460 



Journal of Agricultural Research 



organisms upon which the plants rely for their available nitrogen, pro- 

 vided proper soil texture is maintained. 



Furthermore, it was found that in time the arsenic practically loses 

 its toxic influence toward plants. This was shown by the comparative 

 growth of plants on soils treated at time of seeding and those seeded 

 several months following the application of the arsenic to the soil. There 

 are only two possible explanations of this condition: Either the arsenic 

 reacts with • certain of the soil constituents, resulting in a less toxic 

 combination, or it is rapidly leached from the soil. 



ABSORPTION OF ARSENIC BY THE SOIL 

 When a soluble salt is added to a soil, its ultimate disposition must 

 depend upon certain chemical reactions and physical phenomena. In 

 this case the possibilities involve (i) a combination with or replace- 

 ment of salts already present, resulting in its absorption as a whole; or 

 (2) a selective absorption involving the fixation of only one ion of the salt. 

 In order to determine the fate of arsenic and the effect of irrigation, 

 a set of lysimeter experiments was inaugurated. 



LYSIMETER EXPERIMENTS 



Three types of soil were selected: (i) A ferruginous red clay, (2) a 

 ferruginous brown clay, and (3) a highly organic silt. Twenty-five 

 pounds of soil were placed in each of six lysimeters, two being filled with 

 each type. To each soil were added 3 liters of a solution of sodium 

 arsenite of the same strength as that used for . spraying weeds. One 

 series of three was allowed to stand for two months protected from 

 rain. To the other three i liter of water was added every other day 

 for several weeks, after which the soil was allowed to stand in the lysim- 

 eter until dry enough to sample. 



The object of these experiments was to determine the rate of fixation, 

 the depth to which the arsenic can penetrate, and the leaching effect of 

 irrigation. At the expiration of the above time samples were taken 

 at various depths in the lysimeters and the percentage of arsenic (AsjOg) 

 in the soil at each depth was determined. The results are given in 

 Table I. 



Table I. — Effect of irrigation on arsenic in the soil, giving the percentage of arsenic at 

 various depths 



