631 



CONTRIBUTIONS TO OUR KNOWLEDGE OF SOIL- 

 FERTILITY. 



No. xiii. The Toxicity of Soils. 



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



Society. 



In previous communications, I have shcrwn that when a soil is 

 shaken up with water in the proportion of 100 grams of soil to 

 100 c.c. of water, there is obtained an extract which, under 

 certain conditions, is toxic towards bacteria. The bacteria may 

 be very sensitive to the toxin, such for example as Bac. pro- 

 digiosus, or they may be slightly sensitive, such as a mixture of 

 the soil-bacteria. The toxic effect is shown in the case of Bac. 

 prodigiosus as a distinct diminution in the number of bacteria, 

 and, in the soil-bacteria, as a retardation of the speed of the 

 increase. 



In the soil-extracts, there are present nutritive as well as toxic 

 substances, and the direct evidence of the action of the toxin in 

 destroying bacteria will naturally depend upon the presence of a 

 preponderating amount. Otherwise, if the nutritive substances 

 are in excess, no direct evidence of toxic action will be obtained. 

 It is easy to demonstrate an indirect toxic action. One has only 

 to prepare a soil-extract, to heat one portion, and to compare the 

 rate of growth of bacteria added to each part. It will generally 

 be found that all heated soil-extracts, when seeded with bacteria, 

 will produce a greater number in,^ say, twenty-four hours than 

 an unheated but otherwise similar extract. The reason for this 

 is that the toxins have been destroyed, and, with these out of 

 the way, the growth of the bacteria is much increased. The 

 destruction of the toxins by heat may be analogous to the con- 

 version of the petroleum-soluble and preservative hop-constitu- 



