It will be noted that one-half of the total consists of oxygen, and that nearly 86% (or 47.29% of the 49.98%) of this amount is contained in the solid rocks; nearly 2.50% of the remainder in sea and other water; and .41% in the atmosphere, in the free condition, in which it serves for the respiration of animals and plants, and for the various processes of slow and rapid combustion, or “oxidation.” This relatively small proportion of the whole, is, nevertheless, the most directly important for the maintenance of organic life.
Oxids Constitute Earth’s Crust.—The vast predominance of oxygen in the above list suggests at once that most of the other elements must exist in combination with it, i. e., as “oxids.” H. S. Washington[2] has lately revised the estimates heretofore made, on the basis of a very large number of analyses made by him and others, of rocks within the United States, and gives the following table; alongside of which is placed a revised estimate by Clarke, which also includes rocks from abroad; both being given in terms of oxids of the several elements.
| Washington. | Clarke. | ||
| Silica | SiO₂ | 57.78 | 59.89 |
| Alumina | Al₂O₃ | 15.67 | 15.45 |
| Peroxid of Iron | Fe₂O₃ | 3.31 | 2.64 |
| Protoxid of Iron | FeO | 3.84 | 3.53 |
| Magnesia | MgO | 3.81 | 4.37 |
| Lime | CaO | 5.18 | 4.91 |
| Soda | Na₂O | 3.88 | 3.56 |
| Potash | K₂O | 3.13 | 2.81 |
| Water, basic | H₂O⁺ | 1.42 | 1.52 |
| Water, acid | H₂O⁻ | .36 | .40 |
| Ferric Sulphid | FeS₂ | 1.03 | .60 |
| Phosphoric acid | P₂O₅ | .37 | .22 |
| Manganese Protoxid | MnO | .22 | .10 |
The salient point which at once attracts attention in these tables is the great predominance of the oxid of silicon—silica, silicic acid, quartz, etc.,—over all other substances. While quartz occurs alone in enormous masses, as will be shown later, probably the greater proportion is found in combination with other oxids, notably those of aluminum, calcium, iron, magnesium, and the alkali metals potassium and sodium. Chlorin and fluorin, however, do not occur as oxids.[3]
The Chemical Elements Important to Agriculture.—Of the numerous elements known to chemists, only eighteen require mention in connection with either soil formation or plant growth; and of these only thirteen or fourteen participate in normal plant growth. They are the following:
| Metallic Elements | Non-metallic Elements |
| Potassium | Carbon |
| Sodium | Hydrogen |
| Calcium | Oxygen |
| Magnesium | Nitrogen |
| Iron | Phosphorus |
| Manganese | Sulphur |
| Aluminum | Chlorin |
| Titanium | Fluorin |
| Iodin | |
| Silicon. |
Of this list, titanium, though a very constant ingredient of soils in the form of titanic dioxid, is not known as performing any important function in soils, and is not, so far as known at present, ever taken up by plants. Aluminum, in the form of its compounds with oxygen and silicon, is a very prominent and physically very important soil ingredient, but does not, apparently, perform any direct function in plant nutrition, and is absent from their ash, except in the case of some of the lower plants (horsetails and ferns).
Iodin appears to be normally present in all seaweeds, and occurs in traces in some land plants. Fluorin is a normal ingredient of animal bones, and its presence in plant ashes is often easily shown. The remaining fourteen, however, are always present in plants; carbon, hydrogen, oxygen and nitrogen forming the volatile or combustible part, while the rest occur in the ashes.
It is true that other elements, or rather their compounds, are sometimes found in plants, being taken up by them from solutions existing in the soil. Thus the alkalies caesium and rubidium, also barium, strontium, zinc, copper, boron and some others, may be absorbed when present in soluble form. But they are neither necessary nor beneficial to plant economy, and when in considerable amounts are harmful. Thus fifteen elements, ommiting iodin and titanium, alone require discussion.
The Volatile Part of Plants, as already stated, consists of carbon, hydrogen, oxygen and nitrogen. Of these, carbon is obtained by the plant exclusively from the carbonic (dioxid) gas of the air; hydrogen and oxygen, from the soil in the form of water; nitrogen, directly from the soil but indirectly also from the air, through the agency of certain bacteria. The ash ingredients of course are all derived from the soil through the roots, and must all be present in the latter in an available form, to a sufficient extent to supply the demands of vegetation.