510 THE PRINCIPLES OF SOIL MANAGEMENT 



soils, have reached the conclusion that alfalfa plants 

 excrete a toxic substance which, gradually accumu- 

 lating in the soil, injuriously affects the growth of 

 alfalfa plants. Whitney, Livingston, Schreiner and 

 their associates conclude that certain soils contain toxic 

 substances of organic nature which may be produced 

 by plant roots, or possibly by certain processes of de- 

 composition of organic matter. They have isolated 

 from soils organic compounds that are poisonous to 

 plants. 



It is found, for instance, that cumarin, which is a 

 normal constituent of sweet clover (Medicago alba, L 

 and M., officinalis, P), may be obtained from certain 

 soils, and that it is toxic to wheat seedlings, from 

 which it may be supposed that it is more or less toxic 

 to other plants. Dihydroxystearic acid was isolated 

 from certain soils by Schreiner and Shorey, who found 

 that it is acid to litmus and decomposes BaC0 3 and 

 CaC0 3 , forming the corresponding salts. The extracts 

 of the soil containing this substance were toxic to wheat 

 seedlings. The relation of soil acidity and soil toxicity 

 is thus suggested. 



Working with different media in which wheat and 

 other seedlings were grown, it was shown that, where 

 the nutrient solutions were very dilute, so as not to 

 enable the plant to overcome the effects of small quan- 

 tities of toxic matter, the wheat plants grew much 

 better when following other plants; and that, in spite 

 of a renewal of the supply of nutrients, the wheat plants 

 grew less well when one crop succeeded another. The 

 cause of the lessened growth was attributed to the 



