Our soil is has lots of clay and it makes any type of tuber grow funny. This year I decided to try to grow carrots in a custom soil mix. We are trying two heirloom varieties this year in a section of our 30 foot long grow box.
We mix our carrot seeds in with dry sand and then pour the sand into the area we want the carrots to grow. The carrot seed is added into a mason jar at one part seeds to 100 parts of sand and mixed thoroughly. For our purpose we added 1/2 a teaspoon of carrot seeds to 1 cup of sand.
In the picture below you can see the sand that was applied in a row just over the top of the seed packs. The one cup of sand and seeds ended up being 10 feet long once applied.