only
found in the control which it gives the farmer in the amount of the
soil water. He can add to the rainfall sufficient to take away the
excess of mineral matter. When such soils are first brought under
tillage it is necessary to use a large amount of water from the
canals, in order to wash away the old store of alkali. After that a
comparatively small contribution will often keep the soil in excellent
condition for agriculture. It has been found, however, in the
irrigated lands beside the Nile that where too much saving is
practised in the irrigation, the alkaline coating will appear where it
has been unknown before, and with it an unfitness of the earth to bear
crops.

Although the crust of mineral matters formed in the manner above
described is characteristic of arid countries, and in general peculiar
to them, a similar deposit may under peculiar conditions be formed in
regions of great rainfall. Thus on the eastern coast of New England,
where the tidal marshes have here and there been diked from the sea
and brought under tillage, the dissolved mineral matters of the soil,
which are excessive in quantity, are drawn to the surface, forming a
coating essentially like that which is so common in arid regions. The
writer has observed this crust on such diked lands, having a thickness
of an eighth of an inch. In fact, this alkali coating represents
merely the extreme operation of a process which is going on in all
soils, and which contributes much to their fertility. When rain falls
and passes downward into the earth, it conveys the soluble matter to a
depth below the surface, often to beyond the point where our ordinary
crop plants, such as the small grains, can have access to it, and
this for the reason that their roots do not penetrate deeply. When dry
weather comes and evaporation takes place from the surface, the fluid
is drawn up to the upper soil layer, and there, in process of
evaporation, deposits the dissolved materials which it contains. Thus
the mineral matter which is fit for plant food is constantly set in
motion, and in its movement passes the rootlets of the plants. It is
probably on this account--at least in part--that very wet weather is
almost as unfavourable to the farmer as exceedingly dry, the normal
alternation in the conditions being, as is well known, best suited to
his needs.

So long as the earth is subjected to conditions in which the rainfall
may bring about a variable amount of water in