Soil is not merely powdered rock, but is a collection of broken and weathered rock mixed with organic matter. A soil profile is a vertical cross-section of the soil, which may have distinct layers, called horizons, which aid in classification of the soil.

An organic or ‘O’ horizon may exist above the mineral soil in the forest from dead plants and animals.

The farther down in the soil you go, the less the organic matter and the more the mineral compounds. Old-growth forests develop thick organic layers, and the forest floor is deep and spongy.