ee of porosity of the soil. The nature of the ground air differs
from that of the atmosphere only as it is influenced by its location.
The principal constituents of the air--nitrogen, oxygen, and carbonic
acid--are also found in the ground air, but in the latter the relative
quantities of O and CO2 are different.

AVERAGE COMPOSITION OF ATMOSPHERIC AIR IN 100 VOLUMES

Nitrogen 79.00 per cent.
Oxygen 20.96 "
Carbonic acid 0.04 "

AVERAGE COMPOSITION OF GROUND AIR

Nitrogen 79.00 per cent.
Oxygen 10.35 "
Carbonic acid 9.74 "

Of course, these quantities are not constant, but vary in different
soils, and at different depths, times, etc. The greater quantity of
CO2 in ground air is due to the process of oxidation and decomposition
taking place in the soil. Ground air also contains a large quantity
of bacterial and other organic matter found in the soil.

Ground air is in constant motion, its movements depending upon a great
many factors, some among these being the winds and movements of the
atmospheric air, the temperature of the soil, the surface temperature,
the pressure from the ground water from below, and surface and rain
water from above, etc.

=Ground Moisture.=--The interstices of the soil above the ground-water
level are filled with air _only_, when the soil is absolutely dry; but
as such a soil is very rare, all soils being more or less damp, soil
usually contains a mixture of air and water, or what is called _ground
moisture_.

Ground moisture is derived partly from the evaporation of the ground
water and its capillary absorption by the surface soil, and partly by
the retention of water from rains upon the surface. The power of the
soil to absorb and retain moisture varies according to the physical
and chemical, as well as the thermal, properties of the soil.

Loose sand may hold about 2 gallons of water per cubic foot; granite
takes up about 4 per cent of moisture; chalk about 15 per cent; clay
about 20 per cent; sandy loam 33 to 35 per cent; humus[10] about 40
per cent.

=Ground Temperature.=--The temperature of the soil is due to the
direct rays of the sun, the physicochemical changes in its interior,
and to the internal heat of the earth.

The ground temperature varies according to the annual and diurnal
cha