“In taking up this inquiry, I was led to pass in review the celebrated and highly important experiments, undertaken by Mr. Lawes and Dr. Gilbert, on the continued growth of wheat on the same soil, for a long succession of years, and to examine, likewise carefully, many points, to which attention is drawn, by the same authors in their memoirs on the growth of red clover by different manures, and on the Lois Weedon plan of growing wheat. Abundant and most convincing evidence is supplied by these indefatigable experimenters, that the wheat-producing powers of a soil are not increased in any sensible degree by the liberal supply of all the mineral matters, which enter into the composition of the ash of wheat, and that the abstraction of these mineral matters from the soil, in any much larger proportions than can possibly take place under ordinary cultivation, in no wise affects the yield of wheat, provided there be at the same time a liberal supply of available nitrogen within the soil itself. The amount of the latter, therefore, is regarded by Messrs. Lawes and Gilbert, as the measure of the increased produce of grain which a soil furnishes.
“In conformity with these views, the farmer, when he wishes to increase the yield of his wheat, finds it to his advantage to have recourse to ammoniacal, or other nitrogenous manures, and depends more or less entirely upon the soil, for the supply of the necessary mineral or ash-constituents of wheat, having found such a supply to be amply sufficient for his requirements. As far, therefore, as the removal from the soil of a large amount of mineral soil-constituents, by the clover-crop, is concerned, the fact viewed in the light of the Rothamsted experiments, becomes at once intelligible; for, notwithstanding the abstraction of over 600 lbs. of mineral matter by a crop of clover, the succeeding wheat-crop does not suffer. Inasmuch, however, as we have seen, that not only much mineral matter is carried off the land in a crop of clover, but also much nitrogen, we might, in the absence of direct evidence to the contrary, be led to suspect that wheat, after clover, would not be a good crop; whereas, the fact is exactly the reverse.
“It is worthy of notice, that nitrogenous manures, which have such a marked and beneficial effect upon wheat, do no good, but in certain combinations, in some seasons, do positive harm to clover. Thus, Messrs. Lawes and Gilbert, in a series of experiments on the growth of red-clover, by different manures, obtained 14 tons of fresh green produce, equal to about three and three-fourths tons of clover hay, from the unmanured portion of the experimental field; and where sulphates of potash, soda, and magnesia, or sulphate of potash and superphosphate of lime were employed, 17 to 18 tons, (equal to from about four and one-half to nearly five tons of hay), were obtained. When salts of ammonia were added to the mineral manures, the produce of clover-hay was, upon the whole, less than where the mineral manures were used alone. The wheat, grown after the clover, on the unmanured plot, gave, however, 29½ bushels of corn, whilst in the adjoining field, where wheat was grown after wheat, without manure, only 15½ bushels of corn per acre were obtained. Messrs. Lawes and Gilbert notice especially, that in the clover-crop of the preceding year, very much larger quantities, both of mineral matters and of nitrogen, were taken from the land, than were removed in the unmanured wheat-crop in the same year, in the adjoining field. Notwithstanding this, the soil from which the clover had been taken, was in a condition to yield 14 bushels more wheat, per acre, than that upon which wheat had been previously grown; the yield of wheat, after clover, in these experiments, being fully equal to that in another field, where large quantities of manure were used.
“Taking all these circumstances into account, is there not presumptive evidence, that, notwithstanding the removal of a large amount of nitrogen in the clover-hay, an abundant store of available nitrogen is left in the soil, and also that in its relations towards nitrogen in the soil, clover differs essentially from wheat? The results of our experience in the growth of the two crops, appear to indicate that, whereas the growth of the wheat rapidly exhausts the land of its available nitrogen, that of clover, on the contrary, tends somehow or other to accumulate nitrogen within the soil itself. If this can be shown to be the case, an intelligible explanation of the fact that clover is so useful as a preparatory crop for wheat, will be found in the circumstance, that, during the growth of clover, nitrogenous food, for which wheat is particularly grateful, is either stored up or rendered available in the soil.
“An explanation, however plausible, can hardly be accepted as correct, if based mainly on data, which, although highly probable, are not proved to be based on fact. In chemical inquiries, especially, nothing must be taken for granted, that has not been proved by direct experiment. The following questions naturally suggest themselves in reference to this subject: What is the amount of nitrogen in soils of different characters? What is the amount more particularly after a good, and after an indifferent crop of clover? Why is the amount of nitrogen in soils, larger after clover, than after wheat and other crops? Is the nitrogen present in a condition in which it is available and useful to wheat? And lastly, are there any other circumstances, apart from the supply of nitrogenous matter in the soil, which help to account for the beneficial effects of clover as a preparatory crop for wheat?
“In order to throw some light on these questions, and, if possible, to give distinct answers to at least some of them, I, years ago, when residing at Cirencester, began a series of experiments; and more recently, I have been fortunate enough to obtain the co-operation of Mr. Robert Valentine, of Leighton Buzzard, who kindly undertook to supply me with materials for my analysis.
“My first experiments were made on a thin, calcareous, clay soil, resting on oolitic limestone, and producing generally a fair crop of red-clover. The clover-field formed the slope of a rather steep hillock, and varied much in depth. At the top of the hill, the soil became very stony at a depth of four inches, so that it could only with difficulty be excavated to a depth of six inches, when the bare limestone-rock made its appearance. At the bottom of the field the soil was much deeper, and the clover stronger, than at the upper part. On the brow of the hill, where the clover appeared to be strong, a square yard was measured out; and at a little distance off, where the clover was very bad, a second square yard was measured; in both plots, the soil being taken up to a depth of six inches. The soil, where the clover was good, may be distinguished from the other, by being marked as No. 1, and that where it was bad, as No. 2.
CLOVER-SOIL NO. 1. (GOOD CLOVER).
“The roots having first been shaken out to free them as much as possible from the soil, were then washed once or twice with cold distilled water, and, after having been dried for a little while in the sun, were weighed, when the square yard produced 1 lb. 10½ oz. of cleaned clover-roots, in an air-dry state; an acre of land, or 4,840 square yards, accordingly yielded, in a depth of six inches, 3.44 tons, or 3½ tons in round numbers, of clover-roots.
“Fully dried in a water-bath, the roots were found to contain altogether 44.67 per cent of water, and on being burnt in a platinum capsule, yielded 6.089 of ash. A portion of the dried, finely powdered and well mixed roots, was burned with soda lime, in a combustion tube, and the nitrogen contained in the roots otherwise determined in the usual way. Accordingly, the following is the general composition of the roots from the soil No. 1: