| Where weathering products gradually form or accumulate at the surface of the earth, they form a fine-grained substance that may support plant-life. This 'soil' commonly displays horizontal zones called 'soil horizons' which, together, constitute a 'soil profile.' | Some soil horizons are rich in organic debris. Others are depleted or enriched in certain chemical elements: rain percolating downward through the soil selectively dissolves elements from some layers and precipitates them in other layers. |