522 



Occurrence of Arsenic in Soils 



[July 



TABLE 2 



Data pertaining to ihe water-soluble arsenic in soils mixed with arsenical 



insecticides 



These results show that there may be a great difference in the 

 quantity of water-soluble arsenic existing in the same soil to which 

 various forms of arsenic have been added in equivalent quantities ; 

 and that even a soil comparatively rieh in iron and calcium, to 

 which arsenic has been added in large quantities, may have a high 

 water-soluble content of arsenic. It is much higher when arsenic 

 is added in the form of Paris green, than when added in the other 

 forms mentioned. The solubility of the Compounds, with the excep- 

 tion of arsenic trisulfide, is not greatly changed on standing in a 

 soil containing a large quantity of decomposing organic matter. 



The results also show the superiority of lead arsenate over any 

 of the other arsenical insecticides. Any injurious effect of such 

 Compounds on plants must be proportional to the available amount 

 of water-soluble arsenic, not to the total arsenic. In the use of an 

 arsenical insecticide it should be the rule to select a Compound 

 which would remain insoluble for the greatest possible length of 

 time. Lead arsenate possesses this advantage in high degree. 



Summary. Some virgin soils contain arsenic in appreciable 

 quantities which comes from the decay of the native rocks. Many 

 cultivated orchard soils contain it in large proportions, but there 

 is no uniform relationship between the total quantity of arsenic in 

 different soils, and the water-soluble arsenic of these soils. A soil 

 containing over 100 parts per million of total arsenic contained 

 much less water-soluble arsenic than did a soil carrying only 5 parts 

 per million of total arsenic. The solubility of the arsenic found in 

 a soil is governed largely by the salts in the soil and the form in 

 which the arsenic is applied. Different portions of the same soil, 



