62 IRRIGATION PRACTICE 



tion enabled the plant to produce dry matter at a smaller 

 water cost. 



Cultivation of the soil, therefore, prevents the waste 

 of water by evaporation, and induces soil changes that 

 enable the crops to produce larger yields with a given 

 quantity of water. In truth, cultivation may take the 

 place of irrigation. 



42. Rolling. Rolling is the opposite of cultivation. 

 It compacts the top soil. As a result, excellent capillary 

 connections are established between the top and the sub- 

 soil and water is enabled to move upward, rapidly, from 

 the lower layers to the surface, there to be evaporated 

 into the air. There is no more dangerous practice than 

 this, if evaporation of soil moisture is to be prevented. 

 Moreover, a soil which has been compacted by rolling 

 offers much resistance to the entrance and downward 

 movement of water. Rolling, therefore, (1) prevents the 

 water from entering the soil easily, and (2) allows the 

 water which does enter the soil to evaporate rapidly. 

 From the point of view of water-conservation it is an 

 extremely wasteful process. 



In a few special cases rolling may be permitted in a 

 good system of irrigation agriculture. For instance, in 

 raising sugar beets for factories, the soil is carefully rolled 

 after the planting of the seed, chiefly to insure good 

 germination. This, however, is not necessary except in 

 districts where the spring precipitation is light or where 

 the soils have been so handled as to be too dry for satis- 

 factory germination. By proper methods of fall plowing 

 this precaution would probably be unnecessary. 



A special phase of rolling may be of importance. 

 Campbell recommends highly a sub-surface packer, 

 designed to pack the soil at the bottom of the plow fur- 



