Elements of agricultural chemistry and geology

by Jas. F. W. Johnston

This fact is important to geology, because it enables this science to arrange all the stratified rocks in a certain invariable order,—which order indicates their relative age or antiquity,—since that which is lowest, like the lowest layer of stones in the wall of a building, must generally have been the first deposited, or must be the oldest. It also enables the geologist, on observing the kind of rock which forms the surface in any country, to predict at once, whether certain other rocks are likely to be met with in that country or not. Thus at C ([diagram, No. 1]), where the rock (3) comes to the surface, he knows it would be in vain, either by sinking or otherwise, to seek for the rock (1), the natural place of which is far above it; while at D he knows that by sinking he is likely to find either 2 or 3, if it be worth his while to seek for them.

To the agriculturist this fact is important, among other reasons,—

1. Because it enables him to predict whether certain kinds of rock, which might be used with advantage in improving his soil, are likely to be met with within a reasonable distance or at an accessible depth. Thus if the bed D ([diagram No. 2]) be a limestone, the instructed farmer at E knows that it is not to be found by sinking into his own land, and, therefore, brings it from D; while, to the farmer upon C, it may be less expensive to dig down to the bed D in one of his own fields, than to cart it from a distant spot where it occurs on the surface. Or if the farmer requires clay, or marl, or sand, to ameliorate his soil, this knowledge of the constant relative position of beds enables him to say where these materials are to be got, or where they are to be looked for, and whether the advantage to be derived is likely to repay the cost of procuring them.

2. It is observed, that when the soil on the surface of each of a series of rocks, such as C, or D, or E, in the same diagram, is uniformly bad, it is almost invariably of better quality at the point where the two rocks meet. Thus C may be dry, sandy, and barren; D may be cold, unproductive clay; and E a more or less unfruitful limestone soil: yet at either extremity of the tract D, where the soil is made up of an admixture of the decayed portions of the two adjacent rocks, the land may be of average fertility—the sand of C may adapt the adjacent clay to the growth of turnips, while the lime of E may cause it to yield large returns of wheat.[13] Thus, to the tenant in looking out for a farm, or to the capitalist in seeking an eligible investment, a knowledge of the mutual relations of geology and agriculture will often prove of the greatest assistance. Yet how little is such really useful knowledge diffused among either class of men—how little are either tenants or proprietors guided by it in their choice of the localities in which they desire to live!

And yet here and there the agricultural practice of more or less extended districts, if not really founded upon or directed by, is yet to be explained only by principles such as those I have above illustrated. I shall mention only one example. The chalk in Yorkshire, in Suffolk, and in other southern counties, consists of a vast number of beds, which, taken all together, form a deposit of very great thickness. Now, the upper beds of the chalk form poor, thin, dry soils, producing a scanty herbage, and only under the most skilful culture yielding profitable crops of corn. The lower beds, on the contrary, are marly; produce a more stiff, tenacious, and even fertile soil; and are found in a remarkable degree to enrich the soils of the upper chalk, when laid on as a top-dressing in autumn, and allowed to crumble under the action of the winter’s frost. Hence in Yorkshire, Wiltshire, Hampshire, and Kent, where the lower chalk covers the surface, or is found at no great depth beneath it, it is dug out of the sides of the hills, or pits are sunk for it, and it is immediately laid upon the land with great benefit to the soil. But in parts of Suffolk, where the soil equally rests upon the upper chalk, there is no other chalk in the neighbourhood, or to be met with at any reasonable depth, which will materially improve the land. The farmers find it, from long experience, to be more economical to bring chalk by sea from Kent to lay on their lands in Suffolk, than to cover them with any portion of the same material from their own farms. The following imaginary section will fully explain the fact here mentioned:—

No. 3.

Suffolk.Mouth of the Thames. Kent.

In this diagram 1 represents the London clay; 2, the plastic clay which is below it; 3, the upper chalk with flints, rising to the surface in Suffolk; and 4, the lower chalk, without flints, which is too deep to be reached in Suffolk, but which rises to the surface in Kent,—where it is abundant, is easily accessible, and whence it is transmitted across the estuary of the Thames into Suffolk.

3. The further fact that the several stratified rocks are remarkably constant in their mineral character, renders this knowledge of the order of relative superposition still more valuable to the agriculturist. Thousands of different beds are known to geologists to occur on various parts of the earth’s surface—each occupying its own unvarying place in the series. Most of these beds also, when they crumble or are worn down, produce soils possessed of some peculiarity by which their general agricultural capabilities are more or less affected,—and these peculiarities may generally be observed in soils formed from rocks of the same age—that is, occupying the same place in the series—in whatever part of the world we find them. Hence if the agricultural geologist be informed that his friend has bought, or is in treaty for a farm or an estate, and that it is situated upon such and such a rock, or geological formation, he can immediately give a very probable opinion in regard to the agricultural value of the soil, whether the property be in England, in Australia, or in New Zealand. If he knows the nature of the climate also, he will be able to estimate with tolerable correctness how far the soil is likely to repay the labours of the practical farmer,—nay, even whether it is likely to suit better for arable land or for pasture, and if for arable, what species of white crops it may be expected to produce most abundantly.