THE SOIL AND THE PLANT 141 



of organic compounds likely to occur in the soil was studied 

 by elaborate water cultures ; and a careful search was made 

 in the soil itself for such organic compounds as could be 

 identified. 



The water culture experiments showed that numerous sub- 

 stances, particularly neurine and guanidine, were toxic to plants. 

 In general, oxidation reduced toxicity: choline was less and 

 betain still less toxic than neurine. Tyrosin, again, was harmful 

 but the black oxidation product it affords was actually beneficial. 

 The toxicity of neurine and guanidine and presumably of the 

 other substances also was reduced by addition of charcoal, 

 pyrogallol, calcium carbonate and sodium nitrate. Further, 

 when two crops were grown successively in the same solution 

 the second did better than the first, an effect that may be 

 connected with an oxidising power possessed by plant roots 

 [16]. The combined effect of plant roots and of fertiliser was 

 very potent in overcoming the effects of a toxic body. 



The search for organic compounds in the soil is complicated 

 by the presence of large amounts of sand, silt, clay and highly 

 complex organic substances. In consequence the problem has 

 not attracted workers ; the first serious investigation was made 

 by Schreiner and Shorey [19], who obtained a number of 

 compounds, including two hydroxystearic acids, a piccoline 

 carboxylic acid and several products of protein and nucleoprotein 

 hydrolysis. The harmful effects on plant growth of one of these 

 — dihydroxystearic acid — was investigated at some length [21]. 



Several attempts have been made to discover how these 

 toxic organic substances arise in the soil. Whilst some are 

 supposed to be normal decomposition products of the complex 

 organic matter of the soil, others are considered to be excre- 

 tions from plants. The old hypothesis of de Candolle is revived 

 and these excretions are supposed to be harmful to other 

 plants of the same kind but not necessarily to plants of a 

 different kind. An explanation is thus afforded of what is said 

 to be a common phenomenon : the sterility finally induced 

 when one and the same crop is grown continuously on the 

 same ground. Water culture experiments are quoted to show 

 that wheat seedlings exude from their roots something that 

 makes the solution toxic to a subsequent batch of seedlings. 



Lastly, the part played by fertilisers has been studied. The 

 beneficial effects of fertilisers are not in dispute but it is con- 



