How should I amend the soil in my greenhouse?


I have a 10 x 10 greenhouse with raised beds. I added soil and ProMix to it and have used fertilizer for the five years I’ve had it. I heard that the soil in a greenhouse may become degraded after a while due to the lack of being outdoors. Is it possible that my soil may be depleted in the greenhouse, and I may need to amend it? How would I amend it? It seems some of my crops are not growing as well.


Donna Coffin, Extension Professor, Piscataquis and Penobscot Counties

If you have been adding fertilizer to your greenhouse crops, you may have a build-up of fertilizer salts in the soils. The UMaine Soil Test Lab has a special Greenhouse Soil test that can give you information on the salt level as well as pH and Nitrate Nitrogen. Use the Greenhouse Media Information Form (PDF) to send in with your sample. The organic matter from the ProMix that you added has probably broken down. A soil test is the best way to determine what the nutrient status is in your soils and what you need to add to amend the soils.

Remember to take multiple samples from several places in your greenhouse. Go down 4 to 6 inches. Mix all the samples together and send in about a pint of soil to the soil lab. See our factsheet on soil testing for more info.