114 SOIL CONDITIONS AND PLANT GROWTH 



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, artd 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. 



The Simplification of the Soil Population— Partial Sterilisation 



of Soil. 



The earliest observations £hat soil is altered by an apparently inert 

 antiseptic arose out of attempts to kill insect pests in the soil by means 

 of carbon disulphide. This substance, which for fifty years has been 

 known as an insecticide, was used in 1877 by Oberlin (219), an 

 Alsatian vine-grower, to kill phylloxera, and by Girard (106) in 1887 

 to clear a piece of sugar-beet ground badly infested with nematodes. 

 In both cases the subsequent crops showed that the productiveness of 

 the soil had been increased by the treatment. 



The first piece of scientific work came from A. Koch in 1899 (150), 

 who, working with varying quantities of carbon disulphide, concluded 

 that it stimulates the plant root to increased growth. Four years after 

 Hiltner and Stormer (136) showed that the bacterial flora of the soil 

 undergoes a change. The immediate eff'ect of the antiseptic was to 

 decrease by about 75 per cent, the number of organisms capable of 

 developing on gelatin plates; then as soon as the antiseptic had 

 evaporated, the numbers rose far higher than before, and there was 

 also some change in the type of flora. It was argued that the in- 



