LIMITS OF TOXICITY 135 



again prevail. By determining the ammonifying, nitrify- 

 ing, and nitrogen-fixing power of the organisms they pro- 

 pose to classify land that is being drained as to its ability 

 to grow crops. 



Concentrations of Alkali which Limit Biological Ac- 

 tivities. The quantity of alkali that Will cause injury 

 to the ammonifying and nitrifying bacteria as determined 

 by different investigators varies from a minimum of 250 

 parts per million of sodium carbonate, which was found 

 by Lipman (10) to inhibit growth of these organisms, to 

 a maximum of 4000 parts per million of this salt as found 

 by Kelley (8) . The nature and concentration of the nitrog- 

 enous material used to determine the activity of the or- 

 ganisms has been found to make a great difference in the 

 rate of nitrification. Kelley found that where i per 

 cent of dried blood was used as the nitrogenous material, 

 500 parts per million of sodium carbonate was distinctly 

 toxic, but where only o.i per cent of dried blood was used 

 the organisms were apparently not affected by the presence 

 of 4000 parts per million of sodium carbonate. He also 

 found that while 1000 parts per million of sodium car- 

 bonate were toxic to nitrification in the presence of 0.15 

 per cent of ammonium sulphate, this concentration was 

 markedly stimulating in the presence of 0.0625 per cent of 

 ammonium sulphate. The large discrepancies in the 

 quantities of alkali which these bacteria withstand are 

 probably due in part to the differing quantities and kinds 

 of nitrifying materials used as well as the kind and dif- 

 fering natures of the soils. Dried blood, cottonseed meal, 

 ammonium sulphate, and numerous other materials have 

 been used; this makes comparisons of the different ex- 

 periments difficult. Standard methods are needed in this 

 regard as they are in other alkali work. It is probable 



