| 13 | 27
Cowpea | 78 | 23 | 66
----------------------+-----------+------------+---------

Now suppose we sell the lint of the cotton, keeping all the rest of
the plant, including the seed, on the farm and turning it back into
the soil.

Of the corn suppose we sell one-half the grain and keep the other half
and the fodder for use on the farm.

Suppose the oats be made into oat hay and be fed on the farm and the
cowpeas be turned under.

Assuming that the cowpeas take half their nitrogen from the air.

This will mean that in the course of three years we take out of the
soil of each acre in the crops:

Nitrogen. Phosphoric Acid. Potash.
258 pounds. 103 pounds. 219 pounds.

but we return to the soil in crop refuse and manure from the stock:

Nitrogen. Phosphoric Acid. Potash.
256 pounds. 87 pounds. 197 pounds.

This assumes that we have taken from the farm in products sold:

------------------+-----------+------------+------------+
| Nitrogen. | Phosphoric | Potash. |
| | Acid. | |
------------------|-----------|------------|------------|
Cotton Lint | 2 | 1 | 2 |
Corn | 28 | 12 | 10 |
Animal products | 11 | 3 | 10 |
+-----------+------------+------------+
Totals | 41 | 16 | 22 |
------------------+-----------+------------+------------+

The plant food charged to animal products is twenty per cent. of that
in the grain and forage fed to the stock.

At the end of the three years the plant food account will balance up
with:

Nitrogen a gain of 2 pounds.
Phosphoric Acid a loss of 16 "
Potash a loss of 22 "

This result is of course approximate. There will be some loss of
nitrogen through leaching and denitrification. Some of the potash and
phosphoric acid will be converted into unavailable forms. This can be
made good by applying to the cotton a fertilizer containing twenty
pounds of nitrogen, sixty pounds of phosphoric acid and twenty pounds
of potash.

Additional nitrogen and organic matter can be grown to turn under by
planting crimson clover in the cotton at the last working for a winter
cover crop to be turned under for the corn, and by planting