32 IRRIGATION FARMING. 



is done by loosening the topsoil, not by stirring it to 

 such a depth as to injure the roots of the plants, but in 

 a manner so as to break the tops of the tubes and throw 

 a covering of loose soil over the ground, and at the 

 same time destroy the robber weeds which not only 

 use the moisture but take away plant food as well. 

 This loose soil is a mulch — a blanket which prevents 

 loss of moisture and protects against the direct rays of 

 the sun. There are, of course, certain kinds of cereal 

 crops, such as wheat and oats, which by ordinary 

 planting do not admit of cultivation, and these from 

 necessity naturally require a larger quantity of water 

 than do the cultivated or hoed crops. This subjec5t of 

 cultivation, as well as that pertaining to the fertilizing 

 elements of irrigating waters, will be treated in succeed- 

 ing chapters. 



Addition of Silt. — In most irrigated countries, 

 and especially in the Rocky Mountain region, the 

 principal irrigation of crops is done in the spring or 

 early summer. At this time the water is usually filled 

 with some sediment or silt, which is loosened through 

 erosion by the rapid melting of snow at the higher ele- 

 vations, and the rush of water to the various rivers 

 from which canals for irrigation are taken out. This 

 sediment is of especial value to land in most instances 

 if the water is properly or evenly applied. When water 

 is condudled to the surface by gentle flooding it runs 

 slowly and allows the sediment to settle on the land in 

 greater quantities and more evenly than by any other 

 means, thus giving greater produdliveness to the land. 

 This f a<5l is especially shown by the enormous and con- 

 tinued productivity of the soil on the banks of the Nile, 



