The extent to which the humus of a soil may be derived from the vegetable debris falling or growing upon the surface, varies greatly with the climatic conditions as well with the nature of the soil. In the forests of humid climates with loamy soils, not only does the autumnal leaf-fall, as well as decaying twigs and trunks, become obviously incorporated with the surface soil as decay progresses on the lower surface, but active animal agencies (see below) carry the organic remnants bodily down. But where heavy clay soils prevail, these animal agencies are much restricted by the compactness of the material; only a light surface-layer of mold would be formed, and the humus of the lower soil layers must of necessity be derived from the decay of the roots only. This origin is claimed by Kosticheff[45] for the high content of black humus in the tchernozem or black earth of Russia. Following Hellriegel in determining the weight of roots contained in successive equal layers of soil from the surface downwards, Kosticheff gives for each six inches the following data as found in the tchernozem, taking as 100 the root-content of the surface layer: ===========+======+======+======+======+======+====== Number. | 1 | 1 | 2 | 2 | 3 | 3 Depth. |Roots.|Humus.|Roots.|Humus.|Roots.|Humus. -----------+------+------+------+------+------+------ 6 inches. |100. | 5.42 |100. | 8.11 |100. | 9.64 12 “ | 89.1 | 4.83 | 63.9 | 5.19 | 80.3 | 7.77 18 “ | 66.9 | 3.62 | 48.3 | 3.92 | 70.0 | 6.71 24 “ | 47.3 | 2.56 | 35.0 | 2.84 | 58.4 | 5.61 30 “ | 47.3 | 2.59 | 26.0 | 2.11 | 38.2 | 3.57 36 “ | 34.6 | 1.88 | 18.1 | 1.47 | 33.0 | 3.18 42 “ | 23.9 | 1.29 | 6.3 | .51 | 16.2 | 1.56 48 “ | 14.4 | .78 | | .70 | | 54 “ | 6.7 | .36 | | | | -----------+------+------+------+------+------+------ [45] Abstract in Ann. de la Science Agronomique, Tome 2, 1887. It will be seen that there is a very close correspondence of the humus content with the root development in the several layers, and it seems as if though but little of the humus could be derived from the surface growth, which is that of the grasses of the steppe. The climate of the black-earth country of Russia is, though not properly arid, yet one of rather deficient and uncertain rainfall. But as a consequence of extremely arid conditions, and in sandy lands, it may even happen that the immediate surface soil contains _less_ humus than what, in the farmers’ habitual parlance, would be called the subsoil; because of the penetration of slow combustion for some distance into the porous soils. It will then be lower down that, in the presence of a favorable degree of moisture and lower temperature, the conditions of normal humification are fulfilled.