O horizon

The O horizon is a soil layer found at the land surface, primarily composed of litter and humus. Humus is decomposed organic matter, dark in color and rich in carbon. This horizon is crucial as it represents the uppermost layer of organic material before mineral soil begins.123

Soil horizons are distinct layers within the soil profile, formed by the actions of organisms and water. They are typically designated by letters, with the O horizon being the organic layer, followed by the A horizon (mineral soil with humus accumulation), E horizon (leached layer), B horizon (accumulation of clay), C horizon (unweathered material), and R horizon (bedrock). The O horizon is significant because it is the initial stage of soil formation, contributing organic matter and nutrients to the layers below.12