Best Pottery Clay for making deep botanical imprints?

Hello, Everyone ~

I've been asked to help a community group of teens making a botanical mural. Since I mainly play in the mud (concrete/hypertufa) and not that often with clay, I don't have a lot of experience with different clays. The red pottery clay that the group has now isn't soft enough to hold imprints well. We are pushing real fruit (hard pears, green figs, etc.), branches and twigs, and real leaves into the clay. We'd like to make fairly deep imprints in the clay.

I plan to go to a pottery supply store to buy 50 pounds of clay for the group. Is there a particular type or brand of clay I should ask for? Or should I ask for the softest pottery clay in stock?

Thanks for any help/suggestions you can provide.

Eva