100 THE SOIL SOLUTION 



mentation, growing wheat, and other seedlings in water and agar 

 cultures. 1 It has been shown that wheat renders the culture 

 media unsuitable for subsequent wheat crops, though it can be 

 reclaimed or renovated by treatment with such absorbents as 

 carbon black, or by other methods. 2 Wheat did about as well 

 when grown in a medium which had previously supported a 

 growth of cowpeas as when planted in a fresh medium; poorer 

 results were obtained after oats; no crop produced such poor 

 results in the succeeding wheat crop as did wheat itself. 



It is yet a matter of dispute as to whether the substances thus 

 added to nutrient media are truly excretory products of the 

 plant, sloughed off or otherwise eliminated from the surface of 

 the roots, or further elaborated by bacterial or other agencies 

 before becoming effective. These are important problems for 

 the plant physiologist and the soil chemist alike. It is beyond 

 dispute, however, by reason of a large and increasing weight of 

 evidence, much of it .direct experiment, that, as a result of the 

 growing of plants, soils and the soil water do contain organic 

 substances; harmful to the plant or organism eliminating them; 

 harmful, innocuous, or even stimulating to other plants or 

 organisms. 



For the elimination from the soil of toxic or inhibitory organic 

 substances, whether excreted by roots or otherwise produced, 

 several methods are more or less effective. When, as is some- 

 times the case, the substance is volatile, it may be removed by 

 heating, distilling with steam, or passing a current of air through 

 the soil or cultural medium. These methods, while effective in 

 the laboratory and possibly applicable to green-house conditions, 

 are naturally inapplicable to field conditions. In this last case 

 the obvious procedure is to increase as much as possible the 

 absorptive powers of the soil; to secure the best possible drain- 

 age ; and with these, the best possible aeration of the soil. 



1 Some factors in soil fertility, by Oswald Schreiner and Howard S. 

 Reed, Bull. No. 40, Bureau of Soils, U. S. Dept. Agriculture, 1907. 



* Soil fatigue caused by organic compounds, by Oswald Schreiner and 

 M. X. Sullivan: Jour. Biol. Chem., 6, 39-50 (1909). 



