Section Iv.—of the Chemical Constitution of Soils.

Soils perform at least three functions, in reference to vegetation. They serve as a basis in which plants may fix their roots and sustain themselves in their erect position,—they supply inorganic food to vegetables at every period of their growth,—and they are the medium in which many chemical changes take place, that are essential to a right preparation of the various kinds of food which the soil is destined to yield to the growing plant.

We have spoken of soils as consisting chiefly of sand, lime, and clay, with certain saline and organic substances in smaller and variable proportions. But the study of the ash of plants (see chap. iv.) shews us, that a fertile soil must of necessity contain an appreciable quantity of at least eleven different substances, which in most cases exist in greater or less relative abundance in the ash both of wild and of cultivated plants.

Two well known geological facts lead to precisely the same conclusion. We have seen that the soils formed from the unstratified rocks,—the granites and the traps,—while they each contain certain earthy substances in proportions peculiar to themselves, yet contain also in general a trace of most of those different kinds of matter which are found in the ash of plants. And when to this fact is added the other, that the stratified rocks appear to be only the long accumulating fragments and ruins of more ancient unstratified masses—which, under various agencies, have gradually crumbled to dust, been strewed over the surface in alternate layers, and afterwards again consolidated,—the reader will readily grant, that in all rocks, and consequently in all soils, traces of every one of these substances may generally be presumed to exist.

Actual chemical analysis confirms these deductions in regard to the constitution of soils. It shews that, in most soils, the presence of the several constituents of the ash of plants may be detected, though in very variable proportions. And following up its investigations, in regard to the effect of this difference in the proportion of the generally less abundant constituents of the soil, it establishes certain other points of the greatest possible importance to agricultural practice. Thus, it has found, for example,

1. That as a proper adjustment of the proportions of clay and sand is necessary, in order that a soil may possess the most favourable physical properties—so that the mere presence of the various kinds of inorganic food in a soil is not sufficient to make it productive of a given crop, but that they must be so adjusted in quantity that the plant shall be able readily and at the proper time to obtain an adequate supply of each.

2. That when a soil is particularly poor in certain of these substances, the valuable, cultivated corn crops, grasses, and trees, refuse to grow upon them in a healthy manner, and to yield remunerating returns. And,

3. That when certain other substances are present in too great abundance, the soil is rendered equally unpropitious to the most important crops.

In these facts the intelligent reader will perceive the foundation of the varied applications to the soil which are everywhere made under the direction of a skilful practice, and of the difficulties which, in so many localities, lie in the way of bringing the land into such a state as shall fit it readily to supply all the wants of those kinds of vegetables which it is the special object of artificial culture easily and abundantly to raise.

Chemical analysis is a difficult art,—one which demands much chemical knowledge, and skill in chemical practice (manipulation, as it is called), and calls for both time and perseverance—if valuable, trustworthy, and minutely correct results are to be obtained. I believe it is only by aiming after such minutely correct results that chemical analysis is likely to throw light on the peculiar properties of those soils which, while they possess much general similarity in composition and in physical properties, are yet found in practice to possess very different agricultural capabilities. Many such cases occur in every country, and they are the kind of difficulties in regard to which agriculture has a right to say to chemistry—“These are matters which I hope and expect you will satisfactorily clear up.” But while agriculture has a right to use such language, she has herself preliminary duties to perform. She has no right in one breath to deny the value of chemical theory to agricultural practice, and in another to ask the sacrifice of time and labour in doing her chemical work. Chemistry is a wide field, and many zealous lives may be spent in the prosecution of it without at all entering upon the domain of practical agriculture. It may be that here and there it may fall in with the humour or natural bias of some one chemist to apply his knowledge to this most important art; but hitherto the appreciation of such efforts has, in general, been so small—the reception of scientific results and suggestions by the agricultural body so ungracious—that little wonder can exist that so many have quitted the field in disgust—that the majority of capable men should studiously avoid it.

Hence it has happened that, in England, the analysis of soils has rarely been undertaken, except as a matter of professional business, where so much time was, by a fair calculation, given for so much money, and an analysis made, of that degree of accuracy only which the time allotted to it permitted the analyst to attain.

In order, therefore, to illustrate the deductions which, as above stated, may be drawn from an accurate chemical analysis, I shall exhibit the constitution of three different soils as determined by Sprengel, a German chemist, now at the head of the Prussian Agricultural school, and whose own taste, as well as his professional function, have long directed his attention, and with much success, to scientific agriculture.

