INFLUENCE OF SOIL ALKALI ON BACTERIA 195 



injured. Hence, we find in an alkali soil that the nitrifying bac- 

 teria disappear first, followed by the ammonifying, and finally 

 in the barren waste the nitrogen-fixing microorganisms disappear. 



The first injury is due to increased osmotic pressure of the soil 

 solution, as the osmotic pressure of all these solutions at the point 

 at which they became injurious was between 1.65 and 1.95 at- 

 mospheres. However, as larger quantities of the salts accumulate 

 they poison the soil microorganisms. This is highly fatal to the 

 nitrifiers but less so to the ammonifiers, and still less to the nitro- 

 gen-fixing microorganisms. 



It is interesting to note that alkali salts become poisonous to 

 nitrifying microorganisms considerably before they do to higher 

 plants. Therefore, it is quite possible that the early injurious 

 effect observed when land starts to "go bad," from an accumula- 

 tion of alkali, is due to the action of the salts on the nitrifiers, 

 and only after it becomes more concentrated does it actually 

 injure the plant. Moreover, after a soil has been leached of its 

 alkali it is a number of years before it becomes productive. This 

 unproductive condition is due in a measure to the bad physical 

 condition of the soil. This is not the only factor, however, as the 

 addition of manure or extracts of productive soil greatly shortens 

 the unproductive period by bringing to the soil the necessary bac- 

 terial flora. 



Variation in Toxicity. — ^The toxic or poisonous action of a 

 salt varies not only with the kind and quantity of salt present 

 but also with the soil. A quantity of common salt or other alkali 

 which would be sufficient to greatly retard bacterial growth in a 

 soil of coarse texture may be without effect in one of fine texture. 

 Moreover, a salt is more poisonous in a soil low in organic content 

 than in one of high. It would appear that the greater surface 

 exposed to the action of the salt in the fine textured soil holds 

 the salt in such a way that the bacteria are not injured. This 

 is spoken of as absorption. Moreover, salts are less toxic where 

 there is a mixture than where there is present a single salt. 



Antagonism. — If certain salt-water fish be placed in a solution 

 of common salt having the same osmotic pressure as has sea-water 



