If we look carefully on the sides of a trench made by digging soil from the surface of earth downwards we will find that the soil consists of different layers. Scientists call these layers horizons. 

A horizon                             this is the uppermost lay of soil often refered to as to topsoil lt is dark due to the pressure of humus the particles of this layer are the finest and the roots of only very big trees manage to reach the next layer.                     B horizon