In examining this table, it becomes apparent that while in regard to some of the elements, the quantities removed by different crops do not differ to any marked extent, in others the variation is very great. The cereals and grasses are especially distinguished by the larger quantity of silica they contain, and the exhaustive effect consequent upon the removal of both grain and straw from soils which contain but a limited supply of that substance in an available condition is obvious. It is clear that under such circumstances the frequent repetition of a cereal crop may so far diminish the amount of available silica as to render its cultivation impossible, although the other substances may be present in sufficient quantity to produce a plentiful crop of any plant which does not require that element. Beans and peas, turnips and hay, on the other hand, require a very large quantity of alkalies, and especially of potash.
Looking more minutely, however, into this matter, certain points attract attention which appear to be at variance with commonly received opinions. With the exception of silica, for example, the cereals do not withdraw from the soil so large a quantity of mineral matters as some of the so-called fallow crops, and if their straw be returned to the soil they are by far the least exhaustive of all cultivated plants; and we thus recognise the justice of that practical rule, which lays it down as an essential point of good husbandry that the straw ought, as far as possible, to be consumed on the farm on which it is produced. As regards the general constituents of the ash, it is also to be remarked that though differences in their proportions exist, they are by no means so marked as might be expected; thus there are no plants for which a large quantity of potash, nitrogen, and phosphoric acid is not required; and it is not very easy to see how the substitution of the one for the other should be of much importance in this respect. Indeed, the more minutely the subject is examined, the more do we become convinced of the insufficiency of that view which attributes the necessity for a rotation of crops to differences in chemical composition alone. There can be no doubt that the nature of the plant and the particular mode in which it gathers its nutriment, have a most important influence. Certain plants are almost entirely dependent on the soil for their organic constituents, while others derive a large proportion of them from the air, and a plant of the latter class will flourish in a soil in which one of the former is incapable of growing. In other cases, the structure and distribution of the roots is the cause of the difference. Some plants have roots distributed near the surface and exhaust the superficial layer of the soil, others penetrate into the deeper layers, and not only derive an abundant supply of food from them, but actually promote the fertility of the surface soil by the refuse portions of them which are left upon it. Experience has in this respect arrived at results which tally with theory, and it is for this reason that the broad-leafed turnip, which obtains a considerable quantity of its nutriment from the air, alternates with grain crops which are chiefly dependent on the soil. It is undoubtedly to some such cause that several remarkable instances of what may be called natural rotations are to be attributed. It is well known in Sweden that when a pine forest is felled, a growth, not of pine but of birch, immediately springs up. Now the difference in composition of the ash of these trees is not sufficient to explain this fact, and it must clearly be due to some difference in the distribution of their roots, or the mode in which they obtain their food.
Whatever weight may be given to these different explanations of rotation, there is no doubt about the importance of attending to it, and there are various practical deductions of much importance to be drawn from the facts with which we are acquainted. Thus it is to be observed that the quantities of mineral matters withdrawn by plants of the same class are generally similar, and hence it may be inferred that crops of the most opposite class ought as much as possible to alternate with one another, and each plant should be repeated as seldom as possible, so that, even when it is necessary to return to the same class, a different member of it should be employed. Thus, for instance, in place of immediately repeating wheat, when another grain crop is necessary, it would theoretically be preferable to employ oats or barley, and to replace the turnip by mangold-wurzel or some other root. It is obvious, however, that this system cannot be carried out in practice to its full extent; for the superior value of individual crops causes the more frequent repetition of those which make the largest return. But experience has so far concurred with theory that it has taught the farmer the advantage of long rotations; and we have seen the successive introduction of the three, four, five, and six-course shift, and even, in some instances, of longer periods.
Such is the theory of rotation, and while it will always be most advantageous to adhere to it, it is by no means necessary that this should be done in an absolutely rigid manner. In the practice of agriculture, plants are placed in artificial circumstances, and instead of allowing them to depend entirely on the soil, they are supplied with a quantity of manure containing all the elements they require, and if it be used in sufficiently large quantity, the same crop may be grown year after year. And accordingly the order of rotation, which is theoretically the best, may be, and every day is, violated in practice, although this must necessarily be done at the expense of a certain quantity of the valuable matters of the manure added, and is so far a practice which ought theoretically to be avoided. In actual practice, however, the matter is to be decided on other grounds. The object then is, not to produce the largest crops, but those which make the largest money return, and thus it may be practically economical to grow a crop of high commercial value more frequently than is theoretically advantageous. In such cases the farmer must seek to do away as far as possible with the disadvantages which such a course entails, and this he will endeavour to accomplish by careful management and a liberal treatment of the soil.
But while this system may be adopted to some extent, it must also be borne in mind that the frequent repetition of some crops cannot be practised with impunity, for plants are liable to certain diseases which manifest themselves to the greatest extent when they have been too often cultivated in the same soil. Clover sickness, which affects the plant when frequently repeated on light soils, and the potatoe disease and finger and toe have been attributed to the same cause. Whether this is the sole origin of these diseases is questionable, but there is no doubt that they are aggravated by frequent repetition, and hence a strong argument in favour of rotation. It has been asserted by great authorities in high farming, that with our present command of manures, rotations may be done away with; but this is an opinion to which science gives no countenance, and he would be a rash man who attempted to carry it out in practice.
CHAPTER XIV.
THE FEEDING OF FARM STOCK.
The feeding of cattle, once a subordinate part of the operations of the farm, has now become one of its most important departments, and a large number of minute and elaborate experiments have been made by chemists and physiologists with the view of determining the principles on which its successful and economical practice depends. These investigations, while they have thrown much light on the matter, have by no means exhausted it, and it will be readily understood that the complete elucidation of a subject of such complexity, touching on so many of the most abstruse and difficult problems of chemistry and physiology, and in which the experiments are liable to be affected by disturbing causes, dependent on peculiarities of constitution of different animals, cannot be otherwise than a slow process.