THE MICRO-ORGANIC POPULATION OF THE SOIL 283 



method. There has been much discussion of the view 

 expressed by Russell and Hutchinson, and many investigators, 

 e.g. J. M. Sherman (261), do not admit the presence of any 

 biological factor in soils detrimental to bacteria. The evidence 

 in favour of the view will therefore be set out in some detail. 



The writer's investigation on this subject began in the first 

 instance as the result of an accident. In virtue of its large 

 population of micro-organisms soil absorbs a considerable 

 quantity of oxygen, and evolves a corresponding amount of 

 carbon dioxide. An experiment had been arranged to de- 

 monstrate the well-known fact that soil heated to 130 C, 

 and therefore completely devoid of micro-organisms, lost much 

 of its power of absorbing oxygen. By an accident the auto- 

 clave was not available ; the soil was therefore heated only in 

 the steam oven, and it gave the remarkable result that its 

 power of absorbing oxygen instead of falling, as was expected, 

 considerably increased. Now the steam did not kill all the 

 organisms, but spared those capable of forming spores ; i.e. 

 sterilisation was only partial. Partial sterilisation by means 

 of volatile antiseptics gave the same result. The conclusion 

 was drawn that partial sterilisation increased the bacterial 

 activity, 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 LXXV. contains a 

 typical series of results. 



Further investigations by Russell and Hutchinson (240*:) 

 led to the following conclusions : 



(1) Partial sterilisation of soil, i.e. heating to a tempera- 

 ture of 60 C. or more, or treatment for a short time with 

 vapours of antiseptics, such as toluene, causes first a fall then 

 a rise in bacterial numbers. The rise sets in soon after the 

 antiseptic has been removed and the soil conditions are once 

 more favourable for bacterial development ; it goes on till the 

 numbers considerably exceed those present in the original 

 soil. 



(2) Simultaneously there is a marked increase in the rate 

 of accumulation of ammonia. This sets in as soon as the 



