THE BIOLOGICAL CONDITIONS IN THE SOIL 115 



creased numbers of bacteria must result in an increased food supply for 

 the plant, and it was claimed that the new type of flora was actually 

 better than the old, in that denitrifying organisms were killed, nitrogen- 

 fixing organisms increased, and nitrification only suspended during a 

 period when nitrates were not wanted and might undergo loss by 

 drainage. In a later publication Hiltner (137) shows that other 

 volatile or easily decomposable antiseptics produce the same effect. 

 The important part of this work is unquestionably the discovery that 

 the organisms in the treated soils ultimately outnumber those in the 

 original soil. The hypothesis that the new type of flora is actually 

 more efficient than the old rests on less trustworthy evidence, and has 

 indeed been modified in some of its details by Hiltner himself. 



The effect of heat on the productiveness of the soil was first noticed 

 by the early bacteriologists. It had been assumed that heat simply 

 sterilised the soil and produced no other change, until Frank (99) in 

 1888 showed that it increased the soluble mineral and organic matter 

 and also the productiveness. Later work by Pfeffer and Franke (226) 

 and by Kriiger and Schneidewind (156) showed that plants actually 

 take more food from a heated than from an unheated soil. Heat un- 

 doubtedly causes decomposition of some of the soil constituents quite 

 apart from its effect on the soil flora. 



Experiments by Russell and Darbishire (239) showed that the rate 

 of oxidation was considerably reduced after the soil had been heated 

 to 1 30 C, but was increased by treatment with small quantities of 

 volatile antiseptics, and more than doubled after heating to 100 C. 

 The bacterial activity is therefore increased and consequently the 

 amount of decomposition. The increased quantity of plant food thus 

 formed is shown by the amounts taken up by the plant. Table 

 XLVII. contains a typical series of results : 



TABLE XLVII. WEIGHT AND COMPOSITION OF CROPS GROWN ON PARTIALLY 

 STERILISED SOILS. RUSSELL AND DARBISHIRE. 



