way clover equivalent to about three tons of hay could be plowed
under. Clover hay contains 40 pounds of nitrogen per ton; so this
would supply about 120 pounds of nitrogen in addition to the 173
pounds possible to be supplied in the manure. This would make
possible a total return of 293 pounds, while we figured some 200
pounds removed. Of course if you save only 100 pounds in the manure
the amount returned would be reduced to 220 pounds."

"There are two questionable points in this plan," said Mr. Thornton,
" one is the impossibility, or at least the difficulty, of growing
clover on this land. The other point is, How much of that 120 pounds
of nitrogen returned in the clover is taken from the soil itself? I
remember you figured 86 pounds of nitrogen in two tons of cowpea
hay, but you also assumed that about 29 pounds of it would be taken
from the soil."

"Yes, that is true," Percy replied, "at least 29 pounds and
probably more. You see the cowpeas grow during the same months as
corn and on land prepared in about the same manner. If the soil will
furnish 75 pounds of nitrogen to the corn crop, and 48 pounds to the
oats and wheat, it would surely furnish 29 pounds to the cowpeas. Of
course this particular amount has no special significance, but the
other definite amounts removed in corn, oats, and wheat aggregate
171 and the 29 pounds were added to make the round 200 pounds.
Perhaps 210 pounds would be nearer the truth, in which case the soil
would furnish about half as much nitrogen to the cowpea crop as to
the corn crop. This is reasonable considering that corn is the first
crop grown after the manure is applied. You will remember that only
one-tenth of the total nitrogen of the cowpea plant remains in the
roots and stubble?"