Crops. 203 



tion of rainfall is the most important factor affecting the edible 

 quality of green sweet corn, and that the favorable effects of 

 moderate, well distributed rain-fall indicate that the northern 

 states will continue to produce the best crop outside the irrigated 

 districts. But no special area for sweet-corn growing can be 

 mapped as has been done in the case of the sugar beet. 



Crop rotation should be rationally based upon the varying de- 

 mands of crops for plant food and the characteristic feeding 

 habits of individual species of plants. When the plant food of 

 the surface soil has been exhausted by such shallow rooted crops 

 as corn, grasses and turnips, they should be followed by deep 

 rooted crops, such as wheat, mangels, or alfalfa. Not only will 

 the latter crops obtain their supplies of food from the lower 

 layers of the soil, but they leave a portion of it at the surface 

 in roots and stubble, from which it becomes available to succeed- 

 ing crops. No more striking example of this fact is furnished 

 than that of alfalfa. According to Headden, the roots and stub- 

 ble of alfalfa to a depth of 6% inches contain approximately 

 2.86 tons of dry matter per acre, having the following constit- 

 uents : total ash 172 pounds ; phosphoric acid 24 pounds ; sulphur 

 trioxide 9 pounds ; lime 50.5 pounds ; chlorine 6.5 pounds ; mag- 

 nesia 35.15 pounds; potash 44.5 pounds; and 104 pounds of ni- 

 trogen. Reference to the table in the Appendix which gives 

 "Plant food removed by crops," shows that the stubble of al- 

 falfa alone, places in the surface soil as much plant food as is 

 removed by total cereal crops. 



The waste tops of the mangel crop can also restore to the 

 soil as much food as is required by an average grain crop. Not 

 only will these crops restore fertility to the surface soil, but their 

 deep root systems, and the deep thorough tillage demanded by 

 them, will benefit the physical condition of the soil when they 

 are grown in rotations. 



Increase of soil nitrogen is the most valuable effect produced 

 by legume crops grown in systems of rotation. In this connec- 

 tion the work of Shutt in Saskatchewan, Canada, is of interest. 



