126 Nitrogen-Metabolism on Land and in the Sea. 
during w hich this phenomenon was observed, be two to four times 
as great in quantity as the “producers.” As is well-known, 
however, even tow-nets of the finest miller’s silk fail to catch in 
any quantity the smallest organisms. The disproportion observed 
by Brandt was probably due to the presence of minute Flagellata 
which almost completely escaped the net. 
It is a well ascertained fact that all plants require a minimum 
quantity of certain inorganic substances without which life is 
impossible, and if these are increased to a certain degree, the growth 
and development of the plants are increased also. As most of the 
other substances present in the soil are in sufficient quantity to sup¬ 
ply all the needs of the plant, the production of plant-substance is 
practically dependent on the supply of nitrogen-compounds. By 
increase of nitrogen-compounds, as in manuring, the growth of plants 
can be enormously increased, and by reduction of these compounds 
the growth is correspondingly lessened. It is clear, then, that the 
general metabolism on the land as a whole is dependent on the 
circulation of nitrogen. 
The sources of nitrogen-compounds in the case of land plants 
are three in number. In the first case, on the death of existing 
living beings, the proteid material becomes broken down by putre¬ 
faction into ammonia-compounds, which later become converted into 
nitrites and nitrates. The first process is dependent on the presence 
of putrefactive bacteria and the second on special nitrifying bacteria 
which are found distributed in all soils. Besides this process of the 
formation of nitrites and nitrates in the soil there is also an opposite 
process going on in which nitrogen-compounds are broken down into 
free nitrogen, which escapes into the air and is apparently lost. 
This process is the result of the action of denitrifying bacteria. 
Nitrogen-compounds are also lost to the soil in very large quantities 
in other ways, for owing to their solubility these substances are 
washed out of the soil by rain and carried by the rivers to the sea ; 
also by the wasteful process of running sewage into the sea 
enormous quantities of nitrogen-compounds are continually lost. 
It is clear that there must be some natural process or processes 
by which the stock of nitrogen of the soil can be replenished, for 
green plants by themselves can make no use whatever of the free 
nitrogen which surrounds them in such abundance. There are in 
fact two chief counter-processes which bring again the nitrogen of 
the air into circulation. A certain amount of combined nitrogen 
is returned to the soil dissolved in rain, having been produced by 
