4 Influence of Ar senk in Solls [Oct. 



after which there was a marked retarding influence on the ammoni- 

 fying powers of the soll. From this point the quantity of ammonia 

 formed was nearly constant; only a shghtly greater retarding effect 

 having been noted when the concentration of the arsenic was 90 

 parts per milHon than when it was 20. This is probably due to the 

 fact that the soil was comparatively rieh in calcium and iron, and 

 that the sodium arsenate was changed into the comparatively insol- 

 uble calcium and iron arsenates. At the concentration of 20 parts 

 per million the water in the soil was saturated with these slightly 

 soluble Compounds, so that even though there was an increase in the 

 quantities added, the active masses remained about constant. These 

 results, therefore, cannot be taken to indicate what may happen in 

 a soil devoid of calcium and iron, and in which the arsenic may re- 

 main in a soluble form. They show that large quantities of soluble 

 arsenic, even up to 90 parts per million, may be added to a soil rieh 

 in calcium and iron without stopping ammonification. 



The influence of insoluble arsenic Compounds. The Com- 

 pounds selected for the tests are in use as sprays, and hence find 

 their way into the soil in greater or less quantities. They were lead 

 arsenate, Paris green, zinc arsenite and arsenic trisulfide. The tri- 

 sulfide, while seldom used, is of special interest in this work in that 

 it permits the application of arsenic free from the metallic elements 

 which accompany each of the others, and therefore gives more 

 nearly the influence of the arsenic than do the other Compounds. In 

 each case the quantity of the Compound taken was such as to give 

 equivalent amounts of arsenic. The results reported are in milli- 

 grams of ammonia formed per 100 grams of soil. These results 

 are reported in Table 2. 



On examining the data in Table 2 we find that arsenate stim- 

 ulated ammonification in the lowest concentrations and that the tox- 

 icity gradually increased as the amount of lead arsenate in the soil 

 was increased. For no concentration tested was there any marked 

 retarding action over the previous concentration, but a gradual de- 

 cline in the ammonifying efiiciency. The ammonia was not re- 

 duced to one-half of its original amount until the soil contained a 

 concentration of 760 parts of arsenic per million. Even at a con- 

 centration of 1,120 parts per million of arsenic, there was produced 



