Humification.—While the mechanical action of the roots and the chemical effect of the acids of their root secretions are very efficient in promoting the transformation of mere rock powder into soil material proper, the efficacy does not end with the life of the plant. In the natural process of decay to which the roots are subject after death, and which also affects the leaves, twigs and trunks falling on the surface, the vegetable matter suffers a transformation which must be considered more in detail hereafter, and results in the formation of the complex mixture of dark-tinted substances known as vegetable mold or humus; the remnant of vegetation that imparts to surface soils their distinctive dark tint. Its functions in soils are both numerous, and important to vegetable growth; as regards soil formation, it assists disintegration of the rock minerals both by the formation of certain fixed, soluble acids capable of acting on them with considerable energy, and by the slow but continuous evolution of carbonic acid under the influence of atmospheric oxygen, which has been alluded to above.
Causes influencing chemical action and decomposition.—The chemical processes causing rock decomposition are of course continued in the soil, and there also are materially influenced by climatic and seasonal conditions, which bring about great differences in the kind and intensity of chemical action.
Within the ordinary limits of solar temperatures it may be said that, other things being equal, the higher the temperature the more intense will be chemical action in soil formation. Since, however, water is a potent factor in the majority of these processes, the presence or absence of moisture at the same time with heat will cause material differences in the kind and intensity of chemical action. In view of the importance of carbonic acid as a chemical agent, the presence or absence of vegetable matter or humus, from which by oxidation or decay carbonic and humus-acids are formed, will likewise be of material influence.
The presumption that climatic and seasonal conditions must greatly influence both the kind and rapidity of the soil-forming processes, is fully borne out by observation and practice. Especially is the amount and distribution of rainfall of great importance in this respect, and should therefore be first considered.
INFLUENCE OF RAINFALL ON SOIL FORMATION;
LEACHING OF THE LAND.
In the general consideration of the soil-forming processes, it has been stated that soils formed by the disintegration of rocks “in place,” i. e., without removal from the original locality, are also designated as “residual”; meaning thereby that only a portion of the original rock remains to form the soil mass, while another portion has been removed. To a slight extent this removal occurs by the partial washing-away of the finest clay and silt particles; but the most important action from the agricultural point of view is the removal by leaching with the carbonated water of the atmosphere and soil, of certain easily-soluble compounds formed in the process of chemical decomposition of rocks and resultant soils. The nature of these compounds is exemplified in the subjoined table giving the composition of some waters flowing from drains in unmanured fields, laid at depths of from two to three feet; and for comparison with these, the composition of the water of some of the world’s large rivers, showing what these largest drains carry into the ocean.
The analyses have in all cases, where necessary, been recalculated to parts per million, and to oxids, from the published data.
The letter “c” indicates that the preceding figure has in the absence of a direct determination been stoichiometrically calculated from the data given, in order to complete the comparison.
COMPOSITION OF DRAINAGE WATERS
FROM UNMANURED GROUND.
(PARTS PER MILLION.)
| Rothamsted (Voelker) | Proskau (Krocker) | Mockern (O. Wolff) | ||||
|---|---|---|---|---|---|---|
| Rye Field. | Meadow. | |||||
| Potash, K₂O | 1.7 | 5.4 | 2.0 | 2.0 | 8.5 | 3.4 |
| Soda, Na₂O | 6.0 | 11.7 | 15.1 | 13.7 | 23.3 | 8.2 |
| Lime, CaO | 98.1 | 124.3 | 133.0 | 118.1 | 122.6 | 22.5 |
| Magnesia, MgO | 5.1 | 6.4 | 33.3 | 22.4 | 14.9 | 6.7 |
| Iron Oxid, Fe₂O₃ | 5.7 | 4.4 | 6.6 | 6.6 | 8.0 | 6.0 |
| Alumina, Al₂O₃ | ||||||
| Silica, SiO₂ | 10.9 | 15.4 | 7.0 | 6.0 | 7.0 | 4.0 |
| Carbonic Acid, CO₂ | 48.1 | 44.4 | 75.8 | 82.6 | 121.3 | |
| Phos’ Acid, P₂O₅ | .63 | 9.1 | Trace. | Trace. | Trace. | 19.0 |
| Sulfuric Acid, SO₃ | 24.7 | 66.3 | 122.7 | 67.3 | ||
| Chlorin, Cl | 10.7 | 11.1 | 4.8 | 4.2 | 14.0 | Trace. |
| Nitrogenas, N₂O₅ | 3.90 | 5.10 | ||||
| Nitrogenas, NH₃ | .12 | .13 | ||||
| Total Mineral Matter | 215.9 | 295.5 | 400.3 | 322.9 | 198.3 | 191.1 |
| Less O: Cl | 2.35 | 2.4 | 1.1 | .9 | 3.1 | |
| Corrected Total | 213.3 | 293.1 | 399.2 | 322.0 | 195.2 | 191.1 |
| Organic Matter | 22.9 | 19.3 | 25.0 | 16.0 | 26.0 | 26.0 |
| Total Solids | 235.2 | 312.4 | 424.2 | 338.0 | 221.2 | 217.1 |
| Farnham. (Way.) | Munich. (Zöller.) | Average. | ||||
| Wheat Field. | Hop Field. | Lysemeter Drainage. | ||||
| Potash, K₂O | Trace. | Trace. | 6.5 | 2.4 | 3.2 | |
| Soda, Na₂O | 14.3 | 45.7 | 7.1 | 5.6 | 15.1 | |
| Lime, CaO | 69.3 | 185.0 | 145.8 | 57.6 | 107.6 | |
| Magnesia, MgO | 9.7 | 35.1 | 20.5 | 8.9 | 16.3 | |
| Iron Oxid, Fe₂O₃ | 5.9 | 7.1 | .1 | 6.3 | ||
| Alumina, Al₂O₃ | ||||||
| Silica, SiO₂ | 1.35 | 12.1 | 10.4 | 11.3 | ||
| Carbonic Acid, CO₂ | ||||||
| Phos’ Acid, P₂O₅ | Trace. | 1.7 | 2.2 | Trace. | 0.5 | |
| Sulfuric Acid, SO₃ | 23.5 | 135.8 | 17.5 | 27.1 | 60.8 | |
| Chlorin, Cl | 10.0 | 37.4 | 57.5 | 9.5 | 17.7 | |
| Nitrogenas, N₂O₅ | 102.4 | 163.5 | ||||
| Nitrogenas, NH₃ | .25 | .03 | ||||
| Total Mineral Matter | 248.8 | 623.5 | 267.6 | 128.7 | ||
| Less O: Cl | 2.2 | 8.2 | 12.7 | 2.14 | ||
| Corrected Total | 246.6 | 615.3 | 254.9 | 126.6 | 285.7 | |
| Organic Matter | 100.0 | 105.7 | 20.5 | 12.6 | ||
| Total Solids | 346.6 | 721.0 | 275.4 | 139.2 | 352.6 | |