274 BIOCHEMICAL PROCESSES OF THE SOIL 



field observations with the known activities of bacteria. The 

 recent work at Rothamsted has satisfactorily explained these 

 difficulties by showing that the micro -organic population must 

 be regarded not as one, but as two or more groups, some of 

 which are detrimental to the useful organisms concerned in 

 the processes of ammonia and nitrate formation. 



The investigation 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 demonstrate 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 autoclave was not available, 

 and the soil was only heated in the steam oven, and it gave 

 the remarkable result that its power of absorbing oxygen 

 showed a considerable increase. Now the steam oven 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 decom- 

 position. The increased quantity of plant food thus formed 

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

 XCIII. contains a typical series of results : 



TABLE XCIII. Weight and Composition of Crops grown on Partially 



Sterilised Soils. 



