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Refreshing Container Garden Soil #899875

Asked May 03, 2025, 3:02 PM EDT

Hello, I'm entering the 2nd year of working through my own container garden (in fabric grow bags) and I've saved the leftover potting soil from last year's season. I've already picked up composted soil from my local yard waste site to mix back into last years crop but I was wondering what is an appropriate ratio of old soil to compost soil? Should I add anything else to the mix like perlite? And lastly, the compost still contains large particles of stone, chunks of wood, etc. Is there a suggested screen size I should pass the compost through to filter out debris before I mix? Thanks!

Ramsey County Minnesota

Expert Response

Numerous resources note the ideal ratio of old potting soil to new compost is 3:1. Some things to consider:
  1. If you had any disease in last year's plants it is not recommended to reuse the soil.
  2. If you are growing plants that are heavy feeders (eg: tomatoes) a greater concentration of new compost is recommended.
  3. Without testing the soil it is difficult to recommend anything to add.  
For filtering out your new compost an ideal screen size would have 1/4" openings. 
Amy J. Extension Master Gardener - HC Replied May 04, 2025, 10:42 PM EDT

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