THE BIOLOGICAL CONDITIONS IN THE SOIL 113 



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 (178), by dilution, shaking 

 with calcium carbonate, precipitated ferric or aluminium oxide, animal 

 charcoal, or soil ; results which are explained by supposing that these 

 agents precipitate a toxin. Addition of fertilisers, and especially of an 

 aqueous extract of farmyard manure, improved the solution; these 

 substances also were supposed to precipitate the toxin. 



A double set of experiments was therefore began by Schreiner and 

 his colleagues : a careful search was made in the soil for such organic 

 compounds as could be identified (see p. 72) ; and the effect of these 

 and similar compounds on plant growth was studied by elaborate water 

 cultures. Considerable attention has been devoted to dihydroxy- 

 stearic acid. This substance is toxic to plants in water culture, and is 

 almost invariably present in infertile soils, especially such as are badly 



