FIELD EXPERIMENTS IN THE SOUTH 



481 



TABLE 95. CROP YIELDS IN ILLINOIS SOIL EXPERIMENTS: Du Bois FIELD 



interior streams in glaciated areas. Under ordinary methods of 

 cultivation these lands are subject to serious loss from surface 

 washing, and even when not under cultivation there is and has 

 been more or less rapid erosion taking place. Where this soil has 

 been under ordinary cultivation for several years, it is almost 

 invariably poor in humus and nitrogen, and the dominant problem 

 is to maintain or increase the organic matter in the soil, which will 

 also increase the nitrogen. 



Of course the organic matter must in large part at least be grown 

 upon the land, and legume crops are most suitable for this purpose 

 because their growth is not limited by the small nitrogen content 

 of the soil, and they also furnish green manures or animal manures 

 rich in nitrogen. 



While these soils are not rich in phosphorus, that element is not 

 the chief limit to the yield of crops because the nitrogen limit is so 

 much lower as measured by crop requirements and by culture 

 experiments. Furthermore, by surface washing, the nitrogen, 

 which is contained only in the humus, is rapidly depleted, while 

 the phosphorus is constantly renewed because of the supplies in 

 the underlying materials. It is certain that for the highest crop 

 yields phosphorus must be applied, and very probably it can ulti- 

 mately be applied with profit in the best systems of soil improve- 



