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       |   13       |    27
Cowpea                |  78       |   23       |    66
----------------------+-----------+------------+---------
re>
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     |
------------------+-----------+------------+------------+
re>
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.