It is covered with a thick sheet of black earth, a kind of loess, that is mixed with humus.
The nitrogen in decaying roots, in the dead stems. and leaves of plants, and in humus generally is sooner or later changed into a nitrate, the change being effected by bacteria.
The turf is taken off either with the breast plough - a paring tool pushed forward from the breast or thighs by the workman - or with specially constructed paring ploughs or shims. The depth of the sod removed should not be too thick or burning is difficult and too much humus is destroyed unnecessarily, nor should it be too thin or the roots of the herbage are not effectually destroyed.
The general surface of the interior highland consists of bare rolling moor-like country, with a great amount of red claylike soil, while the valleys have a rich humus of bluish-black alluvium.
Loess, often thin and always containing little humus, also covers large areas on the high, semi-arid plains in the western part of the state.