IRRIGA'TION (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 wits 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 practiced neat ly 2,000 years me., and where great systems of canals 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, anti some other parts of the east; and in such of-these countries as have not entirely lost their ancient prosperity, such works still exist. In many parts of the world the necessity of irrigation, at least at certain seasons of the year, is so strongly felt that the .agriculture even of compara tively rude tribes depends on the facility with which it can be accomplished. borne 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. Irrigation is supposed to have been introduced into Britain by the Romans, but was very little practiced till the beginning of the present century. In Europe, irrigation prevails chiefly in the s., where it was extensively practiced by the Romans, from whom it was adopted by the Lombards; and it is most extensively prat ticed in Lombardy, and in some parts of Spain, and in the s. of France, so that the great plains and valleys of the Po, Adige, Tagus, Douro, and other rivers, are almost entirely subjected to a systematic irrigation, which prodigiously increases their fertility. The extent of irrigated land in the valley of the Po is estimated at 1,600,000 acres; and the increase of rental thus caused at £830,000.
Irrigation in Britain, and in most parts of Europe, except Lombardy, is almost exclu sively employed for the purpose of increasing the produce of grass by converting the laud 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 exten sively practiced 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 fertilizing 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 thorn unless in a state of solution; whilst it is an additional recommendation of irrigation, that the supply of water most favorable to the growth of many valuable plants, is destructive of some which in many places naturally encumber the soil, as heath, broom, etc. 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 practiced in Great Britain as seems desirable. The extent of water-meadows in England is stated to be not more than 100,000 acres. They are mostly confined to the w. and s. of England. Individual farms, irrigated with sewage water, are to be met with in Nottingham shire, Staffordshire, Lancashire, Cheshire, and one or two counties in Wales. The most successful instance, however, 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 £20 to £40 an acre; the grass is cut from 3 to 0 times.a year, and as much as 10 tons an acre have been obtained at a cutting. See SEWAGE, MANURE.