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.
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 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.
Fig. 26. Cowpeas and Corn—August
Let us now see how easily plant food may be saved by the rotation of crops.