ferred to in a following
chapter,[48] show this. A damp soil may in the course of a year absorb
far more ammonia than that washed down in rain.

_Gas-absorbing Power of Soils varies._

The power of different soils to absorb these gases varies. This
variation depends not only on their physical properties, but also on
their chemical as well. Soils containing much organic matter have a
greater capacity for absorbing gases than the more purely mineral ones.

_Absorption of Nitrogen._

The absorption of nitrogen by the soil is a question of considerable
importance. It will be referred to later on under the heading of the
biological properties of soils, as it is fixed by the agency of
micro-organisms.[49]

To recapitulate, the chief physical or mechanical properties of a soil
are its absorptive and retentive powers for water; its capacity for
heat; and its power of absorbing gases. It will be easily seen how
tillage operations are calculated to influence these physical properties
of a soil. Thus, in the case of a stiff soil, tillage increases its
power for absorbing the atmospheric gases, chiefly oxygen, which are so
necessary for rendering its fertilising matters available. On the other
hand, in a light and too open soil it may exert quite a contrary effect.

It may be also well to refer here to the important influence these
physical properties exercise on the growth of the plant.

_Plant-roots require a certain Openness in the Soil._

One of the functions of the soil is to support the plant in an upright
position, and this is a function which requires in the soil a certain
amount of compactness or firmness. On the other hand, however, a soil
must not possess too great compactness, otherwise the plant-roots will
experience a difficulty in pushing their way downwards. This is
especially the case during the earlier periods of growth, when the
plant-roots are as yet extremely tender, and experience great difficulty
in overcoming much resistance. The importance of preparing a mellow
seed-bed will be thus at once seen to be based on sound scientific
principles; and this for a double reason. Not only does the young plant
require every facility for developing its roots, but also, as has just
been pointed out, an abundant supply of oxygen is of paramount
importance during the process of germination.

_Soil and Plant-roots._

The whole question of the influence of the mechanical condition of the
soil on the develo