A soil pit has around 10 baskets of soil in it. After that it looks empty, but there are 3 worms in it. Take them out for fishing. After that you can use the shovel on the soil pit. You get 9 additional baskets of soil. After that the pit will turn into a clay pit. If you wait, the pit will fill slowly with clay. Usually there are a max of 5 clay in the pit. Take as much as you need, it will refesh.
This is great help!