THE BIOLOGICAL CONDITIONS IN THE SOIL 115 



from the grass went straight down to the tree roots. There seemed no 

 possibility of the grass roots in the trays abstracting anything from the 

 soil, and the only explanation appears to be that a toxin is excreted by 

 the grass. Such a toxin, however, must be very readily decomposed, 

 because no toxic properties could be discovered by laboratory tests 

 either in soil that had been removed from the grass roots or in the 

 washings from the above-mentioned trays. Pickering has since shown 

 that the phenomena are general and hold whatever crops are grown in 

 the pots and trays. In consequence we must be prepared to consider 

 possible toxic effects of one plant on another growing alongside of it, 

 and the part such effects may play in determining natural plant 

 associations and in explaining some of the bad effects of weeds. 



Pickering's results agree with the hypothesis already put forward 

 by Whitney, and developed by him and Cameron, Schreiner (306, 

 68, 249, etc.) and their colleagues at the Bureau of Soils, Washington. 

 Certain soils are supposed to contain toxins, which are not necessarily 

 plant excretions, but may arise by decomposition of organic matter in 

 the soil. The genesis of this hypothesis is interesting. Reference has 

 already been made to Whitney's view that the soil solution furnishes 

 the food of plants and is of the same composition and concentration in 

 all soils, from which it follows that infertility of any soil cannot be due 

 to lack of food. But in certain cases this infertility is transmitted to the 

 aqueous extract of the soil, and must, therefore, arise from some soluble 

 toxin. As an example Whitney and Cameron (305) selected two 

 Cecil clays of very different productiveness but of identical chemical 

 and physical constitution, prepared aqueous extracts and used them 

 as culture solutions for wheat seedlings. The extracts contained in 

 parts per million : 



and were thus identical in their content of plant nutrients ; they were 

 also both neutral. Yet they produced very different effects on the 

 wheat seedlings : the " good soil " extract caused a larger and healthier 

 development of root and a somewhat better development of leaves. 

 In other cases it has been found that growth in extracts of poor soils 

 is even worse than in distilled water. The productiveness of the extract 

 could be raised, according to Livingstone (179), by dilution, shaking 



