Take your baby outside and let her experience some dirt, then make the sign. After a few times of playing in the dirt, your baby will learn to make the association to the sign. As your baby grows older, you can extend this understanding and talk about plants growing in the dirt, and ants and worms living in the dirt as well.
To start introducing the dirt sign, take your baby outside and let them get their little hands dirty. You can then introduce the dirt flash card to help them broaden their understanding of the word.