142 



CONTRIBUTIONS TO OUR KNOWLEDGE OF SOIL- 

 FERTILITY. 



No. xvi. The Search for Toxin-Producers. 



By R. Greig-Smith, D.Sc, Macleay Bacteriologist to the 



Society. 



In the earlier papers of this series, it was shown that bacterio- 

 toxie substances were sometimes present in the clear liquid ob- 

 tained by shaking a soil with water and filtering it through 

 porcelain. The toxins were potent, for, when the same portion 

 of soil was extracted a second time with water, the extract was 

 highly nutritive. The first extract undoubtedly contained sub- 

 stances which functioned as toxins, as well as substances which 

 acted as nutrients, and, of these two, the toxins were the more 

 powerful. 



The local soils were found to be toxic during the cold winter 

 months and not in the dry summer. The toxicity was variable, 

 and even in the favourable season, one could not be certain that 

 a soil, undoubtedly toxic at one time, woiild be toxic at another. 

 It seemed to be a matter of chance, but this was undoubtedly 

 due to the fact that the cause of this toxicity was unknown. 

 With the elucidation of the origin of the toxins, it will be better 

 understood when a soil is likely to be toxic or otherwise. 



Soil-toxicity, as exhibited in the extracts of the soils, can be 

 demonstrated directly and indirectly. Directly, by adding a 

 certain number of a test-bacterium, and obtaining a diminution 

 in that nnmber after an incubation-period of, say, 20 hours at 

 22°. Indirectly, by obtaining an increased growth as a result of 

 boiling the extract and also of diluting it. A toxic soil exhibits 

 all three characters. There are other points connected with 

 toxic soils, and these will be found in my former papers. 



During my earlier work, I was led to believe that the cause 

 pf the toxicity would be found in the products of the bacteria, 



