Reference has already been more than once made to the fact that a crop growing in any soil must necessarily exhaust it to a greater or less extent by withdrawing from it a certain quantity of the elements to which its fertility is due. That this is the case has been long admitted in practice, and it has also been established that the exhausting effects of different species of plants are very different; that while some rapidly impoverish the soil, others may be cultivated for a number of years without material injury, and some even apparently improve it. Thus, it is a notorious fact that white crops exhaust, while grass improves the soil; but the improvement in the latter case is really dependent on the fact, that when the land is laid down in pasture, nothing is removed from it, the cattle which feed on its produce restoring all but a minute fraction of the mineral matters contained in their food; and as the plants derive a part, and in some instances a very large part, of their organic constituents from the air, the fertility of the soil must manifestly be increased, or at all events maintained in its previous state. When, however, the plant, or any portion of it, is removed from the soil, there must be a reduction of fertility dependent on the quantity of valuable matters withdrawn by it; and thus it happens that when a plant has grown on any soil, and has removed from it a large quantity of nutritive matters, it becomes incapable of producing an equally large crop of the same species; and if the attempt be made to grow it in successive years, the land becomes incapable of producing it at all, and is then said to be thoroughly exhausted. But if the exhausted land be allowed to lie for some time without a crop, it regains its fertility more or less rapidly according to circumstances, and again produces the same plant in remunerative quantity. The observation of this fact led to the introduction of naked fallows, which, up to a comparatively recent period, were an essential feature in agriculture. But after a time it was observed that the land which had been exhausted by successive crops of one species was not absolutely barren, but was still capable of producing a luxuriant growth of other plants. Thus peas, beans, clover, or potatoes, could be cultivated with success on land which would no longer sustain a crop of grain, and these plants came into use in place of the naked fallow under the name of fallow crops. On this was founded the rotation of crops; for it was clear that a judicious interchange of the plants grown might enable the soil to regain its fertility for one crop at the time when it was producing another; and when exhausted for the second, it might be again ready to bear crops of the first.

The necessity for a rotation of crops has been explained in several ways. The oldest view is that of Decandolle, who founded his theory on the fact that the plants excrete certain substances from their roots. He found that when plants are grown in water, a peculiar matter is thrown off by the roots; and he believed that this extrementitious substance is eliminated because it is injurious to the plant, and that, remaining in the soil, it acts as a poison to those of the same species, and so prevents the growth of another crop. But this excretion, though poisonous to the plants from which it is excreted, he believed to be nutritive to those of another species which is thus enabled to grow luxuriantly where the others failed. Nothing can be more simple than this explanation, and it was readily embraced at the time it was propounded and considered fully satisfactory. But when more minutely examined, it becomes apparent that the facts on which it is founded are of a very uncertain character. Decandolle's observations regarding the radical excretions of plants have not been confirmed by subsequent observers. On the contrary, it has been shewn that though some plants, when growing in water, do excrete a particular substance in small quantity, nothing of the sort appears when they are grown in a siliceous sand. And hence the inference is, that the peculiar excretion of plants growing in water is to be viewed as the result of the abnormal method of their growth rather than as a natural product of vegetation. But even admitting the existence of these matters, it would be impossible to accept the explanation founded upon them, because it is a familiar fact that, on some soils, the repeated growth of particular crops is perfectly possible, as, for instance, on the virgin soils of America, from which many successive crops of wheat have been taken; and in these cases the alleged excretion must have taken place without producing any deleterious effect on the crop. Besides, it is in the last degree improbable that these excretions, consisting of soluble organic matters, should remain in the soil without undergoing decomposition, as all similar substances do; and even if they did, we cannot, with our present knowledge of the food of plants, admit the possibility of the direct absorption of any organic substance whatever. Indeed, the idea of radical excretions, as an explanation of the rotation of crops, must be considered as being entirely abandoned.

The necessity for a rotation of crops is now generally attributed to the different quantities of valuable matters which different plants remove from the soil, and more especially to their mineral constituents. It has been already observed that great differences exist in the composition of the ash of different plants in the section on that subject; and it was stated that a distinction has been made between lime, potash, and silica plants, according as one or other of these elements preponderate in their ashes. The remarkable difference in the proportion of these elements has been supposed to afford an explanation of rotation. It is supposed that if a plant requiring a large quantity of any one element, potash, for example, be grown during a succession of years on the same soil, it will sooner or later exhaust all, or nearly all, the potash that soil contains in an available form, and it will consequently cease to produce a luxuriant crop. But if this plant be replaced by another which requires only a small quantity of potash and a large quantity of lime, it will flourish, because it finds what is necessary to its growth. In the meantime, the changes which are proceeding in the soil, are liberating new quantities of the inorganic matters from those forms of combination in which they are not immediately available, and when after a time the plant which requires potash is again sown on the soil, it finds a sufficient quantity to serve its purpose. We have already, in treating of the ashes of plants, pointed out the extent of the differences which exist; but these will be made more obvious by the annexed table, giving the quantity of the different mineral matters contained in the produce of an imperial acre of the different crops.

Table shewing the quantities of Mineral Matters and Nitrogen in average Crops of the principal varieties of Farm Produce.

The minor constituents, such as oxide of iron, manganese, etc., have been omitted as being of little importance; and the quantity of nitrogen, which is of great moment in estimating the exhaustive effects of various crops, has been added.

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.