taken out
of the soil after a while, and a new wheat crop, if planted on the
field, would not get enough of its proper food to yield a paying
harvest. This same land, however, that could not grow wheat could
produce other crops that do not require so much nitrogen. For example,
it could grow cowpeas. Cowpeas, aided by their root-tubercles, are able
to gather from the air a great part of the nitrogen needed for their
growth. Thus a good crop of peas can be obtained even if there is little
available nitrogen in the soil. On the other hand wheat and corn and
cotton cannot use the free nitrogen of the air, and they suffer if there
is an insufficient quantity present in the soil; hence the necessity of
growing legumes to supply what is lacking.

[Illustration: FIG. 26. COWPEAS AND CORN--AUGUST]

Let us now see how easily plant food may be saved by the rotation of
crops.

If you sow wheat in the autumn it is ready to be harvested in time for
planting cowpeas. Plow or disk the wheat stubble, and sow the same field
to cowpeas. If the wheat crop has exhausted the greater part of the
nitrogen of the soil, it makes no difference to the cowpea; for the
cowpea will get its nitrogen from the air and not only provide for its
own growth but will leave quantities of nitrogen in the queer nodules of
its roots for the crops coming after it in the rotation.

[Illustration: FIG. 27. COWPEAS AND CORN--OCTOBER]

If corn be planted, there should be a rotation in just the same way. The
corn plant, a summer grower, of course uses a certain portion of the
plant food stored in the soil. In order that the crop following the corn
may feed on what the corn did not use, this crop should be one that
requires a somewhat different food. Moreover, it should be one that fits
in well with corn so as to make a winter crop. We find just such a
plant in clover or wheat. Like the cowpea, all the varieties of clover
have on their roots tubercles that add the important element, nitrogen,
to the soil.

From these facts is it not clear that if you wish to improve your land
quickly and keep it always fruitful you must practice crop-rotation?

AN ILLUSTRATION OF CROP-ROTATION

Here are two systems of crop-rotation as practiced at one or more
agricultural experiment stations. Each furnishes an ideal plan for
keeping up land.

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