Chickens not using sand bath correctly

They might just like the texture of the plant pot dirt or are just used to using it. Can you sprinkle some of the dirt from the plant pot onto the sand box and see if they like that? Can you move the pot or cover it so they can't use it and will look for an alternative and find the sand box?

Chickens like to scratch themselves a hole to dust bathe in. For whatever reason the spot is appealing to them. I would fence it off to make it less accessible. That isn't to say they won't figure a way in, like flying. After you have discouraged access to the garden plot, try presenting the sandbox again. They may need a few days to adjust to it being there. You may want to try sinking it into the ground so it has a more realistic feel. Also, make sure it is in a sunny location. They will use it double duty, sunning and bathing. Dirt is no less clean then sand so if they are still resisting the sand, try mixing it with dirt or just use dirt by itself. And do not offer them mud like the other person suggested. That is poor for them and poor for the person who has to give them a bath later. They need to stay dry so that bacteria and parasites are not encouraged. Jessica (a veteran poultry raiser who has had way more than 3 chickens)