72 CORN CROPS 



seventy-five years or more. Erosion also has played an 

 important part in depleting the rolling lands. The 

 supply of organic matter is generally low, and in many 

 cases the lands need underdrainage. Throughout the 

 " Corn Belt " there are also considerable areas in this 

 class. 



(b) Soils naturally not very productive, through lack 

 of one or more mineral elements or of drainage. 



In general, legumes and manure must be the principal 

 means of increasing and maintaining the productivity 

 of this land ; but when a mineral element is lacking, as 

 lime, potassium, or phosphorus, it will usually be neces- 

 sary to add this in the form of commercial fertilizer. 



4. Land producing less than 20 bushels per acre. 



(a) Through the Eastern and Southern States are large 

 areas which, through continuous cropping and erosion, 

 are low in yield. In addition to the prevention of ero- 

 sion, the same general treatment as is recommended for 

 the previous class may be used. 



(6) Land in regions of deficient rainfall. Where there 

 is less than eight inches during the growing season, lack 

 of moisture becomes a limiting factor in corn production. 

 From Dakota to Texas there is a large area with a fertile 

 soil but an annual rainfall of only 18 to 25 inches. In 

 these soils conservation of moisture is the most important 

 phase of soil treatment. 



SUMMARY 



55. The ability of corn to yield is indicated by certain 

 maximum yields, when 150 to 200 bushels per acre have 

 be«n harvested. Regarding climatic factors, there is 

 usually enough sunshine and, in most of the Corn Belt, 

 a sufficient total rainfall ; but the latter is not often dis- 



