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Fertilizer Injectors for Greenhouses

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About Fertilizer Injectors

If you use a greenhouse, then you likely use a fertilizer injector. Fertilizer injectors are used by most growers to apply water-soluble fertilizers to plants. These devices "inject" a small quantity of concentrated fertilizer solution (stock solution) into the irrigation line so that the water leaving the hose (dilute solution) supplies the proper concentration of fertilizer. Most growers apply water-soluble fertilizers at a dilute concentration on a "constant feed" basis (with mostly every watering) to insure an adequate supply of the essential elements for plant growth.

Parts Per Million

Rates of fertilization are often given in parts per million (ppm) of nitrogen (N), which is a way of expressing fertilizer concentration. One ppm is equivalent to 1 milligram/liter (mg/l). It is important to remember that an injector does not deliver a fixed ppm N. You need to determine the amount of fertilizer to dissolve per gallon of water (stock solution) to make the appropriate concentrate for a specific injector setting. An injector setting of 1:100 does not mean that the injector is delivering 100 parts per million (ppm) nitrogen. It.means that 1 gallon of fertilizer concentrate makes 100 gallons of final solution. Note that fertilizer should be measured by weight for mixing, not volume. Also, fertilizer solution color is not a reliable gauge for fertilizer concentration.

Fertilizer Injector Ratio and Injector Settings

Fertilizer stock solutions are mixed according to the fertilizer injector ratio: each injector will deliver a certain amount of stock solution for each increment of irrigation water that passes through the injector. For example, a 1:100 injector will deliver 100 gallons of dilute fertilizer solution for each gallon of concentrated stock solution. A 1:200 injector will deliver 200 gallons of dilute fertilizer for each gallon of concentrated stock solution (or 100 gallons of dilute solution per ½ gallon of stock). If both injectors were to deliver 200 ppm of nitrogen from the same fertilizer, the stock solution for the 1:200 injector would have to be twice as concentrated as the one for the 1:100 injector. Therefore, the injector ratio determines the concentration of the stock solution that is needed to deliver a particular rate of fertilization. Some injectors (Hozon,) have a fixed (nonadjustable) injector ratio whereas other injectors (Dosatron, Anderson, Dosmatic) have adjustable ratios. Many growers prefer injectors with adjustable ratios so that different fertilizer rates can be applied to crops with different nutrient requirements.

Many injectors have dual settings, in percent and ratio. A 1 percent setting is the same as a 1:100 ratio, a 2 percent setting is the same as a 1:50 ratio and a 0.5 percent setting is the same as a 1:200 ratio.

For an easy to use table on injector ratios, ppm Nitrogen and fertilizer calculations see: Ratios and Concentrations for Constant Feeding with Water Soluble Fertilizers

To learn more about how to calculate fertilizer ppm for greenhouse crops see: Fertilizer Calculations for Greenhouse Crops

Checking the Accuracy of Your Fertilizer Injector

Fertilizer injectors should be checked periodically to be sure they are operating accurately. This can be done by testing the electrical conductivity (EC) of the fertilizer solution and comparing the results to an EC chart from the fertilizer manufacturer. To check a fertilizer solution, use a good conductivity meter or send a sample to the University of Massachusetts soil test laboratory.

Procedure to check the EC of a fertilizer solution:

  1. Let plain water (no fertilizer) run a little, then collect water in a clean bucket. Take a sample of the plain water from the bucket. Check the EC of the untreated water supply.
  2. Mix up your fertilizer as you normally do and run it through the injector and hose. Let it run a little to be sure you get an accurate sample. Take a sample of the fertilizer-injected water from the end of the hose and use your meter or soil test laboratory to check the EC of the sample.
  3. Subtract the EC value of the untreated water from the EC value of the fertilizer water.
  4. Compare the results to an EC chart from the fertilizer manufacturer or fertilizer bag. The chart will correlate the EC measurement with the ppm Nitrogen so you can determine if your injector and mixing procedure is accurate.

References and Resources:

Prepared by Tina Smith
Extension Greenhouse Crops & Floriculture Program
University of Massachusetts
Amherst, MA 01003

3/09, reviewed 8/15

Commercial Horticulture
Commercial Horticulture topics: 
Plant Nutrition