IMPROYEMEKT OF SOILS. 37 



All waters are suitable for irrigation except those con- 

 taining mineral substances deleterious to vegetable life — 

 such as the drainage from peat swamps and mineral 

 springs, etc. Water from a running stream or river is 

 generally superior to that from wells or springs ; the 

 former not only containing many salts which it has dis- 

 solved out of the soil or rocks as it j^assed over them, but 

 being also more richly freighted with bodies extracted 

 partly from the air, such as oxygen and ammonia. Of 

 spring and river waters, those denominated *'hard" are 

 the best, and owe this quality to the presence of sulphate 

 and carbonate of lime and magnesia. 



Effects of Irrigation. — The first effect of water, when 

 made to flow over the soil, is to soften it and render 

 it more permeable to the roots of plants, and to the 

 air. Water acts still further in dissolving out the food, 

 and producing those chemical changes which must take 

 place in the manures before they are fitted for nourish- 

 ment. And owing both to this and to its conveyance of 

 fertile matters from a distance which are deposited on 

 the fields overflown, the mcrease from irrigation is some- 

 times four-fold, when the soil, the season, and the water 

 are all favorable ; and it is seldom less than doubled. 

 The quality of grass from irrigated meadows is but 

 slightly inferior to that grown upon dry soils ; and for 

 pasturage it is found that animals do better in dry seasons 

 upon the former, and in wet upon the latter. 



Modes of Irrigating,— These must depend on the con- 

 ^ figuration of the surface, and the supply of water. Some- 

 times reservoirs are made for accumulating water from 

 rain and inundations ; but the usual source of supply is 

 from streams or rivulets, or copious springs which dis- 

 charge their water at a*level above the ground to be irri- 

 gated. The former are dammed up, to turn the water 



