Ask a Master Gardener: How to grow carrots if you have clay soil
If your heart is set on carrots, your best bet is to use a raised bed.
Q: I want to grow carrots, but my soil is dense clay. All the sources I read say carrots prefer a sandy loam. How much sand should I add to get it to be the right texture?
A: This is a question we get pretty often. So many of the homes in our region have clay soil. And the answer is: You really cannot add enough sand to turn clay soil into sandy loam. You would need to replace at least half of the clay with sand. If you do add sand to clay soil, it actually makes the texture worse. You’re essentially creating cement.
The better way to improve your soil texture is to spread a thick layer of organic mulch over it year after year for several years. As the mulch breaks down and you replace it, the soil will gradually improve.
That won’t get you a place to grow carrots this year, though. If your heart is set on carrots, your best bet is to put in a raised bed and fill it with a mix of soil and compost. Carrots grow well in raised beds. If your raised bed isn’t very deep, look for varieties of carrots that don’t grow too long.
Carrots need full sun and prefer cool weather. They take a while to germinate after planting, so many sources recommend planting a few radish seeds with them. The radishes will come up more quickly and will mark where the row is.
Also crucial: Carrots must be thinned. My sister was so disappointed years ago with her failed carrot crop. She got nothing but tiny, skinny roots when she dug them up. I asked, “Did you thin them?” And she said, “Do what now?” Oops.
When the plants are about 2 inches tall, pull out enough of them that the remaining ones are 2-4 inches apart. Thinning is a fussy job, but well worth the effort.
There's more information about why not to add sand to clay soil at https://bit.ly/3LqqZmf .
Written by U of M Extension Master Gardeners in St. Louis County. Send questions to firstname.lastname@example.org.