MANURES. 165 



often supplies ammonia and other organic matt, 

 the soil. Its chief value, however, in most o 

 must depend on the amount of mineral matter which 

 it furnishes. 



The word "irrigation" means simply tl- <i of 

 tottering. In many districts water is in various 

 ways made to overflow the land, and is rem..vi-d ,,r 

 withheld when necessary tor the purposes of cultiva- 

 tion. All river and spring water contains some im- 

 purities, many of which are beiielicial to vegetation. 

 These are derived from the earth over, or through, 

 which the water has passed. Ammonia also i- 

 sorbed by the water from the atmosphere. When 

 water is made to cover the earth, especiallv if its 

 rapid motion be arrested, much of this fertilizing 

 matter settles, and is deposited on or absorbed by 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 the open texture 

 of the land, the water passes through the soil, its im- 

 purities are arrested, and become available in vege- 

 table growth. It is, of course, impossible to say 

 exactly what kind of mineral matter is supplied by 

 the water of irrigation, as that depends on the kind 

 of rock or soil from which the impurities are derive- 1 ; 

 but, whatever it may be, it is generally soluble and 

 ready for immediate use by plants, and is distributed 

 in the most uniform manner possible. 



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 



