Extension, Tift County. Adjustment Factors for Calibrating Liquid Fertilizer Applicators.
- Yara North America. UAN-32 Product Data Sheet.
- USDA Forest Service, Forest Nursery Notes (Landis and others). Fertigation: Injecting Soluble Fertilizers into the Irrigation System.