IRRIGATION.
On what does the benefit arising from irrigation chiefly depend?
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, if the course of the brook were turned, so that it would run in a current over any part of the soil, it might carry away more than it deposited, and thus prove injurious. Small streams turned on to land, from the washing of roads, or from elevated springs, are good means of irrigation, and produce increased fertility, except where the soil is of such a character as to prevent the water from passing away, in which case it should be under-drained.