HOFFMANN: SOIL PRODUCTIVITY. 83 



rado accumulations of nitrates have occurred which have re- 

 sulted in a complete cessation of plant growth. The presence 

 of various organic compounds has also been demonstrated to 

 exert a harmful, if not a toxic action, at least when these sub- 

 stances are dissolved in the soil extracts. Schreiner 5 thus at- 

 tributes to di-hydroxystearic acid a decided toxic property. He 

 states that 



"It would seem that di-hydroxystearic acid is either a direct 

 or an indirect factor in the low productivity of soils." 



Several other organic compounds have since been separated 

 from the soil by him, which in liquid cultures exert a pro- 

 nounced retarding influence upon wheat seedlings grown 

 therein. 



The source of these compounds has been variously at- 

 tributed. Reference has already been made to the root ex- 

 cretions as the responsible agent for their origin. Beyond 

 the evolution of C0 2 however, no evidence has been secured 

 to indicate such excretions from plant roots. Czapek, for ex- 

 ample, was unable even by the most sensitive and delicate 

 methods, to demonstrate the presence of any organic acids in 

 plant-root excretions. That these organic substances are 

 formed as a result of the decomposition of the plant residues 

 in the soil, due to the agency of soil bacteria, is an explanation 

 of the problem that is relatively new but one which, because 

 of its plausibility, is rapidly gaining support. Among the 

 first to suggest this possibility was Dachnowski 6 in connection 

 with his work on the growth of seedlings in peptone solutions 

 inoculated with various soil organisms. 



It is reasonable to assume that continuous cropping will de- 

 posit large quantities of plant tissue of a specific character in 

 the soil. Further, that these tissues will undergo decomposi- 

 tion with the formation of by-products characteristic for the 

 crop grown. Decomposition of wheat refuse undoubtedly will 

 yield different by-products than clover or corn-stubble. Thus 

 the continued one-cropping would tend to cause an accumula- 

 tion of by-products which may prove toxic to the particular 

 crop grown, whereas they may not affect other crops. By 

 changing the rotation these would be destroyed so that when 

 the original crop is again grown no toxicity would exist. An- 



5. Schreiner, Bulletin 80, Bureau of Soils, U. S. Dept. Agriculture. 



6. Dachnowski, Contributions 53, Botanical Laboratory, Ohio State University. 



2 — Univ. Sci. Bull., Vol. IX, No. 7. 



