THE ROTATION OF CROPS
The two essential reasons for a rotation of crops are: (1) The
possibility of obtaining for the soil a supply of nitrogen from the
air by introducing a legume at regular intervals, and (2) the
prevention of injury to the crops from fungous diseases, insect
enemies, weeds or other causes. Other reasons are often advanced, some
of which are entirely erroneous, while others are of quite secondary
importance.
The rotation should be carefully studied with reference to the farm
scheme as previously outlined. Reasons for modifying the rotations are:
(1) To change the kind or proportion of crops grown, (2) to change the
amount of labor required, or (3) to increase the crop-producing power
of the soil.
During 25 years the four crops of maize, oats, wheat, timothy and
clover hay have been taken in rotation from the four tiers of plats at
the Pennsylvania State College, so that the influence of the soil has
been entirely eliminated. At the December farm prices for the decade
ending December 1, 1906, the value of these four crops per acre have
been: Maize, $29.67; oats, $14.49; wheat, $18.49; and hay, $18.05. It
will be noted that during 25 years the average income from an acre of
maize has been almost exactly twice that from an acre of oats. The
region where these results were obtained is relatively unfavorable to a
large yield of maize. It is obvious, therefore, that a modification in
the rotation may modify the average income from the farm materially,
provided such modification does not reduce the fertility of the soil.
Thus, while the average income per acre during 25 years for the
four-course rotation above mentioned was $20.17, if the rotation were
increased to a five-course rotation by the addition of another year of
maize, the average income would be $22.45 an acre.
It may be desirable to modify the rotation in order to increase or
decrease a certain crop usually fed upon the farm. Thus, with a
four-course rotation of maize, oats, wheat, clover and timothy,
one-fourth the area would produce hay; while with a six-course
rotation, composed of maize, oats, wheat, each one year, and hay three
years, one-half the area would produce hay. If it is desired to still
further reduce the area in oats and wheat, a seven-course rotation
could be arranged with maize, two years in succession. This is the
rotation that would be desirable for a dairy farm where it is planned
to keep as many cows as practicable and to buy the concentrates
largely. Either the wheat or the oats could be taken out of this
rotation if either the one or the other were thought undesirable and a
still greater amount of roughage desired.
On the other hand, there are places where the minimum amount of
roughage is wanted. There are certain sections of the central West
where it is possible to sow oats on corn stubble without plowing and
where occasionally a rotation is practiced of maize, oats and mammoth
clover. The clover is plowed for maize, the oats are disked in upon
the corn stubble and the next year the clover is pastured until about
June 1, when it is allowed to go to seed. In this rotation the only
roughage obtained is the corn stover and the oat straw.
Another result reached by this rotation is that only one-third the
land is plowed annually. In the four-course rotation mentioned above
three-fourths of the land must be plowed, while in the six-course
rotation one-half is plowed each year. In other ways the character of
the rotation modifies the labor. For example, the labor and cost of
harvesting an acre of hay is much less than that of producing,
harvesting and threshing an acre of wheat.
Rotations may often be planned with reference to the main or cash
crop. Thus in the Aroostook (Maine) potato district the rotation is
potatoes, oats and clover. The chief purpose of the oats and clover is
to keep down the blight in potatoes and add through the clover
nitrogen and organic matter to the soil.
A system of cropping that is best when the owner operates the farm may
not be desirable when the farmer is a tenant. When a farm is rented,
the lease should provide that clover or other legumes occur with
sufficient frequency to keep up the supply of nitrogen without the
purchase of a considerable quantity in chemical fertilizers. The lease
should be so drawn as to make it necessary for the tenant to keep live
stock in order to realize the largest profit. The landlord should
provide an equitable proportion of the mineral fertilizers when such
are required.
The provisions of the lease and the character of the rotation will
necessarily vary with circumstances, but the following system of
tenant farming which has been employed for many years in Maryland will
illustrate the principles just stated:
The lease provides for a five-course rotation consisting of maize,
wheat, clover, wheat, clover. The landlord and the tenant share the
maize and wheat equally, but the clover for hay or pasture goes
entirely to the tenant, unless hay is sold, when it is divided
equally. They each provide one-half the commercial fertilizer and
one-half the seed, except clover seed, which the tenant is required to
furnish.
This lease provides for two clover crops out of every five crops
raised, thus supplying nitrogen abundantly, and the terms of the lease
are such that it is necessary for the tenant to keep live stock to
consume these clover crops in order to secure the most profitable
returns. The feeding of the clover makes it necessary to feed some or
all the maize and may lead to buying additional concentrates.
Stable manure is thereby supplied for the field which is to raise
maize, while mineral fertilizers may be applied to the fields sown to
wheat. On the limestone soils of the eastern states 50 pounds each of
phosphoric acid and potash per acre applied to the wheat, and 10 loads
of stable manure per acre to the maize will probably be found
sufficient to maintain the crop producing power of the soil.
In laying out a farm for a rotation it is desirable to plan the number
of fields or tracts that will go in a rotation and try to get these as
nearly equal size as possible. Having decided upon the number of years
the rotation is to run and having adjusted the fields or tracts
accordingly, it is quite possible to modify the proportion of crops by
adding one crop and dropping another at the same time. Thus, if there
are six 20-acre fields, any one of the following rotations might be
used and the change from one to another easily made:
1. Maize Maize Maize Maize Maize
2. Oats Maize Maize Maize Barley
3. Wheat Oats Oats Wheat Alfalfa
4. Clover and Wheat Clover and Clover and Alfalfa
timothy timothy timothy
5. Timothy Clover and Timothy Timothy Alfalfa
timothy
6. Timothy Timothy Timothy Timothy Alfalfa
During the first year the 20-acre field could be divided into four
tracts of five acres each, containing potatoes, cabbage, tomatoes and
sweet corn, and then followed for four or five years by any succession
of crops above outlined. The point is that a definite adjustment of
the farm to some general method of rotation and a definite system of
fertilization and soil renovation do not prevent a considerable
latitude in the crops raised. It will be obvious that the longer the
rotation the more flexible it becomes in this particular, which is a
point to be considered in laying out the farm and in adjusting fields
and fences.
In some cases it may be desirable on account of the arrangement of the
farm or the character of the crops to be raised to have two distinct
rotations of crops. For example, if the farm lends itself to be
divided into eight tracts, a five-course rotation of maize, oats,
wheat, each one year, and clover and timothy two years, and a
three-course rotation of potatoes, oats or wheat and clover may be
arranged.