What is the general rule concerning the composition of rocks?
Do these distinctions affect the fertility of soils formed from them?
What do we mean by the mechanical character of the soil?
Is its fertility indicated by its mechanical character?
As a general rule, it may be stated that all rocks are either sandstones, limestones, or clays; or a mixture of two or more of these ingredients. Hence we find that all mineral soils are either sandy, calcareous, (limey), or clayey; or consist of a mixture of these, in which one or another usually predominates. Thus, we speak of a sandy soil, a clay soil, etc. These distinctions (sandy, clayey, loamy, etc.) are important in considering the mechanical character of the soil, but have little reference to its fertility.
By mechanical character, we mean those qualities which affect the ease of cultivation—excess or deficiency of water, ability to withstand drought, etc. For instance, a heavy clay soil is difficult to plow—retains water after rains, and bakes quite hard during drought; while a light sandy soil is plowed with ease, often allows water to pass through immediately after rains, and becomes dry and powdery during drought. Notwithstanding those differences in their mechanical character, both soils may be very fertile, or one more so than the other, without reference to the clay and sand which they contain, and which, to our observation, form their leading characteristics. The same facts exist with regard to a loam, a calcareous (or limey) soil, or a vegetable mould. Their mechanical texture is not essentially an index to their fertility, nor to the manures required to enable them to furnish food to plants. It is true, that each kind of soil appears to have some general quality of fertility or barrenness which is well known to practical men, yet this is not founded on the fact that the clay or the sand, or the vegetable matter, enter more largely into the constitution of plants than they do when they are not present in so great quantities, but on certain other facts which will be hereafter explained.
What is a sandy soil? A clay soil? A loamy soil? A marl? A calcareous soil? A peaty soil?
As the following names are used to denote the character of soils, in ordinary agricultural description, we will briefly explain their application:
A Sandy soil is, of course, one in which sand largely predominates.
Clay soil, one where clay forms a large proportion of the soil.