Dec. 13. 191S Effect of Arsenic Applied as a Spray for Weeds 461 



The columns headed "Not irrigated" show the percentage of arsenic 

 in the soil at the given depth in the lysimeters which were protected 

 from rain and which received no irrigation. The columns headed 

 "Irrigated" show the percentage of arsenic in the soil at the given 

 depth in the lysimeters which were subjected to irrigation. A com- 

 parison of the two columns for each soil will show the strong fixing 

 power of these soils for arsenic, the influence of different soil types upon 

 the fixation, and the danger of its accumulation. Samples of soil No. 3 

 were taken at depths different from those of soils Nos. i and 2, as shown 

 in Table I, because of the concentration of arsenic at the surface in the 

 former. 



In order to determine how nearly these results represent actual field 

 conditions, samples of soil were obtained from a plantation at Nahiku, 

 Maui, which was the first to adopt the use of sodium arsenite as a means 

 of weed control. Weeds on this land have been sprayed for five years, 

 at the rate of three applications per year, using 5 pounds of arsenic 

 (AS2O3) per acre for one application. During this time the soil has 

 received no cultivation whatever and the rainfall averages about 200 

 inches per year. The soil is very porous and there is very little run-off 

 water. Samples were taken at three depths: Every 4 inches of the 

 first foot. The surface 4 inches contained 0.00924 per cent of arsenic 

 (ASjOg), and none was present below this depth. A determination made 

 by boiling the soil with water showed an arsenic content of 0.00006 per 

 cent, or 0.6 p. p. m., soluble in water. That the arsenic fixed by soils in 

 the lysimeters was partly soluble in water indicates that the fixation is 

 due in part to physical influences. 



CHEMICAL REACTIONS INVOLVED IN THE FIXATION 



The composition of the spray as prepared by recommended methods 

 may be either a solution of the acid salt (Na2O.2AS2O3.2H2O) or the 

 neutral salt (Na20.As203), depending on the proportions of soda (either 

 hydrate or bicarbonate) and arsenious acid used. 



H20+2NaOH-f2As203=Na20.2As203.2H20. 

 H20+2NaOH+As203=Na20.As203.2H20. 



For the following experiments in studying the replacement phenomena, 

 a solution of the neutral salt was used. 



One liter of a i per cent solution of sodium arsenite was allowed to 

 act upon 200 gm. of soil, with occasional shaking, for two weeks. Checks 

 were also maintained with 200 gm. of soil and i liter of water. The 

 arsenic extract was then separated from the soil and a partial analysis 

 made to determine the elements with which the sodium arsenite is most 

 active. The results are given in Table II, which shows the composition 

 of a I per cent sodium-arsenite solution after contact with the soil, as 

 compared with the solvent action of water. The percentage of humus 



