pend?

What kind of water is best for irrigation?

How do under-drains increase the benefits of irrigation?]

_Irrigation_ does not come strictly under the head of inorganic manures,
as it often supplies ammonia to the soil. Its chief value, however, in
most cases, must depend on the amount of mineral matter which it
furnishes.

The word "irrigation" means simply _watering_. In many districts water
is in various ways made to overflow the land, and is removed when
necessary for the purposes of cultivation. All river and spring water
contains some impurities, many of which are beneficial to vegetation.
These are derived from the earth over, or through which, the water has
passed, and ammonia absorbed from the atmosphere. When water is made to
cover the earth, especially if its rapid motion be arrested, much of
this fertilizing matter settles, and is deposited on the soil. The water
which sinks into the soil carries its impurities to be retained for the
uses of plants. When, by the aid of under-drains, or in open soils, the
water passes _through_ the soil, its impurities are arrested, and become
available in vegetable growth. It is, of course, impossible to say
exactly what kind of mineral matter is supplied by water, as that
depends on the kind of rock or soil from which the impurities are
derived; but, whatever it may be, it is generally soluble and ready for
immediate use by plants.

[What is the difference between water which only runs over the
surface of the earth, and that which runs out of the earth?

Why should strong currents of water not be allowed to traverse the
soil?]

Water which has run over the surface of the earth contains both ammonia
and mineral matter, while that which has arisen out of the earth,
contains usually only mineral matter. The direct use of the water of
irrigation as a solvent for the mineral ingredients of the soil, is one
of its main benefits.

To describe the many modes of irrigation would be too long a task for
our limited space. It may be applied in any way in which it is possible
to cover the land with water, at stated times. Care is necessary,
however, that it do not wash more fertilizing matter from the soil than
it deposits on it, as would often be the case, if a strong current of
water were run over it. Brooks may be dammed up, and thus made to cover
a large quantity of land. In such a case the rapid current would be
destroyed, and the fertilizing matter would settle; but