PRESIDENTIAL ADDRESS. 81 



expect to find the direct evidence of toxin in soils at any 

 other time than during the cool winter months. 



Conclusion. 



In concluding, I have endeavoured to show that a new 

 stimulus was given to the study of certain aspects of the 

 microbiology of the soil by the work of Russell and Hutch- 

 inson, and although their deductions have not been gener- 

 ally accepted, yet they have done much to advance the 

 study of the protozoa and of other factors in soil fertility. 

 It has not been proved that the protozoa play any part in 

 restricting the gross numbers of the soil bacteria. They 

 do not constitute the limiting factor, and with their claims 

 out of the way, attention can be paid to the work of the 

 other micro-organisms. These are non-phagocytic, and 

 must exercise their activity by the secretion of certain 

 bodies which, being destructive, are called toxins. At 

 present the nature of these toxic bodies is a matter of 

 indifference. Their characters will naturally be determined 

 when we isolate the causative micro-organisms. This may 

 involve a considerable amount of research work, but I 

 believe that the elucidation of the problem is not far distant. 

 So far as we can judge at present, our policy should not be 

 directed to preventing the total formation of the toxic 

 products of micro-organisms, even if it could be done, but 

 to regulating their amount, so that they are present in the 

 soil in the proportion in which they act as microbic and 

 possibly as plant stimulants. 



F— May 3, 1916. 



