Organic matter containing _nitrogen_, such as animal substances, etc., furnish ammonia, and other nitrogenous substances to the roots of plants. FOOTNOTES: [Q] Produce. [R] By absorbing and retaining, we mean taking up and holding. [S] Nitrates are compounds of nitric acid (which consists of nitrogen and oxygen), and alkaline substances. Thus nitrate of potash (saltpetre), is composed of nitric acid and potash: nitrate of soda (cubical nitre), of nitric acid and soda. CHAPTER III. USES OF INORGANIC MATTER. [What effect has clay besides the one already named? How does it compare with charcoal for this purpose?] The offices performed by the inorganic constituents of the soil are many and important. These, as well as the different conditions in which the bodies exist, are necessary to be thoroughly studied. Those parts which constitute the larger proportion of the soil, namely the clay, sand, and limy portions, are useful for purposes which have been named in the first part of this section, while the _clay_ has an additional effect in the absorption of ammonia. For this purpose, it is as effectual as charcoal, the gases escaping from manures, as well as those existing in the atmosphere, and in rain-water, being arrested by clay as well as charcoal.[T] [What particular condition of inorganic matter is requisite for fertility? What is the fixed rule with regard to this? What is the condition of the alkalies in most of their combinations? Of the acids? What is said of phosphate of lime?] The more minute ingredients of the soil--those which enter into the construction of plants--exist in conditions which are more or less favorable or injurious to vegetable growth. The principal condition necessary to fertility is _capacity to be dissolved_, it being (so far as we have been able to ascertain) a fixed rule, as was stated in the first section, that _no mineral substance can enter into the roots of a plant except it be dissolved in water_. The _alkalies_ potash, soda, lime, and magnesia, are in nearly all of their combinations in the soil sufficiently soluble for the purposes of growth. The _acids_ are, as will be recollected, sulphuric and phosphoric. These exist in the soil in combination with the alkalies, as sulphates and phosphates, which are more or less soluble under natural circumstances. Phosphoric acid in combination with lime as phosphate of lime is but slightly soluble; but, when it exists in the compound known as _super_-phosphate of lime, it is much more soluble, and consequently enters into the composition of plants with much greater facility. This matter will be more fully explained in the section on manures. [How may silica be rendered soluble? What is the condition of chlorine in the soil? Do peroxide and protoxide of iron affect plants in the same way? How would you treat a soil containing protoxide of iron? On what does the usefulness of all these matters in the soil depend?]