Soil is composed of broken rock particles, decaying organic matter called humus, water, air, and organic matter. It takes 100 years to form 1 inch of soil. Over time, soil develops distinct horizontal layers called horizons that divide it into a profile. The top layer, A horizon, contains high organic matter and humus. The middle layer, B horizon, is enriched with clay minerals. The bottom layer, C horizon, consists of weathered parent material directly above bedrock and contains no organic material.