The nitrifying bacteria are those which have the power to form nitrates.
In the following brief discussion of this subject we include at least
three species of bacteria which by their combined or successive action
have the power to transform organic nitrogen into nitrate nitrogen, which
is a suitable form of nitrogen for plant food. For the exact information
which we now have regarding the nitrifying bacteria we are indebted to
the researches of Pasteur and Schlösing and Müntz of France, Winogradsky
of Russia, Warington of England, and others.
The nitrogen in the soil is almost entirely in organic compounds; that
is, the nitrogen (which is a gas in the free, or uncombined, state) is
united or combined with other elements, notably with carbon, hydrogen,
and oxygen, in the form of partially decayed vegetable or organic matter.
(By organic matter we mean matter which has been formed by the growth
of some organism, either plant or animal, as grass or flesh.) Plants
cannot use the free nitrogen of the air as plant food, neither can they
use the organic compounds of nitrogen which occur in the soil. There
are at least three different kinds of bacteria, and also three different
steps or stages involved in the process of nitrification, the nitrogen
being changed from the organic compounds first into the ammonia[2] form,
second, into the nitrite form, and third into the nitrate form. During
the process the nitrogen is separated from the carbon and other elements
composing the insoluble organic matter, and is united or combined with
oxygen and some alkaline element (as calcium) to form the soluble
nitrate, such as calcium nitrate, which is one of the most suitable
compounds of nitrogen for plant food. Calcium is the alkaline element
contained in lime or limestone. The name _calcium nitrate_ indicates just
what elements this compound contains; namely, calcium, nitrogen, and
oxygen. (In the names of compounds the ending _-ate_ always means oxygen.)
This is the general process of nitrification in which the nitrifying
bacteria transform or transfer the nitrogen from insoluble organic
compounds into soluble compounds in which it may serve as available
plant food. The nitrate which is thus formed may be calcium nitrate or
magnesium nitrate or potassium nitrate or even sodium nitrate, depending
upon which of these alkaline elements is present in the must suitable
form. If no alkaline element is present in available form then no
nitrates can be made in the soil. One of the reasons for applying ground
limestone to soils which are deficient in lime is to furnish the element
calcium in suitable form for the formation of nitrates in the process of
nitrification. Ground limestone is calcium carbonate (CaCO₃), a compound
containing one atom of calcium (Ca), one atom of carbon (C) and three
atoms of oxygen (O₃). This is the same form of lime which is contained
naturally in limestone soils—soils which are noted for their great
productiveness—and it is generally the most economical form of lime to
use for correcting soil acidity and promoting nitrification.
In the process of nitrification, that is in the formation of nitrates,
there is required, not only the presence of calcium, or some other
alkaline element, in suitable form, but also a good supply of the element
oxygen; for calcium nitrate, Ca(NO₃)₂, contains one atom of calcium
(Ca), two atoms of nitrogen (N)₂, and six atoms of oxygen (O₃)₂, in
each molecule as indicated in the formula, Ca(NO₃)₂. Magnesium nitrate,
Mg(NO₃)₂, potassium nitrate, KNO₃ (K is from the Latin word _Kalium_,
which means potassium), and all other nitrates, also, contain oxygen. The
supply of oxygen for the formation of nitrates in the soil comes from the
air, which consists of about twenty percent oxygen, seventy-eight percent
nitrogen, and two percent of other elements and compounds, as argon,
carbon dioxid, CO₂, water vapor, H₂O, etc. One of the important effects
of cultivation, or tillage, is that it permits the air more freely to
enter the soil, and thus promotes nitrification.