158 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



ter is made up of so-called humus, which will be discussed later. 

 There is still much room for investigation in distinguishing var- 

 ious chemical compounds in the soil. Some of the organic com- 

 pounds mentioned above have been found in only one or two soils, 

 while others are perhaps of general occurrence. Most of them 

 appear to be indifferent toward plants, and their significance to- 

 wards plant growth or soil chemistry is not known. The import- 

 ance of these substances is at present chiefly due to their relation 

 to the toxic theory of soil fertility. 



The Toxic Theory. According to the toxic theory of the 

 Bureau of Soils, "the production of toxic excretions by the 

 roots of plants is undoubtedly a factor in soil fertility." 1 "So- 

 called exhausted soils are poisoned soils." 2 "Practically all soils 

 contain sufficient plant food for good crop yields and this sup- 

 ply will be indefinitely maintained." 3 "The small yields of unpro- 

 ductive soils can be greatly improved by treatments which destroy 

 toxic substances in these soils." 4 "The soil is the one indestruct- 

 ible immutable asset that the nation possesses. It is the one re- 

 source that cannot be exhausted, that cannot be used up." 5 



The evidence offered in support of this theory is based 

 chiefly upon experiments in water cultures and wire-basket tests 

 of about three weeks duration. The aqueous extract from an 

 unproductive soil grew larger wheat plants when ferric hydrate, 

 calcium carbonate, or carbon black were added to it. 6 Pure water 

 is better suited for growing wheat plants than is the soil extract 

 of poor soils. 7 The extracts of the poor soils are benefited by 

 nitrate of soda, carbon black, pyrogallic acid and tannic acid, 

 and the same chemicals have similar effects when added to the 

 soil. 8 Dihydroxystearic acid and picoline carboxylic acid isolated 



1 Bulletin 40, p. 40. 

 '* Bulletin 28, p. 28. 



3 Bulletin 22, p. 63 4. 



4 Bulletin 47, p. 51. 



5 Bulletin 55, p. 66. 



6 Bulletin 28. 

 ' Bulletin 36. 

 8 Bulletin 36. 



