ROTATION IN DRY AREAS 391 



ring of the same, that the water that falls may enter it 

 readily, even though it should fall in large volume. 

 Grass crops are the least favorable to the entrance of 

 water into the soil from above. Alfalfa is in some de- 

 gree an exception, because of the discing that is given 

 to the crop in the springtime. 



The character of the rotation exerts a very marked 

 influence on the retention of moisture in the soil that is 

 already present in the same. When the small cereals 

 follow each other in close succession they not only draw 

 proportionately on the soil moisture to perfect their 

 growth, but there is no means of preventing the sub- 

 soil moisture from escaping into the air subsequent 

 to the cessation of the use of the harrow on the crop. 

 The more frequently that a fallow or a cultivated crop, 

 therefore, can be introduced into the rotation, the more 

 perfectly will the moisture be conserved in the soil. Be- 

 cause of the increase in the power in the soil to hold 

 moisture as a result of putting humus into it, the intro- 

 duction of humus occasionally in some form should never 

 be lost sight of. The ability of the soil, therefore, to 

 hold moisture in these areas will be proportionate: (1) 

 to the infrequency with which the cereal crops are 

 grown ; (2) to the frequency with which cultivated crops 

 are grown, and (3) to the increase in the humus content 

 in the soil. 



These agencies will also have an influence on the 

 increase or decrease of the moisture content in the soil. 

 The summer-fallow is probably the most potent agency 

 that can be introduced in the line of cultivation to in- 

 crease the moisture content of the soil. The increase 

 which it may thus exert may be in itself a sufficient rea- 

 son for introducing the summer-fallow. Next in potency 

 in this respect is probably deep plowing and subsoiling. 

 The alfalfa crop calls for such deep plowing when pre- 

 paring the seed bed for it, and when the ground is broken 



