PRESIDENTIAL ADDRESS. 63 



the action of lime depended upon the character of the soil, 

 each soil absorbing a definite and variable amount before 

 the so-called partial sterilisation effect became evident. 

 This critical CaO point can be determined chemically, as 

 it is the point at which there is no more absorption of lime. 

 Above this point the lime remains as free alkali, and it is 

 the free alkali that functionates as a disinfectant. 



The Soil Protozoa. 

 Our knowledge of the soil fauna is at present very scanty. 

 The economic value of the protozoa is undoubtedly to 

 decompose organic matter, for they feed upon it, and there- 

 fore, if the effect of digestion is similar to that of the higher 

 animals, the bulk of the food is soon resolved into simpler 

 substances. We are not quite sure what protozoa are 

 vegetating in the soil. All forms that are present must 

 surely have vegetated under some conditions, but that they 

 do not always lead an active life appears from the work 

 of several observers. 



Goodey 1 concluded that the large Colpodce exist in the 

 encysted condition, for he noted that when the soil was 

 shaken up with culture media, the first cells to be observed 

 did not have the granular structure of vegetating forms. 

 In culture media infected with soil, the protozoa are found 

 to run in cycles; some forms predominate at one time, and 

 others at another, and Martin and Lewis 2 think it possible 

 that this also occurs in soils, and that two soils with similar 

 water content, may show a different fauna according to the 

 point of the cycle at which the soil had arrived. The cycle 

 may be explained by the protozoa secreting substances 

 which are toxic to themselves, as has been shown by Wood- 

 ruff in the case of Paramecium aurelia. Sun confirmed 

 this, and pointed out that heating the fluids destroyed the 



1 Proc. Boy. Soc., 1911, B. 84, 165. 

 2 Journ. Agr. Sci., 7, 106. 



