Examine the ingredients of soil, Earth’s thinnest and most fragile layer and the frontier between the worlds of geology and biology, in this video from NOVA: Making North America: Human. Kirk Johnson meets with fellow geologist Dave Montgomery, who reveals the recipe that makes life on land and all agriculture possible. Soil is made of sand, silt, clay, and organic matter. It’s primarily a product of minerals and dead organisms that, over time and with the help of living things including worms, are broken down.