As they reach the surface in dry
weather, they give off by evaporation pure water; and the salts,
which they held in solution, remain behind in the upper surface. The
capillary action goes on; and as the pure water is taken off in the
atmosphere in vapour, other water impregnated with more salts comes
up to supply its place; and the salts near the surface either
accumulate or are supplied to the roots of the plants, shrubs, or
trees, which require them.
[* Caused, possibly, by the Vendeya range once extending E. N. E. up
to the Himmalaya chain, which runs E. S. E. It now extends up only to
the right bank of the Ganges, at Chunar and Mirzapoor.]
Rain-water,* which contains no such salts, falls after the dry season
is over, and washes out of the upper surface a portion of the salts,
which have thus been brought up from below and accumulated, and
either takes them off in floods or carries them down again to the
beds below. Some of these salts, or their bases, may become
superabundant, and render the lands oosur or unfit for ordinary
tillage. There may be a superabundance of those which are not
required, or cannot be taken up by the plants, actually on the
surface, or there may be a superabundance of the whole, from the
plants and rain-water being insufficient to take away such as require
to be removed. These salts are here, as elsewhere, of great variety;
nitrates of ammonia, which, combining with the inorganic substances--
magnesia, lime, soda, potash, alumina, and oxide of iron--form double
salts, and become soluble in water, and fit food for plants.
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