16 BULLETIN 1451, U. S. DEPARTMENT OF AGRICULTURE 
SODIUM ARSENITE 
Sodium arsenite is generally considered one of the most effective 
of the common herbicides. The use of the various forms of arsenic 
as insecticides also has resulted in an extensive literature dealing 
with the results produced on plants by various applications of these 
substances. The insoluble forms are very slightly, if at all, toxic to 
plants, but the soluble forms are all toxic, and of these sodium 
arsenite is one of the most toxic. Schulz and Thompson (14) per- 
formed experiments to determine roughly the most toxic form of 
arsenic. This proved to be arsenious acid, H 3 As0 3 . This acid is 
produced by dissolving white arsenic, As 2 3 , in water, but since it is 
only slightly soluble it is not a satisfactory herbicide for this 
purpose, and sodium arsenite was selected for study. Sodium 
arsenite may be in various forms containing more or less sodium to 
the molecule. Some of the possible forms are NaAs0 2 , N"a 3 As0 3 , 
Na 2 HAsO s , and NaH 2 As0 3 . In solution these probably all dis- 
sociate to form arsenious acid and sodium hydroxide. The less 
sodium there is in the molecule the more arsenious acid may be 
formed and the more toxic is the solution. For field use, however, 
the action of the soil solutes on the arsenic must also be considered. 
In some cases it may be an advantage to have a strongly alkaline 
solution. 
The commercial sodium arsenite selected and used in most of the 
experiments was a product known as " 8-pound " material. It is 
formed from 8 pounds of white arsenic and about 2 pounds of 
sodium hydroxide with water sufficient to make a gallon. Each 
gallon weighed about 16 pounds. 
In greenhouse tests 4 cubic centimeters of this material in 80 
cubic centimeters of water killed barberries in 8 -inch pots in eight 
days. This was so satisfactory that field tests were made. Some of 
the field tests with " 8-pound " sodium-arsenite solution are sum- 
marized in Table 2. 
At the time these tests were begun the writers had onl}- a gen- 
eral idea of the root system of the barberry. As there were long- 
roots radiating from the crown, it was thought that by placing the 
chemical in a ring around the barberry, but at a little distance from 
it, the lateral roots and the entire bush might be killed. In most 
of the treatments recorded in Table 2, the sodium-arsenite solution 
was placed in a ring around the bush. An examination of this table 
will show that where the solution was applied in a circle with a 2- 
foot radius only 30 per cent of the total number of bushes treated 
were killed. Where the radius of the circle was only 1 foot about 
45 per cent of the treated plants were killed, and where the solution 
was applied to the base of the plant as a drench about 98.5 per cent 
were failed. This indicated that the barberry was most susceptible 
to chemicals immediately around the base of the shoots. Later 
studies of the root system indicated the reason for this, as has already 
been explained. 
The results reported in Table 2 indicate another point. Sodium 
arsenite was known to be an extremely active plant poison. It was 
suspected that a small quantity would kill a bush if it could reach 
the living tissues. It remained to be determined how killing could 
be done most economically. If a small quantit}' of the substance 
