Elements of agricultural chemistry and geology

by Jas. F. W. Johnston

But there are still other agencies at work by which the surface of stiff soils is made to undergo a change. As the roots penetrate into the clay, they more or less open up a way into it for the rains. Now the rains in nearly all lands, when they have a passage downwards, have a tendency to carry down the clay along with them. They do so, it has been observed, on sandy and peaty soils, and more quickly when these soils are laid down to grass. Hence the mechanical action of the rains,—slowly in many localities, yet surely,—has a tendency to lighten the soil, by removing a portion of its clay. They constitute one of those natural agencies by which, as elsewhere explained, important differences are ultimately established, almost everywhere, between the surface crop-bearing soil and the subsoil on which it rests.

But further, the heats of summer and the frosts of winter aid this slow alteration. In the extremes of heat and of cold, the soil contracts more than the roots of the grasses do; and similar though less striking differences take place during the changes of temperature experienced in our climate in a single day. When the rain falls on the parched field, or when a thaw comes on, the earth expands, while the roots of the grasses remain nearly fixed; hence the soil rises up among the leaves, mixes with the vegetable matter, and thus assists in the slow accumulation of a rich vegetable mould.

The reader has witnessed in winter how, on a field or a by-way side, the earth rises above the stones, and appears inclined to cover them; he may even have seen in a deserted and undisturbed highway, the stones gradually sinking and disappearing altogether, when the repetition of this alternate contraction and expansion of the soil for a succession of winters has increased in a great degree the effects which follow from a single accession of frosty weather.

So it is in the fields. And if a person skilled in the soils of a given district can make a guess at the time when a given field was laid down to grass, by the depth at which the stones are found beneath the surface, it is because this loosening and expansion of the soil, while the stones remain fixed, tends to throw the latter down by an almost imperceptible quantity every year that passes.

Such movements as these act in opening up the surface-soil, in mixing it with the decaying vegetable matter, and in allowing the slow action of the rains gradually to give its earthy portion a lighter character. But with these, among other causes, conspire also the action of living animals. Few persons have followed the plough without occasionally observing the vast quantities of earthworms with which some fields seem to be filled. On a close shaven lawn many have noticed the frequent little heaps of earth which these worms during the night have thrown out upon the grass. These and other minute animals are continually at work, especially beneath an undisturbed and grassy sward—and they nightly bring up from a considerable depth, and discharge on the surface, their burden of fine fertilizing loamy earth. Each of these burdens is an actual gain to the rich surface soil, and who can doubt that in the lapse of years, the unseen and unappreciated labours of these insect tribes must both materially improve its quality and increase its depth?

There are natural causes, then, which we know to be at work, that are sufficient to account for nearly all the facts that have been observed, in regard to the effect of laying lands down to grass. Stiff clays will gradually become lighter on the surface, and if the subsoil be rich in all the kinds of inorganic food which the grasses require, will go on improving for an indefinite period without the aid of manure. Let them, however, be deficient, or let them gradually become exhausted of any one kind of this food, and the grass lands will either gradually deteriorate after they have reached a certain degree of excellence—or they must be supplied with that ingredient—that manure of which they stand in need. It is doubtful if any pasture lands are so naturally rich as to bear to be cropped for centuries without the addition of manure, and at the same time without deterioration.[21]

On soils that are light, again, which naturally contain little clay, the grasses will thrive more rapidly, a thick sward will be sooner formed, but the tendency of the rains to wash out the clay may prevent them from ever attaining that luxuriance which is observed upon the old pastures of the clay lands.

On undrained heaths and commons, and generally on any soil which is deficient in some fertilizing element, neither abundant herbage, nor good crops of any other kind, can be expected to flourish. Laying such lands down, or permitting them to remain in grass, may prepare them for by-and-by yielding one or two average crops of corn, but cannot be expected alone to convert them into valuable pasture.

Finally, plough up the old pastures, on the surface of which this light and most favourable soil has been long accumulating—and the heavy soil from beneath will be again mixed up with it—the vegetable matter will disappear rapidly by exposure to the air,—and if again laid down to grass, the slow changes of many years must again be begun through the agency of the same natural causes, before it become capable of again bearing the same rich herbage it was known to nourish while it lay undisturbed.

Many have supposed that by sowing down with the natural grasses, a thick sward may at once be obtained—and on light loamy lands, rich in vegetable matter, this method may, to a certain extent, succeed—but on heavy lands, in which vegetable matter is defective, disappointment will often follow the sowing of the most carefully selected seeds. By the agency of the causes above adverted to—the soil gradually changes, so that it is unfit, when first laid down, to bear those grasses which, ten or twenty years afterwards, will naturally and luxuriantly grow upon it.