n6 SOIL CONDITIONS AND PLANT GROWTH 



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. 74) ; 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 not obtain any 

 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 normally 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. 



Bacterio-toxins. Several observers, including Greig-Smith (113)* 

 Bottomley (42) and others, have claimed to find soluble bacterio- 

 toxins in soils. Russell and Hutchinson, on the other hand, obtained 

 wholly negative results, and concluded that soluble bacterio-toxins are 

 not normal constituents of soils, but must represent unusual conditions 

 wherever they occur. But the possibility of the existence of toxins in- 

 soluble in water still remains. 



