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?
The neutrals, silica, chlorine, oxide of iron, and oxide of manganese, deserve a careful examination. Silica exists in the soil usually in the form of sand, in which it is, as is well known, perfectly insoluble; and, before it can be used by plants, which often require it in large quantities, it must be made soluble, which is done by combining it with an alkali.
For instance, if the silica in the soil is insoluble, we must make an application of an alkali, such as potash, which will unite with the silica, and form the silicate of potash, which is in the exact condition to be dissolved and carried into the roots of plants.