No. 1 is a very fertile alluvial soil from East Friesland, formerly overflowed by the sea, but for 60 years cultivated with corn and pulse crops without manure.

No. 2 is a fertile soil near Göttingen, which produces excellent crops of clover, pulse, rape, potatoes, and turnips, the two last more especially when manured with gypsum.

No. 3 is a very barren soil from Lunenburg.

When washed with water in the manner described in pages 70 to 73, they gave, respectively, from 1000 parts of soil—

No. 1. No. 2. No. 3.

Soluble saline matter, 18 1 1 Fine earthy and organic matter (clay), 937 839 599 Siliceous sand, 45 160 400 ---- ---- ---- 1000 1000 1000

The most striking distinction presented by these numbers is the large quantity of saline matter in No. 1. This soluble matter consisted of common salt, chloride of potassium, sulphate of potash and gypsum, with a trace of sulphate of magnesia, sulphate of iron, and phosphate of soda. The presence of this comparatively large quantity of these different saline substances,—originally derived, no doubt, in great part from the sea,—was probably one reason why it could be so long cropped without manure.

The unfruitful soil is much the lightest of the three, containing 40 per cent. of sand; but this is not enough to account for its barrenness—many light soils containing a larger proportion of sand, and yet being sufficiently fertile.

The finer portions separated from the sand, and soluble matter, consisted in 1000 parts of

No. 1. No. 2. No. 3.

Organic matter, 97 50 40 Silica, 648 833 778 Alumina, 57 51 91 Lime, 59 18 4 Magnesia, 8½ 8 1 Oxide of iron, 61 30 81 Oxide of manganese, 1 3 ½ Potash, 2 trace. trace. Soda, 4 ” ” Ammonia, trace. ” ” Chlorine, 2 ” ” Sulphuric acid, 2 ¾ ” Phosphoric acid, 4½ 1¾ ” Carbonic acid, 40 4½ ” Loss, 14 — 4½ ------ ------ ------ 1000 1000 1000

1. The composition of No. 1 illustrates the first of those general deductions above stated, that a considerable supply of all the species of inorganic food is necessary to render a soil eminently fertile. Not only does this soil contain a comparatively large quantity of soluble saline matter, but it contains also nearly 10 per cent. of organic matter, and, what in connection with this is of great importance, 6 per cent. of lime. The potash and soda, and the several acids, are also present in sufficient abundance.

2. In the second,—a fertile soil, but one which cannot dispense with manure,—there is little soluble saline matter, and in the insoluble portion we see that there are mere traces of potash, soda, and the important acids. It contains also 5 per cent. only of organic matter, and about 2 per cent. of lime, which smaller proportions, together with the deficiencies above stated, remove this soil from the most naturally fertile class to that class which is susceptible, in hands of ordinary skill, of being brought to, and kept in, a very productive condition.

3. In the fine part of the third soil, we observe that there are many more substances deficient than in No. 2. The organic matter amounts apparently to 4 per cent., and there seems to be nearly half a per cent. of lime. But it will be recollected, that this soil contains 40 per cent. of sand, so that in every hundred of soil there are only 60 of the fine matter, of which the composition is presented in the table, or 100 lbs. of the native soil contain only 2½ lbs. of organic matter and ¼ lb. of lime.

But all these wants would not condemn the soil to hopeless barrenness, because in favourable circumstances, and where it was worth the cost, they might all be supplied. But the oxide of iron amounts to 8 per cent. of this fine matter, a proportion of this substance which, in a soil containing so little organic matter, appears, from practical experience, to be incompatible with the healthy growth of cultivated crops. To this soil, therefore, there requires to be added not only those substances of which it is destitute, but such other substances also as shall prevent the injurious effect of the large proportion of oxide of iron.

In these three soils, then, we have examples, first, of one which contains within itself all the elements of fertility; second, of a soil which is destitute, or nearly so, of certain substances,—which, however, can be readily added by the ordinary manures in general use,—and to which the elements of gypsum are especially useful, in aiding it to feed the potato and the turnip; and, third, of a soil not only poor in many of the necessary species of the inorganic food of plants, but too rich in one which, when present in excess, is prejudicial to vegetable life.

This illustration, therefore, will aid the general reader in comprehending how far rigid chemical analysis is fitted to throw light upon the capabilities of soils, and to direct agricultural practice.