s, suggests a
word or two on how these natural defects may, to a certain extent, be
remedied artificially. It stands to reason, that if organic matter in a
soil renders its absorptive power greater, a simple method of improving
a soil defective in this property is by the addition of organic matter.
One of the benefits of ploughing-in green crops on sandy soils is
undoubtedly due to this fact; the addition of farmyard manure having
also a similar effect. The absence of a sufficient amount of
retentiveness, such as is found in sandy soils, in the same way
suggests, as a remedy, the addition of clay; and, _vice versa_, where
the soil is too clayey, the natural method of improvement will be the
addition of sand.[39]

_Shrinkage of Soils._

In drying, soils shrink. Those which shrink least are sandy and chalky
soils. Humus soils, on the other hand, shrink most.

_Most favourable Amount of Water in a Soil._

The amount of water in a soil most favourable for plant-growth is a
question of considerable difficulty. Too great an amount of moisture
renders the land cold; air cannot obtain access to the soil-particles,
and the plants sicken and die. Hellriegel has found that as much as 80
per cent of what the soil can hold is hurtful to plants, and that from
50 to 60 per cent is the best amount.[40]

_Hygroscopic Power._

A property possessed by soils in relation to water, which is quite
distinct from absorptive power, is their hygroscopic power. By this is
meant their power of absorbing water from the air where it is present in
the gaseous form. This property is identical with the property which
will be adverted to immediately--viz., capacity for absorbing gases. The
extent to which soils possess this hygroscopic property seems to be
regulated very much by the same conditions as regulate their ordinary
absorptive power.[41] This property is considered to be of great
importance in the case of soils in hot climates, where their
agricultural value may be said to depend to a large extent upon it. The
amount of water, however, absorbed in this way is, comparatively
speaking, insignificant. Lastly, it may be observed that there are
certain methods of drying soils afflicted with too much moisture. These
consist in making open ditches, and thus relieving them of their
superabundance of water, or in planting certain kinds of trees, such as
willows and poplars. The amount of green surface presented by the large
number of leaves