microscopic organisms called bacteria, which live in little balls
called tubercles attached to the roots of certain plants called
legumes, like cowpeas and clover. Corn and wheat and such crops
can't get this nitrogen. Now, Mr. Johnston is telling about
nitrification, a process which is entirely different from
nitrogen-fixation. Excuse me, Mr. Johnston, but I wanted to make
this plain to Mrs. Thornton and Miss Russell."

"I am glad you did so," Percy replied. "As I was saying,
nitrification has no connection whatever with the free nitrogen of
the air.

"All plants take their food in solution; that is, the plant food
taken from the soil must be dissolved in the soil water or moisture.
Of the essential elements of plant food, seven are taken from the
soil through the roots into the plant. These seven do not include
those of which water itself is composed. Now, these seven plant food
elements exist in the soil almost exclusively in an insoluble form.
In that condition they are not available to the plant for plant
food; and it is the business of the farmer to make this plant food
available as fast as is needed by his growing crops.

"The nitrogen of the soil exists in the organic matter; that is, in
such materials as plant roots, weeds, and stubble, that may have
been plowed under, or any kind of vegetable maker incorporated with
the soil, including all sorts of crop residues, green manures, and
the common farm fertilizers from the stables. When these organic
materials are decomposed and disintegrated to such an extent that
their structure is completely destroyed, the resulting mass of
partially decayed black organic matter is called humus. The nitrogen