Do all leguminous plants have equal numbers of
nodules? How do these nodules help the farmer?

SECTION XI. THE ROTATION OF CROPS

Doubtless you know what is meant by rotation, for your teacher has
explained to you already how the earth rotates, or turns, on its axis
and revolves around the sun. When we speak of crop-rotation we mean not
only that the same crop should not be planted on the same land for two
successive years but that crops should follow one another in a regular
order.

Many farmers do not follow a system of farming that involves a change of
crops. In some parts of the country the same fields are planted to corn
or wheat or cotton year after year. This is not a good practice and
sooner or later will wear out the soil completely, because the
soil-elements that furnish the food of that constant crop are soon
exhausted and good crop-production is no longer possible.

Why is crop-rotation so necessary? There are different kinds of plant
food in the soil. If any one of these is used up, the soil of course
loses its power to feed plants properly. Now each crop uses more of some
of the different kinds of foods than others do, just as you like some
kinds of food better than others. But the crop cannot, as you can, learn
to use the kinds of food it does not like; it must use the kind that
nature fitted it to use. Not only do different crops feed upon different
soil foods, but they use different quantities of these foods.

Now if a farmer plant the same crop in the same field each year, that
crop soon uses up all of the available plant food that it likes. Hence
the soil can no longer properly nourish the crop that has been year by
year robbing it. If that crop is to be successfully grown again on the
land, the exhausted element must be restored.

[Illustration: FIG. 25. GRASS FOLLOWING CORN]

This can be done in two ways: first, by finding out what element has
here been exhausted, and then restoring this element by means either of
commercial fertilizers or manure; second, by planting on the land crops
that feed on different food and that will allow or assist kind Mother
Nature "to repair her waste places." An illustration may help you to
remember this fact. Nitrogen is, as already explained, one of the
commonest plant foods. It may almost be called plant bread. The wheat
crop uses up a good deal of nitrogen. Suppose a field were planted in
wheat year after year. Most of the available nitrogen would be