CHAPTER XIII.

THE ROTATION OF CROPS.

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of it, is removed from the soil, there must be a reduction of fertility dependent on the quantity of valuable matters withdrawn by it; and thus it happens that when a plant has grown on any soil, and has removed from it a large quantity of nutritive matters, it becomes incapable of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.

Produce per Imperial Acre.Total Weight in lbs.Total Mineral Matters. Potash. Soda. Lime.
Wheat—Grain 28 bushels at 60 lbs. 1,680 34·12 10·11 1·20 1.04
Straw 1 ton 3 cwt. 2,576 114·48 20·70 2·84 8·53
Total ... ... 148·60 30·81 4·04 9·57
Barley—Grain 33 bushels at 53 lbs. 1,749 44·24 9·40 0·30 0·76
Straw 18 cwt. 2,106 99·14 11·24 1·14 5·81
Total ... ... 143·38 20·64 1·44 6·57
Oats—Grain 34 bushels at 40 lbs. 1,360 48.89 11·00 ... 5·31
Straw 1 ton. 2,240 143·53 30·71 6·10 10·29
Total ... ... 192·42 41·71 6·10 15·60
Beans, Peas—Grain 25 bushels at 60 lbs 1,650 55·97 30·00 0·31 3·01
Straw 1 ton. 2,240 108·51 48·61 13·14 29·37
Total ... ... 164·48 78·61 13·45 32·38
Turnips—Bulbs 13-1/2 tons. 30,240 213·75 57·35 44·71 28·60
Potatoes 3 tons. 6,720 55·58 28·92 2·85 1·20
Hay 2-1/2 tons. 5,600 391·31 129·79 4·80 35·46
Magnesia. Chlorine. Sulphuric Acid. Phosphoric Acid. Silica. Nitrogen.
Wheat—Grain 4.80 ... 0.32 16.22 0.43 29.20
Straw 2·23 ... 3·55 3·16 73·47 16·13
Total 7·03 ... 3·87 19·38 73·90 45·33
Barley—Grain 3·10 1·12 0·85 15·52 13·19 34·98
Straw 2·75 1·30 1·10 7·22 68·58 6·03
Total 5·85 2·42 1·95 22·74 81·77 41·01
Oats—Grain 4·04 0·20 ... 26·07 2·27 27·54
Straw 5·50 5·55 5·18 7·35 72·85 14·10
Total 9·54 5·75 5·18 33·42 75·12 41·64
Beans, Peas—Grain 4·00 ... 1·76 16·65 0·24 46·10
Straw 3·74 7·00 2·07 0·74 3·84 26·88
Total 7·74 7·00 3·83 17·39 4·08 72·98
Turnips—Bulbs 4·65 10·35 39·02 22·57 6·50 60·48
Potatoes 2·11 3·21 10·24 5·76 1·29 26·00
Hay 9·62 39·61 16·57 21·79 133·67 56·22

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.