Clays are formed by the disintegration of any of the crystalline rocks; the purest clays being formed from felspar. A pure clay consists simply of silica and alumina, all the other constituents having been washed out. Disintegration, however, seldom reaches such an extent; otherwise clay soils would be completely barren, which they are notably not. The impurities present in clay, which consist of alkalies, especially potash and other mineral ingredients of the plant, are what confer on clay soils their fertility. Clays differ, however, very considerably in their composition. The following is an analysis of a clay soil by Dr Anderson:-- Silica 60.03 Alumina 14.91 Peroxide of iron 8.94 Lime 2.08 Magnesia 4.22 Potash 3.87 Soda 0.06 Water and carbonic acid 5.67 ----- 99.72 ----- NOTE VII. (p. 91). FORMS IN WHICH PLANT-FOODS ARE PRESENT IN SOIL. The forms in which the bases necessary for plant-food are present in the soil, are chiefly as _hydrated silicates_, and in combination with organic acids, forming humates, &c., as well as in the form of sulphates and chlorides. Phosphoric acid is present in combination with iron, alumina, or lime, or possibly also as magnesium-ammonium-phosphate. Sulphuric acid is generally present in a more or less insoluble condition, in combination with iron and lime; whereas chlorine is combined with the alkali bases in an easily soluble form. An important point is as to the form in which the plant absorbs these food constituents. In this connection reference may be made to a theory put forward by a very distinguished French agricultural chemist, Professor Grandeau. His theory is that the necessary ingredients of plant-food are absorbed into the plant as humates, or, at any rate, that the medium of this transference is humic acid, and organic acids of a similar nature. This theory, however, while ingenious, has not yet been supported by sufficient evidence to make its acceptance advisable. It is probable that it is only in the form of soluble salts that the plant can absorb its food. It is quite probable, however, at the same time, that the exact form in which the different food substances enter the plant may be largely determined by circumstances. According to Nobbe, chloride of potassium is the most suitable form of potassium salts, although the plant may absorb its potassium as sulphate, phosphate, or even silicate. FOOTNOTES: [61] Composition of the earth's solid crust in 100 parts by weight:-- Oxygen 44.0 to 48.7 | Calcium 6.6 to 0.9 Silicon 22.8 to 36.2 | Magnesium 2.7 to 0.1 Aluminium 9.9 to 6.1 | Sodium 2.4 to 2.5 Iron 9.9 to 2.4 | Potassium 1.7 to 3.1