ROTATION OF CROPS
(Continued)
“When soil is spoken of as worn out and needing rest, the speaker uses
a figure of speech meaning that the soil has been exhausted by the
crops it has borne. The crops do indeed take from the land a great
quantity of substances necessary for plant-life; and when these
substances are no longer present in sufficient amount, the soil refuses
to produce; it is exhausted. To restore its original fertility would
require a large outlay in fertilizer and hence it is often more
advantageous to accomplish this object by one of the following methods.
“Sometimes the land is allowed to lie fallow; that is to say, it is
left to itself without any care whatever for whole years. Weeds spring
up freely, and at the same time water, air, and frost act on the soil,
disintegrating and mellowing it and inducing the formation of certain
substances necessary to vegetation. The weeds are converted into mold,
and finally the land, rested and recuperated, is ready to bear a new
crop. Restoration by this process is very slow, taking several years,
and hence it is customary to shorten the period of waiting by working
the soil and even manuring it, although it may not yet be the intention
to sow any seed. In these circumstances the land is called fallow land.
“There is, however, one way to obtain an uninterrupted succession of
crops from the same land unless the soil is very poor. All plants
derive their nourishment from the soil and the atmosphere; but some
take more from the atmosphere, others from the soil. The plants that
get their sustenance chiefly from the air are those that have luxuriant
foliage. The potato is one of these. You know that it is through their
leaves that plants obtain the carbonic acid gas diffused in the air.
The greater the spread of foliage, the more abundant will this
absorption be. The plants that depend almost wholly on the soil are
those with only a few small, slender leaves, thus taking but little
carbonic acid gas from the air. Such is wheat.
“Moreover, from the potato plant we take only the tubers, which form
but a small part of the whole, and we turn under the stalks and leaves,
which are thereupon converted into humus. Thus the potato has the
property of enriching the soil at the expense of the atmosphere, and it
gives back more than it takes. It is, then, one of the enriching rather
than impoverishing plants in respect to its action on the soil.
Cereals, on the contrary, are utilized by the harvester both as to seed
and haulm, nothing but the meager roots being left in the ground; and
as, on account of their very scanty foliage, cereals derive almost
their entire sustenance from the soil, they take from it much more than
they give back to it. They accordingly belong to the class of plants
that impoverish rather than enrich the soil in which they grow.
“It is impossible, thus, except by a ruinous expenditure of fertilizer,
to raise a crop of grain every year on the same land. But if we should
let potatoes succeed wheat, and wheat succeed potatoes, what would be
the result? The latter crop, deriving a large part of its nourishment
from the air, would flourish in soil comparatively exhausted by wheat;
and on having its leaves and stalks turned under it would give back to
the soil a part of its former fertility. Wheat could then be
successfully raised again on the same land.
“This practice of raising successively on the same land different crops
as little harmful to one another as possible and capable of utilizing
to the utmost the dressing put on to the land, is nothing but that very
rotation of crops that I have already told you something about. Its
purpose is to economize fertilizer and at the same time to secure an
uninterrupted succession of crops. The underlying principle consists in
making an enriching plant succeed an impoverishing one; that is to say,
a plant with luxuriant foliage is made to succeed one with scanty
foliage. The chief enriching plants are clover, lucerne, sainfoin,
potatoes, turnips, and beets. Cereals, on the contrary, are all
impoverishing plants. It is a general custom to raise on the same land
a more or less extended series of different crops, the series running
four, five, or six years, or even longer, after which it begins over
again in the same order. This rotation of crops is designated according
to the number of years the series covers, as for instance a five-year
or a six-year rotation. A six-year rotation might run, we will say,
somewhat as follows:
1st year—potatoes—enriching crop.
2nd year—wheat—impoverishing crop.
3rd year—clover—enriching crop.
4th year—wheat—impoverishing crop.
5th year—sainfoin—enriching crop.
6th year—oats—impoverishing crop.
“Let us examine in detail this series that we have taken as an example.
The first year the soil is thoroughly manured. One of the effects of
manuring is to start a great crop of weeds that would infest the land
and impoverish the crop were they not carefully removed. Hence the
necessity of weeding. To weed a piece of ground is to destroy the weeds
either by hand or with some implement. But it is not every crop that
admits of weeding: the plants must be a certain distance apart, as
otherwise they will be trampled under foot, cut off, or uprooted in the
weeding process. Wheat cannot be weeded, its stalks are too close
together; but potatoes are far enough apart for weeding without
difficulty. Now, weeding destroys all useless, injurious grasses and
other unwelcome intruders; their future reappearance is prevented by
pulling them up before their seeds ripen, and thus at last the ground
is cleaned and made ready for a choice crop. This will explain to you
the great advantage of letting the potato or some other crop that can
be weeded take precedence of the cereals.
“The second year comes wheat. Cleaned by the tillage that has gone
before, the ground is no longer covered with grass and weeds. Nor does
it need fresh manure, for if the potatoes have consumed certain
elements in the soil, these are not exactly the same that wheat
requires; and, furthermore, the dead plants, turned under and reduced
to vegetable mold, compensate by what they have derived from the
atmosphere for what the tubers may have taken from the soil. Wheat is
therefore just the crop to raise now.
“But it would be much against one’s interest to exact from the soil
another crop of wheat the third year. Exhausted by the grain it has
just produced, the soil would yield but a scanty harvest unless it were
freshly manured, a process that would make of the whole operation, not
a piece of farming, but an example of gardening, and would also entail
too great expense. For that reason the third year is devoted to the
raising of an enriching crop, such as clover. After furnishing a supply
of fodder, what is left of the clover is turned under, and all its
remnants of roots, stems, and leaves are reduced to mold, which renders
the soil fit for another wheat harvest the fourth year. A third
enriching crop to be turned under after the final mowing, is likewise
needed for the fifth year; and this crop may be sainfoin. At the end of
the series comes another cereal, oats, for example. The rotation is now
complete, and the program begins all over again.
“Crop-rotation is capable of innumerable variations, and the series may
be longer or shorter, but there should be the slightest possible
departure from the rule that a cereal crop ought always to be preceded
by some crop that enriches the soil.”