The irrigation of the land is, in general, only a more refined method
of manuring it. The nature of the process itself, however, is different
in different countries, as are also the kind and degree of effect it
produces, and the theory by which these effects are to be explained.
In dry and arid climates, where rain rarely falls, the soil may contain
all the elements of fertility, and require only water to call them
into operation. In such cases, as in the irrigations practised so
extensively in eastern countries, and without which, whole provinces
in Africa and Southern America would lie waste, it is unnecessary to
suppose any other virtue in irrigation than the mere supply of water it
affords to the parched and cracking soil.
But in climates such as our own, there are two other beneficial
purposes in reference to the soil, which irrigation may, and one at
least of which it always does, serve.
2. The occasional flow of _pure_ water over the surface, as in
our irrigated meadows, and its descent into the drains, where the
drainage is perfect, washes out acid and other noxious substances
naturally generated in the soil, and thus purifies and sweetens it.
The beneficial effect of such washing will be readily understood in
the case of peat lands laid down to water meadow, since, as every one
knows, peat soils abound in matters unfavourable to general vegetation,
and which are usually in part drawn off by drainage, and in part
destroyed by lime and by exposure to the air, before boggy lands can be
brought into profitable cultivation.
3. But it seldom happens that _pure_ water is employed for the purposes
of irrigation. The water of rivers, more generally, is diverted from
its course, more or less loaded with mud and other finer particles of
matter, which are either gradually filtered from it as it passes over
and through the soil, or in the case of floods subside naturally when
the waters come to rest. Or in less frequent cases, the drainings of
towns, and the waters from common sewers, or from the little streams
enriched by them, are turned with benefit upon the favoured fields.
These are evidently cases of gradual and uniform manuring. And even
where the water employed is clear and apparently undisturbed by mud,
it always contains saline substances grateful to the plant in its
search for food, and which it always contrives to extract more or less
copiously as the water passes over its leaves or along its roots. Every
fresh access of water affords the grass in reality another liquid
manuring.
In the refreshment continually afforded to the plant by a plentiful
supply of water, in the removal of noxious substances from the soil, or
in the frequent additions of enriching food to the land—the efficiency
of irrigation, therefore, seems entirely to consist.