Under natural conditions the soil does not lose its fixed nitrogen. Green
plants use it to synthesize complicated substances without changing it into
elementary nitrogen. Animals and humans ingest it with the plants and return
it to the soil in fixed form in their excretions and finally with their deceased
remains. Putrefaction and combustion does destroy a certain amount of
fixed nitrogen, but Nature makes good the loss when, during thunderstorms,
lightning combines nitrogen and oxygen in the upper layers of the atmosphere,
which is then washed down by the rain. To this nitrogen-fixing action
of electrical discharge as a source of bound nitrogen is added the activity
of soil bacteria, some of which live free while others are to be found settled
in the root nodules of many plants, converting free nitrogen into bound nitrogen.
Agricultural husbandry essentially maintains the balance of bound nitrogen.
However, with the advent of the industrial age, the products of the soil
are carried off from where the crops are grown to far-off places where they
are consumed, with the result that the bound nitrogen is no longer returned
to the earth from which it was taken.
This has caused the world-wide economic necessity of supplying bound
nitrogen to the soil. This need is increased by national economic considerations,
which, with the denser population of industrialized countries, call for