Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER in. Composition of Soils. A Soil, like a plant, consists of two distinct classes of substances, — first, organic or vegetable, derived, as we have seen, from decaying growths; second, inorganic or mineral constituents, derived from the rocks which form the earth's surface. Organic Substances. — Organic substances are made up of carbon, oxygen, hydrogen, and nitrogen. Of these the nitrogen is of the most direct importance in the growth of the plant, and is the valuable constituent of the humus already described. Inorganic Substances The inorganic or mineral substances of the soil are also identical with the substance of the ash of plants (with the addition of alumina, which is not taken up by the latter), namely, silica, alumina, lime, potash, magnesia, phosphoric acid, soda, iron, chlorine, and sulphuric acid. The first three of these, as has already been noted, are the principal ingredients of soils, and give to them their distinctive character; with the exception of lime, they do not aid materially in furnishing food. The more important constituents are phosphoric acid and potash. Phosphoric Acid. — Phosphoric acid is an ingredient of all fertile soils, but is contained in very small quantitles as compared with other constituents. Its most common combination is with lime, though it is frequently found in combination with iron and aluminum. Bocks which contain "fossils," or fossiliferous rocks, frequently contain high percentages of phosphoric acid. Potash. — Potash is also derived from rocks, and varies in the amount contained in different soils. Those derived directly from granite or trap are the richest in this element; it exists in the soil in combination with silica, forming substances called " silicates," which are of great importance. Lim...