ROTATION IN DRY AREAS 387 



wheat for a time, and by substituting for it certain 

 crops that call for cultivation. Variations in the crops 

 grown in rotations tend to variations in the time and 

 methods of the cultivative processes, and these in turn 

 have a most disturbing influence on insect life, both in 

 the embryo and subsequently. 



Rotation exercises a far reaching influence on the 

 reduction of the hazard incurred by the presence of 

 certain fungous diseases. In the northwestern states, 

 as shown by Bolley, the continued growth of wheat on 

 the same land for many years has led to the extensive 

 prevalence of a fungous disease which preys upon the 

 roots. The remedy proposed is an interchange in the 

 crops grown. When flax wilt reaches a certain soil, in 

 order to remove it flax should not be grown on the 

 soil for a term of years. Likewise when potato scab 

 is introduced into land, it can only be removed by re- 

 fraining from growing potatoes on the same for at least 

 a limited term of years. What is best, therefore, for 

 keeping fungous diseases at bay, is best also, as has 

 been shown, for maintaining a proper equilibrium in the 



The shifting of soils by water and winds is in some 

 instances very serious. The removal of soil by water is 

 facilitated by fineness in the soil particles, by the ab- 

 sence of humus in the soil, by shallowness in the cul- 

 tivated area, and by the violence and quantity of the 

 rainfall. The soils in the semi-arid belt, especially such 

 as are flocculated in character, shift the most readily 

 because of fineness, especially when visited by torrential 

 downpours. Next to these probably are loam soils light 

 in texture. Humus in the soil, especially in the form of 

 vegetation not yet decayed, as previously shown, binds 

 soils. The binding power of vegetation is most strikingly 

 illustrated in the presence of such grasses as Russian 

 brome (Bromus inermis). Shallowness in the cultiva- 



