Normal humification takes place only under the influence of moderate temperature. When the temperature is too low, bacterial and fungous growth are repressed or arrested; when too high, the fungous vegetation assumes a different phase, the result of which is the almost total oxidation of the organic matter, sometimes so accelerated as to initiate rapid combustion “fire-fanging” of dung; leaving in any case but a trifling organic residue of very high ash contents.[44]
Eremacausis.—In the absence of a sufficient degree of moisture to co-operate with the other agencies of humification, the final result in the soil is practically the same as in the “fire-fanging” of dung. The organic matter is almost wholly destroyed by direct oxidation (eremacausis) with or without the aid of minute organisms; leaving essentially only the ash behind to be reincorporated with the soil. This is to a very great extent the predominant process in the arid regions of the Globe; most of the soils formed in these climates being, therefore, very poor in humus-substances, and deriving it almost entirely from the decay of roots only.
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 | 8.11 | 100. | 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 | ||||
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.
It is not always, then, that the commonly recognized distinction between surface soil and subsoil based upon humus content can be maintained. But the observation of everything bearing upon this point is of the utmost importance in determining both the agricultural value and the mode of treatment of the land.
Losses of Humus from Cultivation and Fallowing.—The fact that humus accumulates in woodlands and meadows, where no cultivation is given, would naturally lead to the converse conclusion, viz., that cultivation causes loss of humus and of its constituents. That this is actually the case is recognized and widely acted upon in practice, and there is no question that the general acceptance of stable manure as the most widely useful fertilizer, despite its usually low content of plant-food ingredients, is based upon the fact that it supplies vegetable matter, in a condition highly favorable to its conversion into humus. The most direct and cogent proof of the depletion of the soil of both humus and nitrogen by continuous cultivation of cereal grains has been given by Snyder,[46] who determined the loss both of humus and of nitrogen suffered by a Minnesota soil during eight years’ continuous cultivation of wheat. The total loss of nitrogen was 1700 pounds per acre, while only 350 pounds were utilized by the crop; about 1400 pounds being dissipated as gas or leached out as nitrates. A conservative estimate of the loss of humus suffered during the same period was about a ton per acre annually, and this loss seriously decreased not only the nitrogen-content, but rendered the soil more compact and less retentive of moisture. But by rotation of the wheat with clover in alternate years, very nearly an equilibrium of both humus and nitrogen-content was obtained. In addition, the amount of available mineral plant-food was decreased by continuous grain culture. Ladd has made similar observations in North Dakota, with similar results.
That excessive aeration results in serious losses of humus as well as of nitrogen, is very obvious in the arid region, where it is the habit to maintain on the surface of orchards and vineyards during the dry, hot summers, a thick mulch of well-tilled soil, thus preventing loss of water by evaporation. In the course of years this surface soil becomes so badly depleted of humus that good tilth becomes impossible, the soil becoming light-colored and compacted; while the loss of nitrogen is indicated by the small size of the orchard fruits. Similar losses are of course sustained in the practice of bare summer-fallow, which at one time was almost universal in portions of the arid region. The complete extirpation of weed growth thus brought about, at first considered an unmixed benefit, has ultimately had to be made up for by the practice of green-manuring; since in the arid region the use of stable manure encounters many difficulties.
Estimation of Humus in Soils. It has been usual to determine the amount of humus in soils by means of (dry or wet) combustion, calculating the humus from the carbonic dioxid so formed, while measuring the nitrogen gas directly. But in this process the entire organic matter of the soil, humified and unhumified, is indiscriminately included; and it is wholly uncertain to what extent the latter will ultimately become humus, from the nitrification of which plants are presumed to chiefly derive their nitrogen.[47] In order to obtain definite results, the actual, functional humus must be extracted from the soil mass by some solvent which discriminates between the humified and unhumified organic matter. This cannot be done by direct extraction with caustic soda or potash, which inevitably dissolve unhumified matters and tend to expel ammonia from the humus; besides themselves acting as humifiers ([see this chapter, p. 125].)