Tips for Ensuring Proper Fertilizer Concentration for Greenhouse Crops
May 4, 2023 
Photo: Christopher Currey, Iowa State University
One common question greenhouse growers may ask when troubleshooting a crop problem is, “What am I fertilizing with, and at what concentration is that being applied?” Growers often know what ppm (parts per million) N (nitrogen) they are fertilizing at because they mix their fertilizer according to the chart on the fertilizer bag. Human error and problems with fertilizer injectors can affect concentration, and a grower might not be fertigating to the level that they expect.
But how can growers be sure the fertilizer concentration applied to the crop is the same as the intended concentration? Michigan State University Extension recommends checking the irrigation solution’s EC (electrical conductivity) to help determine the actual ppm N in the crop’s irrigation water.
A high-quality water-soluble fertilizer will provide a chart indicating the EC value to the corresponding ppm N. It is important to note this is not the EC of the concentrated stock solution, but the EC of the diluted solution after fertilizer is injected into clear water. These values listed on the bag are the EC values when fertilizer is added to pure water. Since most growers are irrigating with city or well water (not water treated with reverse osmosis or distilled water), they need to account for the EC value of the irrigation water before fertilizer is added.
Troubleshooting an Incorrect Concentration
In addition to checking the injector for mechanical issues, some other tips to ensure the correct concentration of fertilizer are to:
- Mix the concentrated stock solution with warm water to ensure the fertilizer dissolves completely.
- Do not make a stock solution at a concentration above the solubility listed on the label.
- Double-check the stock tank to ensure the volume accurately matches the mixed ounces per gallon of fertilizer.
- Mix the fertilizer at the appropriate concentration for the target EC and injector ratio listed on the label. Every fertilizer will need different ounces per gallon to achieve the desired ppm N. Do not use the label from a different fertilizer to calculate another’s concentration.
Learn more at canr.msu.edu for tips from Jaden Gimondo, a floriculture educator with Michigan State University Extension.
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