ist on dew or which can take on
an occasional growth at such times as moisture may come upon them.
With a slight increase in precipitation, the soil rapidly increases in
productivity, so that we may say that where as much as about ten
inches of water enters the earth during the summer half of the year,
it becomes in a considerable measure fit for agriculture. Observations
indicate that the conditions of fertility are not satisfied where the
rainfall is just sufficient to fill the pores of the soil; there must
be enough water entering the earth to bring about a certain amount of
outflow in the form of springs. The reason of this need becomes
apparent when we study the evident features of those soils which,
though from season to season charged with water, do not yield springs,
but send the moisture away through the atmosphere. Wherever these
conditions occur we observe that the soil in dry seasons becomes
coated with a deposit of mineral matter, which, because of its taste,
has received the name of alkali. The origin of this coating is as
follows: The pores of the soil, charged from year to year with
sufficient water to fill them, become stored with a fluid which
contains a very large amount of dissolved mineral matter--too much,
indeed, to permit the roots of plants, save a few species which have
become accustomed to the conditions, to do their appointed work. In
fact, this water is much like that of the sea, which the roots of only
a few of our higher plants can tolerate. When the dry season comes on,
the heat of the sun evaporates the water at the surface, leaving
behind a coating composed of the substances which the water contains.
The soil below acts in the manner of a lamp-wick to draw up fluid as
rapidly as the heat burns it away. When the soil water is as far as
possible exhausted, the alkali coating may represent a considerable
part of the soluble matter of the soil, and in the next rainy season
it may return in whole or in part to the under-earth, again to be
drawn in the manner before described to the upper level. It is
therefore only when a considerable share of the ground water goes
forth to the streams in each year that the alkaline materials are in
quantity kept down to the point where the roots of our crop-giving
plants can make due use of the soil. Where, in an arid region, the
ground can be watered from the enduring streams or from artificial
reservoirs, the main advantage arising from the process is comm