TOXIC EXUDATES AND SOIL TOXINS. 151 



Researches of the Bureau of Soils. — The most extensive series of in- 

 vestigations have been carried on by the Bureau of Soils of the United 

 States Department of Agriculture from 1905 to 1915. The pioneer 

 study of this series was by Livingston, Britton and Reid (1905), 

 who grew wheat seedlings in untreated Takoma soil and its aqueous 

 extract, as well as in these when modified by various substances. 

 Native and cultivated plants growing on Takoma soil exhibited 

 structures similar to those of a soil subject to drought, and wheat 

 seedlings were much stunted, even though the water-content was kept 

 constant. When grown in aqueous extracts of the soil they made the 

 same kind of growth as in the soil itself. The dwarfing effect of both 

 soil and extract was reduced by the use of stable manure, pyrogallol, 

 calcium carbonate, etc. It was regarded as very well established that 

 Takoma soil contains some substance or substances toxic to wheat 

 plants, and as also indicated that bodies are given off by the roots of 

 growing wheat plants deleterious to them or to other wheat plants 

 following them. It is suggested that the so-called exhausted soils 

 are really poisoned and that crop rotation is beneficial because it 

 prevents the accumulation in the soil of the injurious excreta of any 

 one form of plant life. In further studies, Livingston (1907) stated 

 that injurious substances similar to those existing in soils are pro- 

 duced by the growth of wheat in water or sand cultures. 



Schreiner and Reed (1907) maintained that the unfavorable con- 

 ditions brought about by root excreta may affect the succeedin g crop 

 if immediately planted. When wheat succeeds wheat the effect is 

 very marked, and it is also marked when wheat follows oats, but there 

 is little or no effect when it follows cowpeas or corn. They tested 

 the effect of more than 30 different organic soil constituents on wheat 

 seedlings, and found that the majority of them caused injury in con- 

 centrations ranging from 1 to 50 parts per milKon. The toxic solu- 

 tions were markedly improved by treatments similar to those that 

 benefit the extracts of unproductive soils. When succeeding crops 

 were grown in the same soil, certain fertilizers were found to act very 

 beneficially upon soils containing the toxic excreta of plant roots. 

 Methods of cultivation that promote the aeration of the soil and the 

 growth of micro-organisms may aid in destroying soil toxins, and their 

 undue accumulation may be prevented by proper crop rotation. 



Schreiner and Reed (1907^) have discussed the role of the toxic 

 excreta of roots, and suggested that they may be of importance in 

 plant succession, as well as in determining the composition of plant 

 communities, such as the characteristic "oak openings." The ex- 

 creta of growing roots are also regarded as one of the main causes of 

 the low yields obtained in improper crop rotations. While the pro- 

 duction of toxic excretions by the roots of plants is undoubtedly a 

 factor of importance in soil fertihty, they probably do not accumu- 

 late to a harmful extent in soils kept in good tilth. Proper aeration 



