Why does humus tend to increase with the clay content of soil? ?

The humus tend to increase with the clay content of the soil because the clay soil absorbs much water.

The soil containing humus is dry and friable and the clay soil is flat and hard where there is no space for further decomposition.

The clay particles stick together and does not allow the particles to move in the soil.

The humus has 30 to 40 per cent more attraction to nutrients than clay soil.

igneous rocks are formed by solidification of cooled magma (molten rock), with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks.  

the composition depends on the source magma. there are almost a thousand types of composition found till now.  

hope this