THE BIOLOGICAL CONDITIONS IN THE SOIL 247 



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 dis- 

 covered by laboratory tests either in soil that had been re- 

 moved 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 ^r^ze/m^ 

 alongside of ity and the part such effects may play in determin- 

 ing 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, 65c, 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 in- 

 fertility of any soil cannot be due to lack of food. But in 

 certain cases this infertility is transmitted to the aqueous ex- 

 tract 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 ; 



