"Yes, that's what we figured on."

"The cowpea is an annual plant. It is planted, produces its seed,
and dies the same season. It has no need to store up material in the
roots for future use. Consequently the substance of the root is
largely taken into the tops as the plan approaches maturity. It is
different with the clover plant. This is a biennial with some
tendency toward the perennial plant. It lives long and develops an
extensive root system, and its stores up material in the roots
during part of its life for use at a later period. About one-third
of the total nitrogen content of the clover plant is contained in
the roots and stubble. This means that the roots and stubble of a
two-ton crop of clover would contain about forty pounds of nitrogen,
or more than we assumed was taken from the soil by the cowpeas. But
there is still another point in favor of the clover. The cowpeas
make their growth during the summer months when nitrification is
most active, whereas the clover growth we have counted on occurs
chiefly during the fall and spring when nitrification is much less
active, consequently the clover probably takes even a larger
proportion of its nitrogen from the air than we have counted on."

"That is rather confusing," said Mr. Thornton, "you say the cowpea
grows when nitrification is most active, and yet you say that it
takes less nitrogen from the air than clover. Isn't that somewhat
contradictory?"

"I think not," said Percy." Let me see.--Just what do you understand
by nitrification?"