Fertilizer Injector Calculator
Enter the target ppm nitrogen, fertilizer analysis, injector ratio, stock tank volume, flow, and runtime. The calculator sizes the stock solution and checks how long that tank can feed the irrigation event.
Fertilizer to add66.67 oz (4.17 lb)
- Mixing rate per gallon
- 13.33 oz/gal
- Stock solution nitrogen concentration
- 20,000 ppm N
- Finished fertilizer solution from full tank
- 500 gal
- Injector stock draw rate
- 3 gal/hr (6.4 fl oz/min)
- Stock used during this irrigation
- 3 gal
- Full-tank operating time
- 1 hr 40 min
- Tank remaining after this irrigation
- 2 gal (40%)
- Stock shortfall
- 0 gal
Check the fertilizer label for solubility at your water temperature before mixing this much product into the stock tank.
Add the fertilizer first, dissolve it fully, then bring the stock tank up to the stated final volume.
Use a measured injector ratio when possible because nominal settings can drift.
How to use this calculator
- Enter the target ppm nitrogen for the finished irrigation water.
- Enter the first number from the fertilizer label and the actual injector ratio.
- Enter the final stock tank volume after the product is dissolved and the tank is filled to its mark.
- Add the irrigation flow and runtime to estimate draw, tank capacity, and any stock shortfall.
How the stock solution is calculated
This calculator uses the greenhouse fertilizer procedure published by UMass Extension and Virginia Cooperative Extension for products labeled by percent nitrogen. It works from the finished-water nitrogen target back to the concentrated stock solution that an injector must pull from the tank.
Let P be the target concentration in ppm nitrogen, D be the larger number in the injector ratio 1:D, A be the fertilizer nitrogen analysis as a whole percent, V be the final stock volume in gallons, Q be finished-water flow in gallons per minute, and T be runtime in minutes.
The dry fertilizer weight is:
fertilizer ounces = P × D × V ÷ (A × 75)
The constant 75 is the conversion factor used in the cited greenhouse references when fertilizer analysis is entered as a whole percentage and stock volume is in U.S. gallons. After the total fertilizer ounces are known, the calculator divides by V for the per-gallon mixing rate and by 16 for pounds.
Injector draw and tank runtime
The same inputs also describe the hydraulic side of the event. Stock concentration is P × D. One full tank makes V × D gallons of finished fertilizer solution. Injector stock draw is 60 × Q ÷ D gallons per hour, also shown as fluid ounces per minute. Event stock use is Q × T ÷ D, and full-tank runtime is V × D ÷ Q minutes.
If the planned event uses more stock than the tank contains, the result shows the shortage and the maximum runtime one full tank can support at the entered flow. A target of 0 ppm nitrogen returns a zero fertilizer weight, but the flow, draw, capacity, runtime, and tank-balance checks still work.
What moves the result most
The fertilizer weight rises in direct proportion to the ppm target, injector ratio, and stock-tank volume. A 1:200 injector needs twice the stock concentration of a 1:100 injector for the same finished-water ppm, so the same tank requires twice as much fertilizer. A higher nitrogen analysis lowers the required product weight because more of each ounce is nitrogen.
What this calculator leaves out
The calculation uses label nitrogen and the injector ratio you enter. It does not account for nitrate, ammonium, or urea forms separately; nutrients already in source water; acid injection; alkalinity correction; blended products with insoluble carriers; compatibility between fertilizers; or product-specific solubility limits. Always check the label or manufacturer guidance for maximum solubility at your water temperature, and fill the stock tank to the final marked volume after dissolving the fertilizer rather than adding fertilizer to an already-full tank.
Worked example
Suppose a grower wants 200 ppm nitrogen from a 20-10-20 fertilizer through a 1:100 injector and wants 5 gallons of final stock solution. The fertilizer weight is 200 × 100 × 5 ÷ (20 × 75), or 66.67 ounces. That is 4.17 pounds, and the mixing rate is 13.33 ounces per final gallon of stock.
At 5 gallons per minute for 60 minutes, the injector draws 3.00 gallons of stock during the event. The full 5-gallon tank makes 500 gallons of finished fertilizer solution and supports about 1 hr 40 min of irrigation at that flow, leaving 2.00 gallons, or 40%, in the stock tank.
Common questions
How much fertilizer goes in a 1:100 injector stock tank?
Multiply the target ppm nitrogen by 100 and by the final stock-tank gallons, then divide by the fertilizer nitrogen percent and by 75. For 200 ppm nitrogen, 20% nitrogen fertilizer, and a 5-gallon final stock tank, the result is 66.67 ounces, or 4.17 pounds.
What does a 1:200 fertilizer injector ratio mean?
A 1:200 ratio means one part stock solution is mixed into about 200 parts finished irrigation water. For the same finished ppm target, the stock solution must be twice as concentrated as a 1:100 setup. Use a measured ratio if you have calibrated the injector.
How do I calculate ppm nitrogen from an N-P-K label?
Use the first number on the fertilizer label as the nitrogen percentage. The stock-tank formula is target ppm nitrogen times injector ratio times final stock gallons, divided by the nitrogen percentage and 75. The calculator applies that formula and also reports the concentrated stock ppm.
Why must I fill the stock tank to a final volume?
The formula assumes the stock tank volume is the finished volume after the fertilizer is dissolved. If you add fertilizer to an already-full tank, the final volume is larger than entered and the concentration will be weaker than planned. Dissolve the product first, then bring the tank up to its mark.
Can this formula be used for phosphorus or potassium?
The calculator is written for ppm nitrogen from the first number on an N-P-K label. Phosphorus and potassium labels are reported as phosphate and potash equivalents, so they need different nutrient conversions if you want elemental P or K. Follow extension guidance or the fertilizer manufacturer when dosing nutrients other than nitrogen.
Why should stock-solution solubility be checked?
Two fertilizers with the same nitrogen percentage can have different solubility limits, and solubility changes with water temperature. If the required ounces per gallon exceed the product label, some fertilizer may stay undissolved or precipitate. Split the batch, use a larger stock tank, or adjust the injector setup instead of forcing an overloaded stock solution.