52 



R. GREIG-SMITH. 



when applied to the soil, acted in the same way. 1 These 

 ♦ must disappear from the soil before a crop is grown, and 

 this occurs, either by the evaporation of the volatile disin- 

 fectant, by fixation of the metallic oxides, or by the decom- 

 position of the benzene-ring compounds, which in the 

 presence of lime and soda, act as sources of carbon for the 

 soil bacteria. The volatile disinfectants, which contain no 

 sulphur, act in a manner similar to carbon disulphide, and 

 so the oxidation of the sulphur to sulphuric acid cannot be 

 the chief cause of the increase of crop. 



The action of mustard, either as a crop or as green 

 manuring, in preserving the soil nitrogen, for it cannot fix 

 nitrogen like the leguminous plants, is believed to be due 

 to the action of the mustard-oil in the soil. The product 

 of fermentation, allyl isothiocyanate, is supposed to act 

 like the volatile disinfectants, and thus a reason has been 

 advanced for the beneficial effect which has long been known 

 to follow the growth of mustard. 



The Limiting Factor, 

 In 1909, Russell and Hutchinson 2 published a very com- 

 prehensive paper upon the effect of sterilisation by heat 

 and by toluene upon soils. Although, like Hiltner and 

 Stormer, these authors found a difference in the types of 

 bacteria in the toluened as compared with the raw soil, 

 they did not consider that any change in type was respons- 

 ible for the increased activity which follows the treatment. 

 The increase in the numbers of bacteria was alone believed 

 to be of significance. They considered the behaviour of the 

 soils from a new standpoint. Instead of looking at the 



1 Eecent work upon the stimulating effect of the metallic poisons is 

 included in an admirable address by F. B. Guthrie, read before the Aus- 

 tralasian Association for the Advancement of Science in 1913. The 

 address includes an appendix by L. A. Musso in which the bibliography 

 is noted and briefly summarised, 



2 Journ. Agr. Scl, 3, 111. f ; / 



