■248 SOIL CONDITIONS AND PLANT GROWTH 



they were also both neutral. Yet they produced very dif- 

 ferent effects on the wheat seedlings : the "good soil" extract 

 caused a larger and healthier development of root and a some- 

 what 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 

 with calcium carbonate, precipitated ferric or aluminium oxide, 

 animal charcoal, or soil ; results which are explained by sup- 

 posing 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 begun by 

 Schreiner and his colleagues ; a careful search was made in 

 the soil for such organic compounds as could be identified 

 (see p. 141); 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 drained, badly aerated, too compact, and deficient 

 in lime (251); soils, in short, that in England are called 

 *' sour". 



On the other hand, Russell and Petherbridge could obtain 

 no aqueous extract toxic to plants from greenhouse " sick " 

 soils. These soils, however, are rich in organic matter, in 

 plant food, and in calcium carbonate. 



The present position may briefly be summed up as follows : 

 There is no evidence of the presence of soluble toxins in nor- 

 mally aerated soils sufficiently supplied with plant food and 

 with calcium carbonate, but toxins may occur on "sour" 

 soils badly aerated and lacking in calcium carbonate, or on 

 other exhausted soils. There is no evidence of any plant 

 excretions conferring toxic properties on the soil, but the 

 Woburn fruit-tree results show that a growing plant may 

 poison its neighbour. The effect does not appear to be speci- 



