In what manner does water operate in fertilizing barren soils ? As an element, it furnishes direct supplies of the food which is partly required for plants ; as a medium for moderating the temperature of the climate, it lessens the rigour of cold in some regions, and of heat in others ; as a destroyer of some noxious weeds, it extirpates, when un der proper direction, many plants which are hurtful to agri culture, such as broom, heath, and moss; and it may be used in order to convey over the soil fertilizing and enriching sub stances. In each of these modes of operating there occur various features, and these must be well considered, in or der to secure most of those beneficial effects which water is qualified to promote by irrigation.
It follows of course, that great attention ought to be given to the qualities of water intended for this purpose ; and that, before incurring labour and expence in the application of it, these qualities ought first to be known. This may be done partly by analysis ; but most certainly by experiment con ducted on a moderate small scale. In general, spring waters are fertilizing, and possess also an equable temperature ; but impregnations of iron are frequent and hurtlul even in springs : petrify ing springs, which hold lime in solution, are commonly fertilizing : mossy waters are generally pour; and such as convey enriching sediment, promise the most effec tual and permanent improvement. These last ought not, however, to be confined in basins,otherwise they will deposit their sediment there, instead of conveying it directly over the lands ; but in particular cases, when these basins are to be occasionally cleared out, this caution may not be applica ble.
The natural produce for which irrigation is best adapted, in particular soils and climates, conies next under consider ation. Here the mere operator is not always a competent judge, and it may require the combined lights of history and tradition, observation, and experience. to ascertain the fact ; but there is no doubt that it may be ascertained with a consi derable degree of assurance before expensive efforts are made.
The qualities of the water, and the natural produce which it nourishes most effectually, being discovered in the first place, it belongs to the farmer to consider the population and markets of the district, his own demands, and the nature and extent of the irrigated crops by which he may expect to derive most profit. A prudent man will be determined in a considerable degree by these considerations, before lie embark deeply in the undertaking.
The water meadows of England present a fine view of ir rigation for grass, a description of which belongs rather to other portions of this work, and will partly be found under the article AGRICULTCRE, and in those works to which re ference is there made. A similar account of French irriga
tion will be found under the article FRAS'CE of the present work ; and that of Lombardy and other nations will fall to be introduced under other heads.
In Scotland there have been many successful essays in irrigation on a lesser scale ; but the greatest attempt was made by the late Duke of Buccleuch on his pastoral estate on the rivers Esk, Ewes, Tiviot, Etterick, and Yarrow. Neither the water nor the climate in general, were favour able to this liberal effort on the great scale on which it was made ; and the wants and population of the district hardly warranted that scale ; besides, it was conducted mostly in that mode which requires most water and most expellee ; and it was necessary for this purpose to sacrifice too much of those level soils which are most valuable for other purpo ses in that alpine part of Scotland. The art of irrigations is best promoted when success as well as failure are properly recorded ; and the failure of this great attempt is particular ly accounted for in the Farmer's Magazine, 1315. page 42, &c. But many good specimens of irrigation occurred under this attempt, some of which are still preserved ; and many other successful efforts have been made by watering for im quartet s throughout S( tit! ,nd, proving the meadows in all tin- climate and richer waters of England being, however, generally prefer able.
Thei e, in favourable circumstances, it is to ob tain by irrigation three successive crops, viz. two of pasture grass, and one of grass cut for hay or soiling, all in on: year. "Phis fact leads us to consider the climate in connection with soil, and water, and other circumstances ; and in applying; this art to the agriculture of many foreign parts, a similar advantage is obtained by nourishing various crops in tile same year.
The resources which may yet be opened to many nations by means of irrigation, fully understood and practised, ap pear to exceed all present calculations ; and what is already kn of this art is Amply sufficient to attract towards it the no•ice of the legislator as well as that of the husbandman. Water may be natele to furnish a rich and extensive supply of manute to the soil, merely by giving it a proper direction, which t%ill have the further good effects of preventing that waste and ruin which frequently accompany this element when it is left without any direction. Even the drainage of wet soils may thus be rendered valuable for the improve ment of other lands, and what naturally tends to do harm may be turned to advantage.