Explanation

In soil profile, A-horizon is present under the litter zone and is called as top-soil. It is the the zone of eluviations that contains a relatively high content of organic matter but mixed with mineral water. It is further divided into three sub-zones : (i)$\mathbf{A}_{\mathbf{1}}$ region : It is dark and rich in organic matter. Finely divided organic matter here, becomes mixed with the mineral matter and is known as humus. It is dark brown or black coloured. (ii)$\mathbf{A}_{\mathbf{2}}$-region : It contains less humus and is called as the zone of maximum leaching. (iii)$\mathbf{\ }\mathbf{A}_{\mathbf{3}}$-region : It is transitional to B-zone but is more like the A-zone than B. Sometimes, it is totally absent.