xhausting
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fertility dependent on the quantity of valuable matters withdrawn by it;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