194 



BACTERIA AND SOIL FERTILITY 



legitimate at times to use such substances, but the farmer should 

 do so with the understanding that he is using a stimulant — ^not a 

 plant-food — and that he is getting a temporary — ^not a perman- 

 ent — ejffect. 



Toxicity of Alkali Salts. — If we gradually increase the quan- 

 tity of alkali salts in a soil there will come a time when the bac- 

 terial activities are decreased* The quantity of alkali which must 

 be added to a soil to become poisonous to bacteria varies with the 

 salt, the soil, and the bacteria. The quantity, stated as pounds 

 per acre, which it was found necessary to apply to a calcareous 

 loam in order to become toxic to ammonifying and nitrifying 

 organisms, is given below: 



Saif 



Ammonifier 

 {Pounds per Acre) 



Nitrifier 

 {Pounds per Acre) 



Calcium sulfate 

 Sodium carbonate 

 Sodium sulfate 

 Magnesium sulfate 

 Magnesium chloride 

 Sodium nitrate 

 Sodium chloride 

 Calcium chloride 



24,500 

 22,890 



2,200 

 1,700 



1,530 



263 



61 



57 



79 



270 



29 

 47 



3^3 



These results are not to be taken as indicating that neither 

 ammonia nor nitrate is produced in a soil containing the above 

 quantities of alkali, as these are the concentrations at which the 

 activity of the microorganisms are slowed down. 



Ammonia is produced in a soil containing considerable alkali, 

 but the production of nitrates is interfered with even when small 

 quantities of alkali are present. The great difference shown be- 

 tween ammonifiers and nitrifiers is due to the fact that many 

 molds ammonify, whereas it is only bacteria which nitrify. 

 Molds are considerably more resistant to alkali than are bacteria. 



Much larger quantites of alkali than those reported above 

 would have to be in a soil before the nitrogen-fixing bacteria are 



