84 



CROP PRODUCTION AND SOIL FERTILITY 



soy beans, beans, peas and other legumes may be due to no other 

 cause than the lack of proper nodule-forming organisms. In 

 hundreds of cases soils fertile for corn, potatoes, and grains, fail 

 to grow alfalfa because of both & lack of calcium (carbonate of 

 lime) and alfalfa nodule-forming organisms. 



On many long-cropped prairie soils deficient available phos- 

 phorus is a common cause of low fertility and poor crops. 



Soil Exhaustion. Some soils which were at one time productive 

 are now regarded as exhausted, depleted or "worn out/' because 



FIG. 34. Depleted lands (in background), typical of the Appalachian and Blue Ridge 

 . es. Wooded hillsides are cleared of timber, cultivated, and through erosion and exhaus- 

 tive cropping are rendered useless, and allowed to revert to timber. In foreground, new land 

 recently cleared is in tobacco. (Kentucky.) 



they no longer respond to cultivation as they formerly did (Fig. 34) . 

 An exhausted or depleted soil implies that something has been 

 used up or taken out of it. That " something" is commonly 

 understood to mean the fertilizing elements; viz., nitrogen, phos- 

 phorus and potassium. In some instances soil exhaustion may be 

 attributed largely to the removal, mainly through cropping and 

 leaching, of some one or all of the three named elements; not the 

 removal of the " total" amount, for this is impossible, but the 

 removal of the ' ' available " supply. If no phosphorus, for example, 

 were lost through leaching, it would require only about sixty years 



