FERTILITY IN SOILS 357 



toluene or by heat. The alteration in the flora and the change 

 in the course of the soil decomposition are observed and, as 

 far as may be, correlated. 



The Effect of Partial Sterilisation 



The earliest observations that 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, an Alsatian vine-grower, to kill phylloxera ; Girard 

 used it in 1887 to clear a piece of sugar-beet ground badly 

 infested with nematodes. In both cases the subsequent crops 

 were such as to show that the productiveness of the soil had 

 been increased by the treatment. The experiments appear to 

 have been made quite independently and the results were 

 published in 1894; they naturally attracted a good deal of 

 attention, and similar trials were made by a number of 

 investigators. 



The first piece of scientific work came from A. Koch in 1899. 

 He worked with varying quantities of carbon disulphide, and 

 concluded that it stimulates the plant root to increased growth. 

 Four years later Hiltner and Stormer showed that the bacterial 

 flora of the soil undergoes a change. The immediate effect of 

 the antiseptic was to decrease by about 75 per cent, the number 

 of organisms capable of developing on gelatine 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. They argued that the increased numbers of 

 bacteria must in any case result in an increased food supply 

 for the plant, and they further claimed that the new type of 

 flora was actually better than the old in that denitrifying or- 

 ganisms 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 shows that other poisonous substances 

 which can be removed from the soil behave like carbon di- 

 sulphide. 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 



