These facts are true to a greater or less degree in the case of nearly all of the farm crops.  The grain crops are often considered as humus makers because of the stubble turned under, but Professor Snyder, of Minnesota, found that five years’ continuous culture of wheat resulted in an annual loss of 171 pounds of nitrogen per acre, of which only 24.5 was taken by the crop, the remaining 146.5 pounds were lost through a waste of organic matter.

THE ROTATION OF CROPS

Now, suppose that instead of growing cotton on the same soil year after year, we select four crops—­cotton, corn, oats and cowpea—­and grow them in regular order, a rotation practiced in some parts of the South.

We will divide the farm into three fields and number them 1, 2 and 3, and will plant these crops as indicated by the following diagrams: 

[Illustration:  Plan of farm.]

Plan for planting.

FIELD 1.         FIELD 2.          FIELD 3.
+----------------+----------------+----------------+
|                |     OATS,      |      CORN,     |
1st year  |                |  harvested in  |   followed by  |
or 1905.  |     COTTON     |    spring,     |      oats,     |
|                |  followed by   |   planted in   |
|                |    COWPEAS.    |      fall.     |
+----------------+----------------+----------------+
|      CORN,     |                |      OATS,     |
2d year   |  followed by   |                |  harvested in  |
or 1906.  |      oats,     |     COTTON.    |    spring,     |
|   planted in   |                |  followed by   |
|      fall.     |                |    COWPEAS.    |
+----------------+----------------+----------------+
|      OATS,     |     CORN,      |                |
3d year   |  harvested in  |  followed by   |                |
or 1907.  |    spring,     |     oats,      |     COTTON.    |
|  followed by   |  planted in    |                |
|    COWPEAS.    |     fall.      |                |
+----------------+----------------+----------------+

Each of these crops occupies one-third of the farm each year, and yet the crop on each field changes each year so that no one kind of crop is grown on any field oftener than once in three years.  The cotton is grown for market, the corn partly to sell, partly to feed, the oats to feed and the cowpeas to plow under.  All cotton and corn refuse is plowed under.

What effect will such a system have on the conditions necessary for plant growth?  Suppose we follow the crops on Field 1.  Cotton, corn, and oats are humus wasting crops but the pea crop which is grown the third year is plowed under, and largely, if not entirely, remedies the loss by furnishing a new supply of organic matter, and the ill effects which we noticed would follow the loss of organic matter due to the continuous growing of cotton are avoided, soil texture is preserved, soil ventilation is not injured, and the power of the soil over water is preserved.