Amending Hardpan Clay Soil

Amending clay soil with lots of organic matter yearly will turn it around! (Image by Jessie Keith).

“I have hard clay soil. I have been adding one plant at at time and even that is hard digging holes large enough to amend soil around roots. But now I want to start a large bed by putting down cardboard, putting a bed border around it, and using everything in our mulch pile to cover the cardboard and leave over winter. Would this help or is there a better way?” Question from Erin of Kirkwood, Missouri.

Answer: I used to live in the Midwest, and hardpan clay soils are the worst. Covering bad soil won’t help your plants or the soil down below. Plants grow and resist drought best when their roots can grow deeply, which means either transforming your clay bed or building raised beds. If you want to transform your bed, here’s what I suggest.

Dig Deeply and Amend Like Crazy: Wait until the soil is lightly moist (not wet), and dig deeply into your bed bit by bit, putting portions of the clay soil in a wheel barrow. Then add loads of organic matter into the dug area–compost, leaf mulch, peat moss, earthworm castings, etc. (Check out Black Gold’s amendments.) Then slowly mix your clay soil back in. If you have a small tiller to really work it in and blend it well, that would be best. Continue to add lots of organic matter over several seasons, and your soil will be wonderful.

I also recommend taking all of your leaves at the end of the season, chopping them up, and laying them thickly over your bed like mulch. Once springtime comes they will have begun to break down and feed the soil. Using compost instead of bark mulch will also give your soil more enrichment.

We also encourage you read this article about Double Digging!

Happy gardening! Jessie


Plants are the lens Jessie views the world through because they’re all-sustaining. (“They feed, clothe, house and heal us. They produce the air we breathe and even make us smell pretty.”) She’s a garden writer and photographer with degrees in both horticulture and plant biology from Purdue and Michigan State Universities. Her degrees were bolstered by internships at Longwood Gardens and the American Horticultural Society. She has since worked for many horticultural institutions and companies and now manages communications for Sun Gro Horticulture, the parent company of Black Gold. Her joy is sharing all things green and lovely with her two daughters.

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