NITRIFICATION 119 



was added enough arsenic in the form of zinc arsenite to make 100 

 grams of arsenic. These were carefully mixed and allowed to stand 

 for some time, after which an examination was made for soluble 

 arsenic. The analysis revealed the fact that 14 per cent, of the lead 

 arsenate, 30 per cent, of the zinc arsenite, and over 80 per cent, of 

 the Paris green were in the water-soluble form. 



Arsenic being in the soil, some soluble and some insoluble, very 

 naturally suggests the question as to what effect it has upon the 

 bacteria of the soil. Any factor which influences the bacterial 

 activities must indirectly influence the crop yield. 



Extensive studies have been made on the influence of various 

 arsenic compounds upon the bacterial flora of the soil with the result 

 that arsenic was found to be a stimulant in low concentration and 

 toxic only in larger quantities. The extent of stimulation and toxic- 

 ity varies greatly with the specific type of organism and the form 

 in which the arsenic is applied. 



Ammonifiers. Experiments on ammonifiers show that this class 

 of bacteria are not at first poisoned by the arsenic, but their speed 

 of action is increased. The actual results showed that whereas the 

 untreated soil produced in unit time 100 parts of ammonia, soil to 

 which 60 pounds of arsenic an acre was applied produced 103 parts 

 of ammonia in the same length of time. And it was not until 2500 

 pounds of arsenic an acre was applied to the soil that the production 

 of ammonia was reduced to one-half. The Paris green, on the other 

 hand, retarded the action of this class of bacteria even in the lowest 

 concentration added, and by the time .600 pounds an acre had been 

 applied the ammonia produced in unit time had been reduced to 

 one-half normal. This poisonous action of arsenic on bacteria is in a 

 direct relationship to its solubility. An extremely large quantity 

 of lead arsenate would have to be applied to a soil before it would 

 interfere with ammonification. 



Nitrification. The nitrifying flora of a soil are more resistant and 

 are stimulated to a greater extent by arsenic than are the ammoni- 

 fiers. Tests made in soil have shown that whereas untreated soil 

 produced 100 parts of nitrates in unit time, the same soil to which 

 had been added arsenic in the form of lead arsenate at the rate of 

 120 pounds an acre produced 178 parts of nitrates. In other words, 

 in place of being injured by the arsenic, the bacteria were nearly 

 twice as active in the presence of this quantity of arsenic as they 

 were in its total absence. It was not until more than 700 pounds 

 of arsenic, in the form of lead arsenate, an acre, had been applied 

 to the soil that the bacterial activity fell back to 100. Even when 

 arsenic in the form of lead arsenate was applied at the rate of 3500 

 pounds an acre there was 68 per cent, as much ammonia produced 

 as in the untreated soil. The Paris green gave similar results. The 

 untreated soil produced 100 parts of nitrates in given time, while 



