PERMANENT SOIL FERTILITY 



action in increasing the yields of the cereal crops in the rota- 

 tion. In addition to overcoming the acidity of the soil, lime- 

 stone provides the necessary calcium and magnesium as plant 

 foods. 



Various forms and kinds of limestone materials are avail- 

 able, but the work of Doctor Hopkins has clearly demonstrated 

 that the most economic form to use is the finely ground nat- 

 ural limestone the natural material occurring in the soil. The 

 abundance of data obtained by him from various experimental 

 fields, particularly in southern Illinois, provides the best infor- 

 mation the world now affords regarding the great benefit from 

 the use of limestone for the production of common farm crops. 



In most normal soils, such as the brown silt loam of the 

 corn belt, potassium occurs in such large quantities that it will 

 be sufficient for the maximum production of crops for indefi- 

 nite periods of time. Hence in the case of potassium Doctor 

 Hopkins has pointed out that the problem of the farmer is not 

 one of addition, but rather of the liberation of this element 

 from the insoluble compounds contained in the soil. In both 

 the live-stock and the grain systems of farming, his provision 

 for the turning under of freshly decomposing organic matter 

 insures, in normal soils, a sufficient amount of available potas- 

 sium to meet the requirements of crops for this element. The 

 results obtained from various experimental fields have shown 

 clearly that when used on such soils potassium not only does 

 not pay for itself but gives little actual increased yield. 



Certain abnormal types of soil, however, are deficient in 

 potassium. Such soils are peaty soils and soils deficient in 

 organic matter. On peaty soils, potassium is the limiting ele- 

 ment of plant food and is often the limiting factor of crop 

 production. The addition of potassium, therefore, on such 

 soils is an absolute necessity. On soils deficient in organic 

 matter, such as many of those occurring in southern Illinois, 

 potassium may be used with profitable results until the soil 

 has been built up in its organic-matter content. 



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