TOXICITY OF VARIOUS SALTS 149 



that the speed of the nitrification process is controlled and dependent 

 upon the speed of ammonification. The results clearly indicate that 

 there are other side reactions taking place which are influenced by 

 the salts but which are not measured by the ordinary bacterio- 

 logical method. 



It is evident, however, from the results reported in Fig. 21 that 

 those compounds which are most active as stimulants to the higher 

 plants are also most active in stimulating bacteria. It is likely that 

 the effect upon the, plant is due in a large measure to the action of 

 the compound upon the bacteria, which in turn render available 

 more plant-food. 



Toxicity of Various Salts. There is an extremely wide variation in 

 the concentration at which various salts become toxic to soil bac- 

 teria (Fig. 22). Some must occur in soils in large quantities before 

 becoming toxic, whereas others are toxic when present in only minute 

 quantities. The toxicity of the salts to ammonifying organisms are 

 controlled largely by the electronegative ion, but this is not as 

 pronounced in the case of the nitrifiers. The latter class of organisms 

 is, however, more sensitive to salts than are the ammonifiers. The 

 ammonifiers represent more nearly the higher plant. 



It is apparent from these results that the increased osmotic 

 pressure exerted by a salt within the soil plays a part in retarding 

 the bacterial activity of such a soil, but it is not the only factor. 

 The main factor is probable a physiological one, due to the action 

 of the substance upon the living protoplasm of the cell changing 

 its chemical and physical properties in such a way that it cannot 

 function normally. 



REFERENCE. 



Greaves, J. E.: "The Influence of Salts on the Bacterial Activity of the Soil" 

 (Soil Science, 1916, ii, 443-480). 



