646 proceedings of section k. 



Toxic Substances in Soils. 



That substances are formed in the soil either as the result of the 

 decomposition (chemically or by means of micro-organisms) of 

 crop-residues, or excreted by the growing plant seems to be abun- 

 dantly proved. (Schreiner and Shorey, Bull. 74, U.S. Bureau of 

 Soils.) O. Schreiner was the first to show the toxic effect of dihyd- 

 roxystearic acid, and to isolate this substance from soils on which 

 wheat failed to grow. (Bull. 53, U.S. Bureau of Soils.) Further 

 experiments by the United States Bureau of Soils (Schreiner and 

 Reed, Bull. 47, Bureau of Soils) have shown that quite a large 

 number of organic substances exercise a toxic action on plant 

 growth. 



F. Fletcher {Journal Agricultural Science IV., p. 245) 

 describes experiments showing the extraordinary influence of the 

 neighbourhood of sorghum and of maize upon the growth of 

 " Sesamum indicum." This is not due to the removal of moisture 

 or of plant-food by the maize crop, as both these essentials were 

 abundantly supplied to the sesamum, and must, he concludes, be 

 attributed to the excretion of a toxin by the roots of the maize 

 plants. Fletcher believes this to be a salt of dihydroxystearic 

 acid. 



Among the numerous toxic organic compounds which Schreiner 

 and his fellow-workers have found to be present in the soil, three 

 or four have been more particularly studied in relation to their 

 action upon plants provided with varying quantities of the recog- 

 nised fertilizing ingredients. 



Schreiner and Skinner (Bull. 70, U.S. Bureau of Soils) have 

 shown that in water cultures with wheat, dihydroxystearic acid is 

 least harmful when the p'lant is provided with fertilizing substances 

 relatively rich in nitrogen (such as nitrates), and that in the pre- 

 sence of this soil-toxin the plant removed less phosphoric acid and 

 potash than under normal conditions, but that its absorption of 

 nitrogen was more nearly normal. 



The action of other soil-toxins was made the subject of further 

 study (Schreiner and Skinner, Bull 77, U.S. Bureau of Soils), and 

 the following very interesting and rather remarkable results were 

 obtained. Vanillin (an aldehyde) behaves in very much the same 

 way as dihydroxystearic acid in its general effect upon roots and 

 leaves, and its effects are least when the plant is supplied with 

 nitrate. It is pointed out that nitrates increase root-oxidation, 

 whereas both dihydroxystearic acid and vanillin, being capable of 

 further oxidation, are themselves reducing agents. 



Quinone is another organic substance whose presence affects the 

 growth of plants. Unlike the two substances mentioned above, 



