1920.] 



Air Pollution by Coal Smoke. 



73 



yield, an increased libre content, indicating that the grasses 

 were indigestible, and a decreased protein content, indicating 

 a low feeding value. The effect of the acidity in the soil itself 

 was shown most markedly in the reduction in the number and 

 activity of the soil bacteria, of which the most valuable and at 

 the same time the most sensitive are the nitrifying organisms. 



The results of the bacteriological investigation of the soils 

 revealed the fact that while the activity of the " ammonia- 

 producing " organisms was cut down by 42 per cent, by the 

 application of the acid waters, the activity of the nitrifying 

 organisms was cut down by more than 88 per cent. 



Effect of Acidity on Soil Bacteria. 



Under normal conditions, the protein present in the organic 

 matter in the soil is seized on by the first group of bacteria, 

 the putrefactive organisms, and it is their work to break the 

 protein down into ammonium compounds. The ammonium 

 compounds, however, should not accumulate. A normal soil 

 rarely contains more than four parts per million of nitrogen in 

 the form of ammonia. Other organisms are waiting for these 

 ammonium compounds and convert them first of all into nitrites 

 and finally into the finished products nitrates. While in a 

 normal soil the ammonium compounds are converted first into 

 nitrites and then into nitrates almost immediately, other 

 conditions may prevail in abnormal soils. The nitrifying 

 organisms are much more sensitive and more easily damaged 

 than are the putrefactive organisms, and particularly are they 

 susceptible to acid conditions. Hence, if the workers in the 

 nitrate factory of the soil have to put up a fight against hostile 

 acid conditions, it will be the nitrifying department which will 

 close down first ; and, if the ammonia-producing organisms 

 are still able to go on working, this will result in an accumulation 

 of ammonium compounds in the soil Such or similar conditions 

 are to be found in many of our town gardens, and on the farms 

 in and near industrial areas. The acid rainfall is sufficient to 

 keep in check the nitrifying organisms, though not perhaps 

 powerful enough to prevent the more vigorous putrefactive 

 organisms from continuing their work ; and the amount of 

 nitrogen in the form of ammonia may rise to as much as twenty 

 parts per 100,000. 



On the other hand, the acid conditions in the soil may become 

 so unfavourable to the growth of bacteria that even the putre- 

 factive organisms are unable to work. In this case there will 

 remain in the soil an accumulation, not of nitrates — the 



