D.—No. 14.

20

ANNUAL REPORT ON THE

2. In recognition of this principle, therefore, the analyses of the soils have been arranged in a tabular form according to their absorbent power, as deduced from their proportional richness in organic or clay matters, those in which this power is theoretically highest being placed first. 3. The organic and clay matter are possibly not the only absorbents present, but they are, by general consent, allowed to be those most concerned in the manifestation of this absorbent property, and are therefore the only ones practically taken into account. It may be thought that the direct quantitative determination of this actual absorptive power by the application of the proper agents would be a simpler process, and more accurate in its results; but as no method has yet been designed by which to measure the amount of absorption which has primarily taken place in the sample, such a method cannot yet be applied. In deducing their relative absorptive power from these data, precedence is given to clay over organic matter, clay being a more energetic and permanent absorbent. Strictly speaking, the appended table, from being constructed in this manner, presents these soils more in the order of their possible than their actual fertility; while the few exceptional cases hereafter to be noted, in which a rigid adherence to this method has led to palpable misplacements, owe their existence entirely to particular mechanical conditions or to local causes. No other arrangement could well be devised for presenting these analyses in one continuous table in so practical and connected a form, while allowing of their subordinate grouping under those general terms which in practice are so conveniently and constantly employed. A few of the more common constituents of these soils have been determined but only partially, about which it is necessary to make a few explanatory remarks. Sulphuric Acid. —Sulphuric acid enters largely into the composition of many vegetable substances* particularly wheat, oats, and turnips; but though in the case of every soil examined its presence has been demonstrated, yet frequently it has been in such small quantities as to confirm certain views which are opposed to the current idea that the sulphur of plants is solely derived from this acid. In consequence of this the exact estimation of sulphuric acid was left out altogether, as being calculated to mislead. The more correct view appears to be that sulphuretted hydrogen, is the main source of the sulphur of plants, and that sulphuric acid cannot, except by chemical decomposition, afford the sulphur necessary. The facts upon which this view is based are these : 1. The elements of sulphuric acid are very firmly united; while those of sulphuretted hydrogen are only feebly so, and are besides in such a form as possibly to admit of direct assimilation without necessitating decomposition. 2. Hydrous coal and lignite, to which the organic matter of soil most nearly approximates, are generally highly charged with sulphuretted hydrogen, showing the general presence of this body, and they rapidly absorb it from solution. This is fully described in a paper by Mr. Skey, read before the Royal Society of Edinburgh, and published in Proceedings, Vol. V. No. 68. 3. Sulphuric acid and those of its compounds present in soils are generally very soluble, and not at all amenable to physical absorption, so far as yet discovered ; while sulphuretted hydrogen, being readily absorbed, is not liable like the latter to be carried by rains out of the reach of vegetation. It would be quite out of place to go further into this question here, but it was necessary to make the foregoing remarks, so that a seeming omission or incompleteness of analysis might be explained; for it is obvious that if there are reasonable grounds for the view that sulphuretted hydrogen is the source of sulphur contained in plants, the determination of sulphuric acid alone would be of little use. Chlorine. —Chlorine enters only to a very small extent in the composition of plants, and it is doubtful whether its presence may not be purely accidental. Chlorine is not found in any of the proximate constituents of vegetable substances as a component part, and for this reason no attempt has been made to determine its presence in the soils. Ammonia — Carbonic Acid. —Neither ammonia nor carbonic acid have been determined, for reasons already given in the introductory part. They were constantly found —carbonic acid always occurring among the constituents soluble in water. Manganese. —Manganese being found only in traces in most vegetable ashes, has not been estimated. Peactical Deductions. The most essential elements of the food of plants are now understood to be the mineral or inorganic matters which are absorbed in the growth of their several parts. The fertility of a soil is therefore controlled by the minimum proportion of any one of the mineral elements which are essential to the growing of its plants, however abundant the remainder may be. The principal assistance which chemical analysis affords is by ascertaining where such deficiency or want of proportion among the inorganic elements of the soil exists, in order that it may be rectified by artificial applications. But it is not merely necessary that the soil should contain these elements, but also that they should be present in a form that admits of their being rapidly absorbed by the plants. Por this purpose they must be easily dissolved by the feeble re-agents which effect this natural process of disintegration of the rocks. We must further bear in mind that were it not for another property of good soil—namely, its powers of mechanically absorbing and retaining these soluble elements from those solutions —it is obvious that the best soils, as judged by the standard of the proportion of soluble matter which they contain, would within a brief period become sterile, from having all the most valuable elements abstracted by a process of simpile mechanical percolation. This property of absorption is effected in any soil; first by the clay which it contains, and secondly by the decomposed organic matter which it contains. The former retaining within itself, the solutions of the inorganic salts, while the latter has a predisposition to select the organic matters which are held in solution. The quality of a soil is therefore determined by the amount of soluble inorganic salts absorbed or held in a state of physical combination, or in other words by the proportion which there is of clay to absorb, and of nutritive inorganic compounds to be absorbed by it.