_Retentive Power of Soils for Water._

Now closely connected with this absorptive power of soils, which we have
just been considering, is the power soils possess of holding or
retaining the water they absorb. This power, it will be seen at a
glance, must have an important bearing on the fertility of a soil.

_Importance of Retentive Power._

As a considerable interval often elapses between the periods of
rainfall, soils, if they are to support vegetable growth, must be able
to store up their water-supply against periods of drought. This is all
the more necessary when we remember that, in the case of heavy crops,
the rainfall would often be inadequate to supply the water necessary for
their growth. In fact, it has been estimated that the average
evaporation from soils bare of any cultivation is equal to the rainfall.
That the evaporation from soils covered with vegetation is very much
greater, has been strikingly shown by a calculation made by the late
eminent American botanist, Professor Asa Gray, who calculated that a
certain elm-tree offered a leaf-surface, from which active transpiration
constantly went on, of some five acres in extent; while it has further
been calculated that a certain oak-tree, within a period of six months,
transpired during the daytime eight and a half times more water than
fell as rain on an area equal in circumference to the tree-top.[36] Just
as the state of the fineness of the soil-particles has an important
influence on the absorptive power of soils, so, too, it is found, it has
an important bearing on the rate at which evaporation takes place.
Evaporation goes on to the greatest extent in soils whose particles are
compacted together, capillary action in this case taking place more
freely, and effecting evaporation from a greater depth of soil. The
stirring of the surface portion of the soil, as for example by hoeing or
harrowing, has for this reason an important influence in lessening the
amount of evaporation, and minimising the risks of drought, by breaking
the capillary attraction. The amount of evaporation which takes place
from a soil covered with a crop, depends largely on the nature of the
crop; a deep-rooted crop, since it draws its moisture from a wider area
of soil, being more effective in drying a soil than a shallow-rooted
crop. The difference in the amounts evaporated from a cropped and a bare
fallow soil has been shown at Rothamsted to equal a rainfall of nine
inches