Elements of agricultural chemistry and geology

by Jas. F. W. Johnston

Granting that grass lands do thus generally increase in value, three important facts must be borne in mind before we attempt to assign the cause of this improvement, or the circumstances under which it is likely to take place for the longest time and to the greatest extent.

1. The value of the grass in any given spot may increase for an indefinite period—but it will never improve beyond a certain extent—it will necessarily be limited, as all other crops are, by the quality of the land. Hence the mere laying down to grass will not make all land good, however long it may lie. The extensive commons, heaths, and wastes, which have been in grass from the most remote times, are evidence of this. They have in most cases yielded so poor a herbage as to have been considered unworthy of being enclosed as a permanent pasture.

2. Some grass lands will retain the good condition they thus slowly acquire for a very long period, and without manuring, in the same way, and upon the same principle, that some rich corn lands have yielded successive crops for 100 years without manure. The rich grass lands of England, and especially of Ireland, many of which have been in pasture from time immemorial, without, it is said, receiving any return for all they have yielded, are illustrations of this fact.

3. But that others, if grazed, cropped with sheep or meadowed, will gradually deteriorate, unless some proper supply of manure be given to them,—which required supply must vary with the nature of the soil, and with the kind of treatment to which it has been subjected.

In regard to the acknowledged benefit of laying down to grass, then, two points require consideration,—what form does it assume?—and how is it effected?

1. The improvement takes place by the gradual accumulation of a dark-brown soil on the surface, rich in vegetable matter: and which soil thickens or deepens in proportion to the time which elapses from its being first laid down to grass.

If the soil be very light and sandy, the thickening is sooner arrested; if it be moderately heavy land, the improvement continues for a longer period; and some of the heaviest clays in England are known to bear the richest permanent pastures. On analyzing the soils of the richest of these pastures, whatever be the degree of tenacity of the clays or loams (the subsoils) on which they rest, or their deficiency in vegetable matter,—they are found to be generally characterized by containing from 8 to 12 per cent. of organic, chiefly vegetable matter, from 5 to 10 only of alumina, and from 1 to 6 per cent. of lime.

Thus the soil formed on the surface of all rich old pasture lands is possessed of a remarkable degree of uniformity,—both in physical character and in chemical composition. This uniformity they gradually acquire, even upon the stiff clays of the Lias and of the Oxford clay, which originally, no doubt, have been,—as many clay lands still are,—left to natural pasture from the difficulty and expense of submitting them to arable culture.

2. But how do they acquire this new character, and why is it the work of so much time? When the young grass throws up its leaves into the air, from which it derives so much of its nourishment, it throws down its roots into the soil in quest of food of another kind. The leaves may be mown or cropped by animals, and carried off the field, but the roots remain in the soil, and, as they die, gradually fill its upper part with vegetable matter. It is not known what average proportion the roots of the natural grasses bear to the leaves; no doubt it varies much, both with the kind of grass and with the kind of soil. When wheat is cut down, the quantity of straw left in the field, in the form of stubble and roots, is sometimes greater than the quantity carried off in the sheaf. Upon a grass field two or three tons of hay may be reaped from an acre; and if we suppose only a tenth part of this quantity to die every year in the form of roots or parts of roots, or of excretions from roots, we can easily understand how the vegetable matter in the soil thus gradually accumulating, should at length become very considerable in quantity. In arable land this accumulation is prevented by the constant turning up of the soil, by which the vegetable fibres being exposed to the free access of air and moisture, are made to undergo a more rapid decomposition.

But the roots and leaves of the grasses contain inorganic earthy and saline matter also. Dry hay leaves from an eighth to a tenth part of its weight of ash when burned. Along with the dead vegetable matter of the soil, this inorganic matter accumulates also on the surface, in the form of an exceedingly fine earthy powder; hence one cause of the universal fineness of the surface mould of old grass fields. And the earthy portion of this inorganic matter consists chiefly of silica and lime, with scarcely a trace of alumina, so that, even on the stiffest clays, a surface soil may be ultimately formed, in which the quantity of alumina will be comparatively small.