What layer in the soil profile has the most organic material?

A is the layer of the soil profile in which most organic material is found.

Specifically, there are five layers in the soil profile. The first, A layer of organic matter claims the first two inches and contains plant debris. The second, A layer of surface soil comprises the next ten inches down. Below it will be found the third, B layer of subsoil containing aluminum, clay, iron and organic compounds and going down 30 inches; the fourth, C layer of parent rock going down 48 inches; and the fifth, final, R layer of bedrock.