animals again when they studied zoology. Orton says it is easy to
tell a cow from a cabbage, but impossible to assign any absolute,
distinctive character which will divide animal life from plant life.

"The oxygen is essential for nitrification, because that is an
oxidation process. That is, it is a kind of combustion, so to speak.
The organic matter is oxidized or converted into substances
containing more oxygen than in the original form. In ammonification
the carbon is separated or divorced from the nitrogen and united
with oxygen. Some of the hydrogen of the organic matter remains
temporarily with the carbon, and some is held temporarily with the
nitrogen in the form of ammonia.

"The nitrite bacteria replace two of the hydrogen atoms in ammonia
with one of oxygen, and insert another oxygen atom between the
nitrogen and the remaining hydrogen, thus forming nitrous acid;
H-O-N=O, or HNO2.

"The nitrate bacteria then cause the direct addition of another
oxygen atom, which is held by the two extra bonds of the nitrogen
atom, which you will remember is a five-handed atom.

"Thus you will see the absolute need of free oxygen in the
nitrification process; and we can control the rate of nitrification
to a considerable extent by our methods of tillage. In soils
deficient in organic matter, excessive cultivation may still
liberate sufficient nitrogen for a fairly satisfactory crop; and the
benefits of such excessive cultivation for potatoes and other
vegetables is more often due to increased nitrification than to the
conservation of moisture, to which it is frequently ascribed by