Irrigation (Lat., 'watering'), a method of producing or increasing fertility in soils by an artificial supply of water, or by inundating them at stated periods. Irrigation was probably first resorted to in countries where much of the land must otherwise have remained barren from drought, as in Egypt, where it was extensively practised nearly 2000 years before Christ, and where great systems of canals and artificial lakes were formed for the purpose. Extensive works, intended for the irrigation of large districts, existed in times of remote antiquity in Mesopotamia, Persia, India, China, and some other parts of the East; and in such of these countries as have not entirely lost their ancient prosperity such works still exist. Some plants also require a very abundant supply of water, and irrigation has become general where their cultivation prevails. This is particularly the case with rice, the principal grain of great part of Asia. In Europe irrigation prevails chiefly in the south, where it was extensively practised by the Romans; and it is most extensively practised in northern Italy, and in some parts of Spain and southern France.
Irrigation in Britain, where it was hardly practised till the 19th century, and in most parts of Europe except Italy, is almost exclusively employed for the purpose of increasing the produce of grass by converting the land into water-meadows. The value of it, even for this one purpose, does not seem to be sufficiently understood. Poor heaths have been converted into luxuriant meadows by means of irrigation alone. But in the countries in which irrigation is most extensively practised the production of all crops depends on it.
The irrigation of land with the sewage water of towns is, under another name, the application of liquid manure. In no small degree the water of rivers and of springs depends on its organic and mineral constituents for its fertilising properties, so that the application of it is not in principle different from that of liquid manure; but it must be borne in mind that the mere abundance of water itself is of great importance for many of the most valuable plants, as the most nutritious substances brought into contact with their roots are of no use to them unless in a state of solution: whilst it is an additional recommendation of irrigation that the supply of water most favourable to the growth of many valuable plants is destructive of some which in many places naturally encumber the soil, as heath, broom, &c. The water which is used for irrigation should be free from mud and such impurities as mechanically clog the pores of leaves, or cover up the hearts of plants, and interfere with their growth. Irrigation is far from being so extensively practised in Great Britain as would seem desirable. There are few farms in the British Isles which would not give a handsome return for artificial watering in a dry year—i.e. if the water could be obtained and applied at a reasonable cost. In many instances the produce might be increased two, three, or even fourfold. The amount of moisture which farm crops require to ensure their full development is greater than would be readily conceived. At Rothamsted it was found by Lawes and Gilbert that an acre of wheat in five months and eighteen days evaporated through its leaves no less than 335 tons of water. Light porous soils benefit most from irrigation; sandy soils, with a little admixture of clay and marl, usually most of all. Except in tropical countries, stiff retentive clay would not as a rule be benefited by irrigation, and might be injured by it, at any rate for arable farming. Thorough drainage, natural or artificial, is a necessary accompaniment of successful irrigation—necessary so that the soil may not become 'water-logged,' but benefited by the water percolating through it. Soil wholly or partially uncovered by vegetation is liable to be robbed of nitrogen by the rain or irrigation water washing nitrates into the drains or down beyond the reach of the plants. This is avoided in grass land by the roots of the grasses engaging the nitrogen. Irrigation may benefit the land in various ways, most usually (1) by softening and disintegrating the soil in percolating through it; (2) by bringing additional plant food into it; (3) by facilitating the dissolving, preparing, and distribution of the plant food already in the soil; and (4) by the oxidation of any excess of organic matter in the soil, leading thereby to the production of useful carbonic acid and nitrogen compounds. The extent of water-meadows in England is stated to be not more than 100,000 acres. They are mostly confined to the west and south of England. Individual farms, irrigated with sewage water, are to be met with in many parts of England, but the most successful instance of sewage irrigation in Great Britain is to be found near Edinburgh, where an extensive tract of meadows lying between Portobello and Leith yields a rent of £15 to £35 an acre; the grass is cut from three to five times a year, and over ten tons an acre have been obtained at a cutting. See SEWAGE, MANURE.
The methods most generally pursued are what are known as bed-work irrigation, catch-work irrigation, and subterranean irrigation. The first method can be conveniently applied only to ground which is nearly level, and may cost from £20 to £40 per acre. The catch-work method is very much less costly, and can be applied to land whether it is level or not. By the last system the soil is saturated with water from below.
In some parts of the United States irrigation is of vital importance; in 1890 the total area of the arid lands of the west was 1,331,151 sq. m. In the east the principal use of irrigation is in the rice-fields of South Carolina and Georgia; but such western states as Colorado and Utah are altogether dependent on it, owing to the scarcity of the rainfall. This is true also, to a great extent, of southern California. In all these arid districts hundreds of miles of canals and ditches have been constructed in addition to the mining flumes utilised for irrigation purposes; and as a result wide tracts of desert have been turned into a productive farming country. More recently irrigation has been introduced in western Kansas, largely by canals from the Arkansas River; although here, as in eastern Colorado and California, a great part of the water-supply is obtained from artesian wells. In Arizona, also, it is expected that wide tracts now uninhabitable will be rescued with the aid of irrigation.
In Australia irrigation on an American scale, and according to American methods, has transformed hundreds of thousands of acres, once covered with scrub, into luxuriant vineyards, orchards, and orangeries, especially in the lower basin of the Murray. The importance of irrigation to the Cape is noticed in the article on that colony; and the irrigation works of India and Ceylon are referred to at INDIA, CEYLON. Egypt (q.v.) is the land most entirely dependent on systematic and careful irrigation.