, the crop being barley. The increase, of course, is due to the
water which the crop transpires.[37]

It may be generally said that the greater the absorptive power of a
soil, the greater is its retentive power; for soils that most largely
absorb water are the most reluctant to part with it.

While these properties are undoubtedly necessary for fertile soils, it
is needless to add that they may be possessed by a soil to too great an
extent. The soil that is unable to throw off any excess of water becomes
cold and damp, and does not admit of proper tillage. Its pores become
entirely choked up, and the circulation of air, which, as we shall see,
is of so much importance, is rendered impossible. Plants in such a soil
are apt to sicken and die, the water becomes stagnant, and certain
chemical actions are caused which give rise to poisonous gases, such as
sulphuretted hydrogen, &c. A stiff clayey soil offers a good example of
the disadvantage of over-retentiveness. Owing to the difficulty such
soils experience in throwing off their excessive water, they are
extremely difficult to till; and sowing operations are on that account
apt to be delayed.

_Power Plants have of absorbing Water from a Soil._

It is a strange fact, and one worth noticing in this connection, that
the power plant-roots have of drawing their moisture from a soil, seems
to depend on the retentive power of the soil. By this is meant that
plants have not the means of exhausting the water in a retentive soil to
such an extent as in a non-retentive soil.

In some extremely interesting experiments, carried out by the well-known
German botanist Sachs, it was found that plants wilted in a loamy soil,
whose water-holding capacity was 52 per cent, when its moisture reached
8 per cent; while in a sandy soil--water-holding capacity 21 per
cent--the same species of plant did not wilt until its moisture reached
1-1/2 per cent. Here, then, we see that on one kind of soil the plant
was able to live, and obtain sufficient water for its needs, while it
died of thirst in another soil, although that soil contained quite as
much moisture.

Speaking generally, we may say that Hellriegel's experiments have shown
that any soil can supply plants with all the water they need so long as
its moisture is not reduced below one-third of the whole amount it can
hold.[38]

_How to increase Absorptive Power of Soils._

The absence or presence, in excess, of the above propertie