82 THE SOIL SOLUTION 



results which has yet suggested itself is that the toxic organic 

 substances present were less effective on dilution until the con- 

 centration reached a point where they actually became stimula- 

 tive, as is common with toxins of every character. 



Another set of experiments confirmed the conclusion that the 

 poor solution contained some organic substance inhibitory to 

 plant growth. A number of water cultures was prepared from 

 the aqueous extract of the poor soil, and lime in various forms 

 was added to the cultures. To two of the cultures lime car- 

 bonate and lime sulphate respectively were added in excess, so 

 that there was in each case a powdered solid at the bottom of 

 the containing vessel. At the end of two days the wheat seed- 

 lings which were growing in the vessels containing the pow- 

 dered solids had decidedly outstripped those growing in all the 

 others, the tops having the appearance of unusually good and 

 healthy plants. The roots were of a very remarkable character, 

 being exceptionally long, very turgid, clear, clean and translu- 

 cent. 



At once, new experiments were carried out in which there 

 were added to the poor solution, precipitated ferric hydroxide 

 freed from all adhering salts, precipitated alumina, shredded 

 filter-paper, absorbent cotton, or carbon black. In every case the 

 same result was obtained as before, a much improved growth 

 of top and a vastly better root development. Since, by no 

 possibility could these various added substances have increased 

 the concentration with respect to mineral nutrients, another ex- 

 planation must be sought. Aside from their insolubility, the 

 one properly common to these various substances was the large 

 amount of surface they brought into contact with the solution. 

 The one obvious explanation of their effects on the growth of 

 the wheat seedling, therefore, is that they withdrew or ab- 

 sorbed from the solution some substance or substances deleteri- 

 ous to plant growth. As diluting with respect to mineral nutri- 

 ents could not possibly be expected to improve the cultural value 

 of the solution, the conclusion seems evident that the effect pro- 

 duced by these various absorbents was due to more or less com- 

 plete removal from the solution of organic substances inhibitory 



