dear teacher and, even if we finally learn the lesson, it may be too
everlasting late for us to apply it. Now we all want to learn about
that process called nitrification."

"It is an extremely interesting and important process," said Percy.
"It includes the stages or steps by which the insoluble organic
nitrogen of the soil is converted into soluble nitrate nitrogen, in
which form it become available as food for all of our agricultural
plants."

"Excepting the legumes?" asked Mr. Thornton.

"Excepting none," Percy replied. "The legume plants, like clover,
take nitrogen from the soil so far as they can secure it in
available form, and in this respect clover is not different from
corn. The respect in which it is different is the power of clover to
secure additional supplies of nitrogen from the air when the soil's
available supply becomes inadequate to meet the needs of the growing
clover. If the conditions are suitable for nitrogen-fixation, then
the growth of the legume plants need not be limited by lack of
nitrogen; whereas, nitrogen is probably the element that first
limits the growth and yield of all other crops on your common
soils."

"Now, what do you think of that, Girls? With millions of dollars'
worth of nitrogen in the air over every acre, our crops are poor
just because we don't use it. I wish you would tell me something
about the suitable conditions for nitrogen-fixation, Mr. Johnston.
You understand, Girls, that nitrogen-fixation is simply getting
nitrogen from the inexhaustible supply in the air by means of little