BY R. GREIG-SMITH. 635 



The general tendency is for the bacteria in the soils to decrease 

 with an increase in the moisture-content. This may have been 

 caused by the water gradually displacing the air in the soil- 

 spaces, but, as the non-toxic and non-nutritive soil of Expt. a 

 did not show this tendency, we may be justified in assuming 

 that there was a sufficiency of air under the conditions of the 

 experiments, viz., a shallow layer of soil of about 2 mm. thick- 

 ness. It appears that an increase in the amount of moisture 

 increases the toxic effect of the soil-water. 



When the results of these experiments are considered in con- 

 junction with those of the experiments with soil-extracts, it is 

 seen that the effect of adding water in progressive amounts to 

 the soil is to cause a certain degree of toxicity to become mani- 

 fest. As the quantity of water relative to the soil is increased, 

 the toxicity becomes less marked. This is when the ratio of soil 

 to water is about 1 : 0*5. Then the toxicity increases, and is 

 generally most pronounced when the ratio is 1 :1. In one case,* 

 this occurred at the 1:1*5 ratio. With the addition of more 

 water, the toxicity diminishes. 



Moisture and Temperature iu Formation of Toxin. — In at- 

 tempting to produce bacteriotoxins in soils in the laboratory, 

 one realises that certain conditions should favour their produc- 

 tion. The moisture-content of the soil and the temperature are 

 clearly of outstanding importance. An attempt was accordingly 

 made to investigate these. 



A soil was divided into 200 gram portions, and water was 

 added to bring the moisture-contents up to 5, 10, 15, and 20%. 

 Three sets were prepared and stored in bottles in the laboratory. 

 Stumps of matches were set at the sides of the corks to ensure a 

 limited communication with the outside air. At intervals, each 

 set was taken, and the soils shaken up with 200 c.c. of 0*2% of 

 potassium sulphate (added to hasten filtration)^ and due allow- 

 ance was made for the water present as moisture. The extracts 

 were, as usual, filtered through paper and porcelain, and seeded 



* These Proceedings, 1913, p. 773, Experiment xi. 



