CHAPTER XII. 

 INFLUENCE OF ARSENIC ON BACTERIAL ACTIVITY. 



Occurrence of Arsenic. Kunkel showed the presence of arsenic 

 in many rocks and water, while Czapek states that traces are nearly 

 always present in soils. Herzfeld and Lange found arsenic in certain 

 German raw sugars and traced it to the lime which had been used 

 in the manufacture of sugar. Headden found some virgin prairie 

 soils relatively rich in arsenic, an observation in accord with my own 

 experience. I have found arsenic to the extent of 4 parts per million 

 in virgin soil; and, as in the cases referred to by Headden, it did not 

 result from smelter fumes or any such source, but was derived from 

 the decay of native rocks. On the other hand, Headden found 

 arsenic in some cultivated orchard soils to the extent of 138 parts per 

 million. He claims that in many places arsenic from spray is accu- 

 mulating in sufficient quantities to become injurious to vegetation. 

 Francis, however, thinks there is little danger of the soil's becoming 

 unfit for vegetation from the proper use of insecticides. Grunner, 

 who found arsenic to the extent of from 0.026 per cent, to 1.426 per 

 cent, in the Reichenstein soil, is not so optimistic. An extensive 

 analysis of the sprayed orchard soils of western America showed 

 arsenic to be present in all of those soils and varying from mere 

 traces to 500 pounds an acre. In some cases it occurred to a depth 

 of three or four feet. The most interesting fact is that in some of 

 these soils there were as much as 17 pounds per acre of water-soluble 

 arsenic. It is not, however, always the case that the greatest 

 quantity of water-soluble arsenic is found in those soils which con- 

 tain the greatest total quantity of arsenic, for often soils are found 

 which contain only a few pounds to the acre-foot, probably two- 

 thirds of which is in a soluble form. So the conclusion has been 

 reached that some virgin and many cultivated soils contain arsenic 

 in large quantities, but the proportion in a soil is no index of the 

 amount that is soluble in water. The latter is probably governed 

 by many factors; for example, kind of soil, water-soluble salts, and 

 form in which the arsenic was applied to the soil. 



Factors Influencing Solubility. That the form in which the arsenic 

 is applied govern largely its solubility is shown by an experiment in 

 which 100 grams of arsenic in the form of lead arsenate was applied 

 to a soil, and to another portion of the same soil was added 100 

 grams of arsenic in the form of Paris green. To still another soil 



