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.

What is the effect on plant food in the soil?

Before answering this question let us see what amounts of plant foods
these crops take out of the soil.

We will assume that the soil is a good loam at the start and will
produce:

One bale of five hundred pounds of lint cotton per acre, sixty bushels
shelled corn per acre, thirty bushels oats per acre, or two tons
cowpea hay per acre.

Such a yield of crop would take from the soil the following amounts of
plant food per acre:

----------------------+-----------+------------+------------
| | Phosphoric |
| Nitrogen, | Acid, | Potash,
| pounds. | pounds. | pounds.
----------------------+-----------+------------+---------
Cotton (whole plant) | 103 | 41 | 65
Corn (whole plant) | 84 | 26 | 61
Oats (whole plant) | 32