[Sidenote: Subsoil air is saturated with aqueous vapour and contains
large quantities of carbonic acid.]
The pores of soil are either partly or wholly filled with air, which
as a rule is saturated with aqueous vapour. This air consists very
largely of carbonic acid (from 0·2 to 14 per cent., on an average from
2 to 3 per cent.) and to a small extent of oxygen, which has been used
up for the formation of carbonic acid. It also contains traces of
ammonia and gases of decomposition.
The movements of subsoil air need not be considered here, and beyond
these few general observations it will not be necessary to deal with
the subject.