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of it, is removed from the soil, there must be a reduction of fertility dependent on the quantity of valuable matters withdrawn by it; and thus it happens that when a plant has grown on any soil, and has removed from it a large quantity of nutritive matters, it becomes incapable of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