Below the line at which the land appears to meet the sky, there are other horizons. These “hidden” horizons are the descending layers within a soil. Horizons are the visible evidence of soil-forming factors at work—climate, organisms, relief (topography), parent materials, and time.
Scientists group soils based on the arrangement and properties of horizons. Soils of the same type share horizons with similar properties such as color, texture, types of minerals, and organic content.
In some soils, the boundaries between layers may be sharp; in others, the composition of the layers gradually changes. Not all soils have horizons, but by “reading” soil layers, along with observing moisture content and temperatures, scientists can classify or describe soils.