Fallows in Dry Countries 269 



vapor and carried away by the air currents. Further- 

 more, the undisturbed soil becomes covered with a 

 growth of weeds which may pump out of the ground 

 very considerable quantities of water. Adding to this 

 the great absorptive power of fallowed land that reduces 

 the losses of water by surface-drainage and percolation, 

 it is easy to account for the observed differences. We 

 become better able to appreciate the true significance of 

 bare fallows in the conservation of moisture. 



It appears, then, that in countries of slight rainfall, 

 where moisture is the controlling factor in crop-pro- 

 duction, bare fallows are not only necessary but fre- 

 quently indispensable. Every effort is made to husband 

 the limited store of water by frequent stirring of the 

 surface soil and by keeping it free of vegetation for an 

 entire season or a portion of it. Provision is made in 

 this manner not only for supplying the succeeding crop 

 with sufficient moisture for its growth but also for per- 

 mitting the soil bacteria to accomplish this work in the 

 production of plant-food. The growing of fallow crops 

 is inadvisable under such conditions, for they would 

 deplete the store of soil-moisture and leave the soil 

 too dry for the succeeding crop. This is true of hoed 

 crops, as well as of green-manuring crops. Altogether, 

 the area of cultivated soils where the soil-moisture must 

 be conserved by alternate years of fallowing, is compara- 

 tively small. 



Aeration and fallows. — From the standpoint of 

 aeration, again, bare fallows are hardly indispensable 

 even on refractory clay soils. It was formerly thought 

 that the proper tilth for such soils could be secured only 



