A-horizon

A soil horizon is a layer parallel to the soil surface, also the decaying matter on it (plant litter), whose physical, chemical and biological characteristics differ from the layers above and beneath. Horizons are defined in many cases by obvious physical features, mainly colour and texture. These may be described both in absolute terms (particle size distribution for texture, for instance) and in terms relative to the surrounding material, i.e. ‘coarser’ or ‘sandier’ than the horizons above and below. https://en.wikipedia.org/wiki/Soil_horizon#A_horizon