──────────────────────┬─────┬────────┬───────┬─────────── │CaO. │ Na₂O. │ K₂O. │P₂O₅. │ │ │ │ ──────────────────────┼─────┼────────┼───────┼─────────── Granite │ │ │ │ Quartz poryhyries} │ 1–2 │ 2–3 │ 4–5 │ 0.05–0.15 Liparite │ │ │ │ │ │ │ │ Syenite │ │ │ │ Orthoclase porphyries}│ 3–5 │ 2–6 │ 4–7 │ trace. Trachyte │ │ │ │ │ │ │ │ Nepheline syenites} │ 2–4 │ 3–7 │ 4–6 │ 0.15 Phonolites │ │ │ │ │ │ │ │ Diorites │ │ │ │ Porphyrites} │ 5–7 │ 2–4 │ 1–2 │ 0.1–0.3 Andesites │ │ │ │ │ │ │ │ Gabbros │ │ │ │ Norites} │6–10 │ 2–4 │ 0.5–2 │ 0.1–0.33 Melaphyrs │ │ │ │ │ │ │ │ Theralites │ │ │ │ Tephrites} │6–10 │ 5–7 │ 2–4 │ trace. Basanites │ │ │ │ │ │ │ │ Peridotites │ │ │ │ Picrite porphyrites} │ 1–4 │ 0–4 │trace. │ 0.0 Limburgites │ │ │ │ │ │ │ │ Pyroxenites} │8–15 │ │ │ Augitites │ │ │ │ │ │ │ │ Leucite rocks │5–10 │ 3–5 │ 5–7 │ 0.5–2 │ │ │ │ Nepheline rocks │4–10 │ 4–8 │ 1–3 │ 0.2 ──────────────────────┴─────┴────────┴───────┴─────────── =37. Origin of Soils.=—The soils in which crops grow and which form the subject of the analytical processes to be hereinafter described have been formed under the combined influences of rock decay and plant and organic growth. The mineral matters of soils have had their origin in the decay of rocks, while the humic and other organic constituents have been derived from living bodies. It is not the object of this treatise to discuss in detail the processes of soil formation, but only to give such general outlines as may bear particularly on the proper conceptions of the principles of soil investigation. =38. The Decay of Rocks.=—The origin and composition of rocks are fully set forth in works on geology and mineralogy. Only a brief summary of those points of interest to agriculture has been given in the preceding pages. The soil analyst should be acquainted with these principles, but for practical purposes he has only to understand the chief factors active in securing the decay of rocks and in preparing the débris for plant growth.