Of these bacteria, among the most important are
those which are the active agents in the process known as
"nitrification"--_i.e._, the process whereby organic nitrogen and
ammonia salts are converted into nitrites and nitrates. The presence of
these organisms, it would appear, is indispensable to the fertility of
any soil. There are organisms, on the other hand, which have the power
of reversing the work of the nitrification bacteria by converting
nitrates into other forms of nitrogen. The reduction of nitrates in the
soil is often the source of much loss of valuable nitrogen, which
escapes in the free state, so that the action of bacteria is not
altogether of a beneficial nature.

_Three Classes of Organisms in the Soil._

So far as the subject has been at present studied, the micro-organisms
in the soil may be divided into three classes.[57]

_First Class of Organisms._

We have, first of all, those whose function it is to oxidise the soil
ingredients. Organisms of this class may act in different ways. They may
assimilate the organic matter of the soil and convert it into carbonic
acid gas and water; or, on the other hand, they may oxidise it by giving
off oxygen. Some of these organisms, whose action is of the first kind,
choose most remarkable materials for assimilation. One has been found to
require ferrous carbonate for its development, which it oxidises into
the oxide (Winogradsky); while another,[58] the so-called sulphur
organism, converts sulphur into sulphuretted hydrogen according to some,
and according to others into sulphates. To this class of organism the
nitrifying organisms belong. As will be seen more fully in a subsequent
chapter, two distinct organisms connected with this process have already
been isolated and studied--one of these effecting the formation of
nitrites from organic nitrogen or ammonia salts, and the other the
conversion of nitrites into nitrates. The second method in which these
oxidising organisms act is by giving off oxygen. There is much interest
attaching to this fact, as it was supposed till quite recently that all
evolution of oxygen in vegetable physiology was dependent on the
presence of light, and also intimately connected with chlorophyll, or
the green colouring matter of plants. It would seem, however, that among
the soil organisms these conditions are not necessary, and the evolution
of oxygen may be carried on in the case of colourless organisms as well
as in the c