" What's inside potting soil? Everything but soil. danchooalex/Getty Images If you find it shocking that there's no actual dirt in potting soil (also known as potting mix), you're not alone. Much of the confusion is probably because gardening experts don't use the word "soil" the same way that regular people do. "Soil is a broad term and is often used interchangeably with planting medium. Soil in general is composed of minerals and organic matter (decomposing plant, animal or microbial matter)," explains Paul Cunningham, hard goods buyer for Pike Nurseries & Armstrong Garden Centers in an email interview. "A planting medium refers to a variety of materials used for planting in containers or the ground. Depending on the recipe, potting soil is soil because it contains all ingredients mentioned."