Calibrating Fertilizer Injectors

Injecting FertilizerRead More in The Sun Gro’er Issue 1/1 (2003)

So you’ve just used that brand new conductivity meter to measure the EC of your customer’s fertigation solution, and it doesn’t match the value that corresponds to the ppm N that the grower thinks they’ve been applying forever. What to do now? Since you’re already sure that the EC meter is properly calibrated, you better check that fertilizer injector! Fortunately, it’s not that hard to do.

Items Needed to Calibrate Injectors:

One fluid ounce measuring cup
One gallon jug
Five gallon bucket
Twenty gallon trash barrelThe easiest way to check the injection rate is by measuring how much solution is pulled into the injector, while collecting a measured amount coming out of the injector, and then determining the ratio of these volumes. To do this you’ll need: a cup that measures in fluid ounces, a 1-gallon jug, a 5-gallon bucket, and a 20-gallon trash barrel. You must also be able to collect all of the water that comes out of the injector. Not a problem if the injector is portable, but if the injector is in-line this usually requires a faucet coming out of the waterline after the injector and the ability to valve off the rest of the irrigation lines.
Start by accurately determining a 20- gallon mark on the inside of the barrel, using the cup to mark a gallon in the jug, and the jug to mark 5 gallons in the bucket, etc. Next, put the injector’s siphon tube into the gallon jug full of water. Turn on the water and run the injector to get everything (injector, pipes, hose) full and working. Turn off the system and refill the jug to the very top. Turn the system on again and run the injector until you’ve collected exactly 20 gallons of water from the injector into the barrel.Finally, measure the volume of water pulled into the injector. Do this by using the cup to measure how many fluid ounces of water it takes to refill the jug to the top without removing the siphon tube. To improve accuracy in measuring the amount injected, you may need to collect more than 20 gallons from the injector at low injection ratios like 1:300.Now that you have the data, do a quick calculation. The injection rate will be the ounces of water collected (20 gallons x 128 fluid ounces per gal = 2,560 fluid ounces) divided by the fluid ounces injected. If the injection ratio is 1:100 and the injector is working correctly, it should take 25.6 fl. oz. to refill the jug. If the injection ratio is significantly off, run this test again. If it’s still off, make the necessary adjustment to the injector and re-run the test. Once the injection rate is correct, re-test the EC of the fertigation solution. If the EC is still not what you expect at that injection ratio for a particular fertilizer and ppm N, the problem may lie with the makeup of the fertilizer stock solution.Save that barrel with the 20-gallon mark and this becomes an easy way to routinely check injectors.~Ron Walden