16 



bacteria] multiplication and causes the numbers to rise. Variations 

 in moisture content also produce the normal results on partially 

 sterilised, but not on untreated, soils. 



The detrimental organisms are killed by any antiseptic vapour, 

 or by heating the soil for three hours to 55°-60°C : they suffer con- 

 siderably when the soil is maintained at lower temperatures (40°C) 

 for a sufficient length of time. Cooling to low temperatures also 

 depresses them, although it fails to kill them. 



The completeness of the process can be accurately gauged by 

 the extent to which the bacteria suffer. Whenever the treatment is 

 sufficiently drastic to kill the nitrifying organisms and to reduce 

 considerably the numbers of the other bacteria (as shown by the 

 counts on gelatine plates) it also kills the detrimental organisms. 

 If the soil conditions are now made normal, and the antiseptic is 

 completely removed, rapid increase is observed in the bacterial 

 numbers and the rate of production of ammonia. A temporary or 

 partial suppression of the factor is, however, possible without 

 extermination of the nitrifying organisms. 



Once the detrimental organisms are killed, the only way of 

 introducing them again is to add some of the untreated soil. But 

 the extent of the transmission is apt to be erratic, being sometimes 

 more and sometimes less complete than at others ; occasionally the 

 infection fails altogether. We have not yet learned the precise 

 conditions governing the transmission. 



Provisionally we identify the detrimental organisms with the 

 active protozoa of the soil, but as the zoological survey is yet 

 incomplete we do not commit ourselves to any particular organism 

 or set of organisms, or to any rigid and exclusive definition of the 

 term protozoa. 



The increase in bacterial numbers following after partial steri- 

 lisation by volatile antiseptics is accompanied by an increase in the 

 rate of ammonia production until a certain amount of ammonia or 

 of ammonia and nitrate has been accumulated, when the rate falls. 

 Thus two cases arise : (1) when only small amounts of ammonia 

 and nitrate are present there is a relationship between bacterial 

 numbers and the rate of ammonia production ; (2) when large 

 amounts of ammonia or of ammonia and nitrate are present there 

 is no relationship. The limit varies with the composition and con- 

 dition of the soil. 



Complications are introduced when the soil has been partially 

 sterilised by heat, because heat effects an obvious decomposition of 

 the organic matter, thus changing the soil as a medium for the 

 growth of micro-organisms. The bacterial flora is also very con- 

 siderably simplified through the extermination of some of the varie- 

 ties. These effects become more and more pronounced as the 

 temperature increases, and their tendency is to reduce the numbers 

 of bacteria. We find maximum bacterial numbers in soils that 

 have been heated to the minimum temperature necessary to kill the 

 detrimental organisms (about 60°C). Both bacterial numbers and the 

 rate of decomposition in such soils approximate to those obtaining 

 in soils treated with volatile antiseptics, and the above-mentioned 

 relationships between these quantities also hold. 



Although bacterial numbers are at a minimum in soils heated 

 to 100°, the decomposition effected is at a maximum. 



