Sand is a surface type that can be found in a garden in Viva Piñata: Trouble in Paradise. The relevant Surface Packet can be obtained by capturing a piñata in the Dessert Desert and then placing it in the garden. Only a few plants can be grown on sand.
In Viva Piñata: Pocket Paradise sand only exists in the Playground as a visual replacement of soil, it otherwise does the same thing as soil.