"Do you mean to say that the roots and stubble of the cowpeas would
add no nitrogen to the soil? Surely that does not agree with the
common talk."

"It is even worse than that," said Percy. "The cowpea roots and
stubble would contain less nitrogen than the cowpea crop takes from
a soil capable of yielding thirty bushels of corn or oats. Only
about one-tenth of the nitrogen contained in the cowpea plant is
left in the roots and stubble when the crop is harvested. Suppose
the yield is two tons per acre of cowpea hay! Such a crop would
contain about 86 pounds of nitrogen, and about 10 pounds of nitrogen
per acre would be left in the roots and stubble."

"Well, that wouldn't go far toward replacing the 171 pounds removed
from the soil by the corn, oats, and wheat, that's sure," was Mr.
Thornton's comment.

"It is worse than that," Percy repeated. "Land that will furnish 48
pounds of nitrogen for a crop of oats or wheat will furnish more
than 10 pounds for a crop of cowpeas. At the end of such a four-year
rotation such a soil would be about 200 pounds poorer in nitrogen
per acre than at the beginning, if all crops were removed and
nothing returned."

"How much would it cost to put that nitrogen back in commercial
fertilizer?" asked Mr. Thornton.

"That depends, of course, upon what kind of fertilizer is used."