Elements of agricultural chemistry and geology

by Jas. F. W. Johnston

SECTION VII.—OF THE IRRIGATION OF THE LAND.

The irrigation of the land is, in general, only a more refined method of manuring it. The nature of the process itself, however, is different in different countries, as are also the kind and degree of effect it produces, and the theory by which these effects are to be explained.

In dry and arid climates, where rain rarely falls, the soil may contain all the elements of fertility, and require only water to call them into operation. In such cases, as in the irrigations practised so extensively in eastern countries, and without which, whole provinces in Africa and Southern America would lie waste, it is unnecessary to suppose any other virtue in irrigation than the mere supply of water it affords to the parched and cracking soil.

But in climates such as our own, there are two other beneficial purposes in reference to the soil, which irrigation may, and one at least of which it always does, serve.

2. The occasional flow of pure water over the surface, as in our irrigated meadows, and its descent into the drains, where the drainage is perfect, washes out acid and other noxious substances naturally generated in the soil, and thus purifies and sweetens it. The beneficial effect of such washing will be readily understood in the case of peat lands laid down to water meadow, since, as every one knows, peat soils abound in matters unfavourable to general vegetation, and which are usually in part drawn off by drainage, and in part destroyed by lime and by exposure to the air, before boggy lands can be brought into profitable cultivation.

3. But it seldom happens that pure water is employed for the purposes of irrigation. The water of rivers, more generally, is diverted from its course, more or less loaded with mud and other finer particles of matter, which are either gradually filtered from it as it passes over and through the soil, or in the case of floods subside naturally when the waters come to rest. Or in less frequent cases, the drainings of towns, and the waters from common sewers, or from the little streams enriched by them, are turned with benefit upon the favoured fields. These are evidently cases of gradual and uniform manuring. And even where the water employed is clear and apparently undisturbed by mud, it always contains saline substances grateful to the plant in its search for food, and which it always contrives to extract more or less copiously as the water passes over its leaves or along its roots. Every fresh access of water affords the grass in reality another liquid manuring.

In the refreshment continually afforded to the plant by a plentiful supply of water, in the removal of noxious substances from the soil, or in the frequent additions of enriching food to the land—the efficiency of irrigation, therefore, seems entirely to consist.

SECTION VIII.—OF PARING AND BURNING,
AND OF BURNED CLAY.

A mode of improvement often resorted to is the paring and burning of poor land. The efficacy of burned clay, also, even in superseding manure on good lands, has been highly extolled by some practical men.

1. The effect of paring and burning is easily understood. The matted sods consist of a mixture of much vegetable with a comparatively small quantity of earthy matter. When these are burned the ash of the plants only is left, intimately mixed with the calcined earth. To strew this mixture over the soil is much the same as to dress it with peat or wood ashes, the beneficial effect of which upon vegetation is almost universally recognised. And the beneficial influence of the ash itself is chiefly due to the ready supply of inorganic food it yields to the seed, and to the effect which the potash and soda it contains exercise either in preparing organic food in the soil, or in assisting its digestion and assimilation in the interior of the plant.

Another part of this process is, that the roots of the weeds and poorer grasses are materially injured by the paring, and that the subsequent dressing of ashes is unfavourable to their further growth.

2. Much greater uncertainty hangs over the alleged virtues of burned clay. That benefits are supposed to have been derived from its use there can be no doubt, though in many cases the better tillage of the land generally prescribed along with the use of burned clay, may have had some share in producing the good results actually experienced during its use.

By the burning, in kilns or otherwise, any organic matter the clay may contain will be consumed, and the texture of the clay itself will be mechanically altered. It will crumble down like a burned brick into a hard friable powder, and will never again cohere into a paste as before the burning. It will, therefore, render clay soils more open, and may thus, when mixed in large quantity, produce a permanent amelioration in the mechanical texture of many stiff wheat soils. It cannot itself undergo any chemical change that is likely so to alter its constitution as to make it a more useful chemical constituent of the soil than before. Any saline matter we may suppose to be set free could be far more cheaply added in the form of a top-dressing to the soil.

