THE MICRO-ORGANIC POPULATION OF THE SOIL 281 



to kill phylloxera, and by Girard l in 1887 to clear a 

 piece of sugar-beet ground badly infested with nematodes. 

 In both cases the subsequent crops showed that the pro- 

 ductiveness of the soil had been increased by the treatment. 



The first piece of scientific work came from A. Koch 

 in 1 899 (l 5 1 a\ who, working with varying quantities of carbon 

 disulphide, concluded that it stimulates the plant root to in- 

 creased growth. Four years later Hiltner and Stormer (135*:) 

 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 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 increased 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 (135^) shows that 

 other volatile or easily decomposable antiseptics produce the 

 same effect. The important part of this work is unquestion- 

 ably the discovery that the organisms in the treated soils 

 ultimately outnumber those in the original soil. 2 The hy- 

 pothesis 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 (100) in 1888 showed that it increased 



1 Bull. Soc. Nat. d'Agric., 1894, 54, 356. 



2 Sewage investigators have found that the disinfection of sand filters leads 

 to increases in the numbers of bacteria. This was observed in the typhoid 

 epidemic at Lincoln in 1904 and also in the experimental sewage filter at Guild- 

 ford in 1907 (Houston and McGowan, Fifth Report, Sewage Commission* 

 Appendix 4, Cd. 4282, 1910, p. in). 



