of the soil is one of the constituents of this humus or other
organic matter. It is not contained in the mineral particles of the
soil. On the other hand the other six elements of plant food are
contained largely in the mineral part of the soil, as the clay,
silt, and sand. thus the iron, calcium, magnesium, and potassium,
all of which are called abundant elements, are contained in the
mineral matter, and usually in considerable amounts, while they are
found in the organic matter in very small proportion. The phosphorus
and sulfur are found in very limited quantities in most soils, but
they are present in both organic and mineral form.

"Practically the entire stock or store of all of the elements in the
soil is insoluble and consequently unavailable for the use of
growing plants; and, as I said, some of the chief plans and efforts
of the farmer should be directed to the business of making plant
food available.

"The nitrogen contained in the insoluble organic matter of the soil
is made soluble and available by the process called nitrification.
Three different kinds of bacteria are required to bring about the
complete change."

"Are these bacteria different from the nitrogen fixing bacteria?"
asked Mr. Thornton.

"Entirely different," Percy replied, "and there are three distinct
kinds, one for each of the three steps in the process.

"The first may be called ammonia bacteria. They have power to
convert organic nitrogen into ammonia nitrogen; that is, into the