Bricks, however, are generally more porous than the clay from which they are formed; burned clay is so also. And all porous substances suck in and condense much air and many vapours in large quantities into their pores. In consequence of this property, porous substances, like charcoal and burned clay, are supposed, when mixed with the soil, to be continually yielding air to decaying vegetable matter on the one hand, and as continually re-absorbing it from the atmosphere on the other, and by this means to be of singular service in supplying the wants of plants in the earlier seasons of their growth. The vapours of nitric acid and of ammonia, which float in the air, they are also supposed to imbibe, and by the beneficial action of the substances believed to be thus conveyed by burned clay into the soil, the fertilizing virtues ascribed to it are attempted to be explained.

It must be confessed, however, that on this point considerable obscurity still rests. It is in some measure doubtful what the true action of charcoal and of burned clay is, both in kind and in quantity. It is the part of science, therefore, to decline offering more than a mere conjecture till the facts to be explained are more fully and satisfactorily demonstrated.

SECTION IX.—PLANTING AND LAYING
DOWN TO GRASS.

1. Planting.—It has been observed that lands which are unfit for arable culture, and which yield only a trifling rent as natural pasture, are yet in many cases capable of growing profitable plantations, and of being greatly increased in permanent value by the prolonged growth of wood. Not only, however, do all trees not thrive alike on the same soil, but all do not improve the soil on which they grow in an equal degree.

Under the Scotch fir, for example, the pasture is not worth 6d. more per acre than before it was planted—under the beech and spruce, it is worth even less than before, though the spruce affords excellent shelter;—under ash, it gradually acquires an increased value of 2s. or 3s. per acre. In oak copses, it becomes worth 5s. or 6s., but only during the last eight years (of the twenty-four), before it is cut down. But under the larch, after the first thirty years, when the thinnings are all cut, land not worth originally more than 1s. per acre, becomes worth 8s. to 10s. per acre for permanent pasture.[20]

The cause of this improvement is to be found in the nature of the soil, which gradually accumulates beneath the trees by the shedding of their leaves. The shelter from the sun and rain which the foliage affords, prevents the vegetable matter which falls from being so speedily decomposed, or from being so much washed away, and thus permits it to collect in larger quantities in a given time, than where no such cover exists. The more complete the shelter, therefore, the more rapid will the accumulation of soil be in so far as it depends upon this cause.

But the quantity of leaves which annually falls has also much influence upon the extent to which the soil is capable of being improved by any given species of tree, as well as the degree of rapidity with which those leaves, under ordinary circumstances, undergo decay. The broad membranous leaf of the beech and oak decay more quickly than the needle-shaped leaves of the pine tribes, and this circumstance may assist in rendering the larch more valuable as a permanent improver.

We should expect likewise that the quantity and quality of the inorganic matter contained in the leaves,—brought up year by year from the roots, and strewed afterwards uniformly over the surface where the leaves are shed,—would materially affect the value of the soil they form. The leaves of the oak contain about 5 per cent. of saline and earthy matter, and those of the Scotch fir less than 2 per cent.; so that, supposing the actual weight of leaves which falls from each kind of tree to be equal, we should expect a greater depth of soil to be formed in the same time by the oak than by the Scotch fir. I am not aware of any experiments on the quantity of ash left by the leaves of the larch.

The improvement of the land, therefore, by the planting of trees, depends in part upon the quantity of organic food which the trees can extract from the air, and afterwards drop in the form of leaves upon the soil, and in part upon the kind and quantity of inorganic matter which the roots can bring up from beneath, and in like manner strew upon the surface. The quantity and quality of the latter will, in a great measure, determine the kind of grasses which will spring up, and the consequent value of the pasture in the feeding of stock. In the larch districts of the Duke of Atholl, the most abundant grasses that spring up are said to be the holcus mollis and the holcus lanatus, (the creeping and the meadow soft-grasses.)

2. Laying down to grass.—On this point two facts seem to be pretty generally acknowledged:

First, that land laid down to artificial grasses for one, two, three, or more years, is in some degree rested or recruited, and is fitted for the better production of after-corn crops. Letting it lie a year or two longer in grass, therefore, is one of the received modes of bringing back to a sound condition a soil that has been exhausted by injudicious cropping.

Second, that land thus laid down with artificial grasses deteriorates more or less after two or three years, and only by slow degrees acquires a thick sward of rich and nourishing herbage. Hence the opinion, that grass-land improves in quality the longer it is permitted to lie,—the unwillingness to plough up old pasture,—and the comparatively high rents which, in some parts of the country, old grass lands are known to yield.

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.

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.