While we know of some seventy elements present in the earth's crust, it is practically made up of only some sixteen. These sixteen are—oxygen, silicon, carbon, sulphur, hydrogen, chlorine, phosphorus, iron, aluminium, calcium, magnesium, sodium, potassium, fluorine, manganese, and barium.[61] Of these, oxygen is by far the largest constituent, forming, roughly speaking, about 50 per cent.
The main mass of the rocks consists of silica, and this is generally combined with alumina, as in clay, forming aluminium silicate, and with the commoner alkalies and alkaline earths. Another extremely abundant compound is carbonate of lime, which, as limestone, chalk, and marl, forms one-sixth of the earth's total rocks.
The word "mineral" means a definite chemical compound of natural occurrence. The number of minerals is very great, and it is impossible to go into the subject here. Reference can only be made to a few of the more prominent ones, which are chiefly concerned in the formation of soils.
Those formed out of silicates are, from the agricultural point of view, the most important, as they form a very large group; and it is by their disintegration that soils are chiefly formed. They consist of silica and alumina, along with various other substances, chiefly alkalies and alkaline earths. It is important to note one peculiarity about the solubility of silicates. We have two classes of silicates: the one, which is called "acid," and contains an excess of silica; the other, "basic," and which contains an excess of base. Now, while the former of these is more or less insoluble, the second is soluble. This fact has an important signification in the process of the disintegration of the silicate minerals we are about to consider.
The first and most important class are the Felspars. Felspar is not really a definite mineral, with a definite chemical composition, but rather the name of a class of minerals of which there are several different kinds. The felspars are composed of silica and alumina, along with potash, soda, and lime, with traces of iron and magnesia. Their principal constituents, however, are silica and alumina, along with either potash, soda, or lime. According as the base potash, soda, or lime predominates, the felspar is known as Orthoclase, Albite, and Oligoclase, respectively.
The following are the analyses of the three minerals (by the late Dr Anderson):—
| Orthoclase. | Albite. | Oligoclase. | ||||
| 1. | 2. | 1. | 2. | 1. | 2. | |
| Silica | 65.72 | 65.00 | 67.99 | 68.23 | 62.70 | 63.51 |
| Alumina | 18.57 | 18.64 | 19.61 | 18.30 | 23.80 | 23.09 |
| Peroxide of iron | traces | 0.83 | 0.70 | 1.01 | 0.62 | none |
| Oxide of manganese | traces | 0.13 | none | none | none | none |
| Lime | 0.34 | 1.23 | 0.66 | 1.26 | 4.60 | 2.44 |
| Magnesia | 0.10 | 1.03 | none | 0.51 | 0.02 | 0.77 |
| Potash | 14.02 | 9.12 | none | 2.53 | 1.05 | 2.19 |
| Soda | 1.25 | 3.49 | 11.12 | 7.99 | 8.00 | 9.37 |
| 100.00 | 99.47 | 100.08 | 99.83 | 100.79 | 101.37 | |
According as these various felspars are present in a soil, so will the quality of the soil be. It stands to reason that as the presence of potash in a soil is one of the distinguishing features of its fertility, much will depend on the extent to which the orthoclase felspar is present; and also, not only on the extent, but on the state and degree of its disintegration. It is important to note the method of this disintegration. It is effected by the absorption of water. This water is not merely absorbed mechanically, but actually enters into the composition of the mineral. It is not present as moisture merely, capable of being expelled at ordinary boiling temperature, but it forms what is known as water of composition. In this process of hydration, the mineral loses its lustre and crystalline appearance, crumbles away into a more or less—according to its state of disintegration—powdery mass. A very great change is also effected in its chemical composition; it loses nearly all its base. This is effected in the following way. As water enters into the mineral's composition, it sets free a certain portion of the base; there is thus formed a basic silicate, which, being soluble in water, is washed away in solution. This change may be illustrated by quoting the analysis of a kaolin clay formed by the disintegration of orthoclase felspar.
| Kaolin Clay formed by disintegration of Orthoclase. | |
| Silica | 46.80 |
| Alumina | 36.83 |
| Peroxide of iron | 3.11 |
| Carbonate of lime | 0.55 |
| Potash | 0.27 |
| Water | 12.44 |
| 100.00 | |
The chief difference here is the almost total loss of potash and a portion of the silica, and the gain of water. The other constituents practically remain insoluble.