The C and R horizon
Anywhere from "none" to "lots and lots". Calcareous soil is soil high in calcium (lime) content. it has nothing to do with the percentage of humus, which is decayed organic material.
Sand and humus.
Humus
It is rich in mineral nutrients but lacking in organic matter - humus.
The first is organic material ,second is humus, third is oxygen; but don't forget water and minerals which are also essential.
topsoil.
All soil contains some humus. If it doesn't it is not soil.
Humus
the two layers are sub soil The layer of soil that has little or no humus in it is the subsoil. The top layer of the soil has the most humus in it because the dead leaves fall from the plant and go back into the soil.
It is substratum because it has little or no moisture and little or no humus.
Topsoil contains organic matter (humus)
Humus
bed rock
Humus is located in the top layer of soil. It is a type of organic soil matter and aids in nutrition retention in the soil.
the O-Horizon is the topmost soil so it has the most humus
topsoil
topsoil