QUESTIONS 



269 



than a good clover sod, as clover not only leaves a good supply of 

 nitrogen, but is also supposed to bring considerable from the sub- 

 soil by means of its deep roots. 



A short rotation sometimes used is wheat, clover, potatoes. A 

 longer rotation quite common in the East is oats, mixed clover, grass, 

 potatoes. A third rotation on hill lands too acid to grow clover is 

 oats, grass, grass, grass, potatoes, the grass not cut the third year but 

 plowed under to furnish organic matter. 



FIG. 106. Intensive potato culture on a Long Island farm, under the "Skinner system" 



of irrigation. 



Applying Fertilizer. When fertilizer is applied the plant- 

 ing is usually done by machinery. In this case the fertilizer is dis- 

 tributed in the row, either above or below the seed, but not in direct 

 contact. However, it is not considered best to apply more than 

 1000 pounds per acre in the row, but if more than this is used, to 

 spread a part broadcast, either just before or just after planting. 



QUESTIONS 



1. Give the relative rank of potatoes and other crops in the world. 



United States. 



2. Give the relative production of potatoes in the continents. 



3. Compare potatoes and wheat as food crops. 



4. Give the principal potato growing States, and yield per acre. 



In the 



