520 



Occiirrence of Arsenic in Solls 



[July 



extracting, for eight days, 500 grams of soil with 2000 c.c. of 

 carbon-dioxide-free distilled water, and then using an aliquot part, 

 while the total arsenic was determined by extracting the soil with 

 nitric and sulfuric acids, and applying the Marsh method so modi- 

 fied that the iron did not interfere.'' The results are given in the 

 accompanying table ( i ) as parts per million of dry soil. 



TABLE I 



Data pertaining to the quantities of arsenic in soils: parts per million 



From the data in the table it may be seen that some orchard 

 soils contain large quantities of arsenic and some carry compara- 

 tively large proportions of it in the water-soluble form; but there 

 is no uniform relationship between the total arsenic in soils and the 

 water-soluble arsenic. If, as is most likely the case, the injury to 

 plants is due to the water-soluble arsenic, greater injury would 

 result in a soil containing only 5 parts per million of total arsenic 

 with 61.60 per cent. of the arsenic water-soluble, than in a soil con- 

 taining 102 parts per million, in which only 0.9 per cent. of the 

 arsenic is water-soluble. It may be seen, further, that one of the 

 above soils, which contains 63 parts per million of total arsenic, 

 has only 1.62 per cent. in soluble form, while another soil, having 

 exactly the same amount of total arsenic, contains 10.92 per cent. 

 in soluble form. 



Arsenic in soil is not confined to the surface. One soil, obtained 



"^ Greaves : Jour. Amer. Chem. Soc, 1913, xxxv, p. 150. 



