excretions from leaves as a factor in arseni¬ 
cal INJURY TO PLANTS 1 
By C. M. Smith 2 
Assistant Chemist, Insecticide and Fungicide Laboratory, 
Miscellaneous Division, Bureau of Chemistry, 
United States Department of Agriculture 
INTRODUCTION 
Calcium arsenate has been applied as a dust for the control of the 
cotton-boll weevil for several years, with such successful results that 
its use is increasing rapidly, about 16,000,000 pounds having been used 
during the season of 1922. Irregularities in the physical and chemical 
properties of commercial brands and plant injury by material which 
conformed to the specifications of the Bureau of Entomology, United 
States Department of Agriculture, led to a physical and chemical study 
of this arsenical. The progress made on one phase of the problem during 
the summer of 1922 is reported in this paper. 
CAUSES OF ARSENICAL INJURY 
The compounds of arsenic which are soluble in water injure the plants 
to which they are applied with varying degrees of seriousness. Holes 
may appear in the leaves, partial defoliation may occur, or the entire 
plant may be killed, depending upon the concentration of the solution 
used and the susceptibility of the plant treated. The compounds of 
arsenic that are only slightly soluble in water, however, are much less 
toxic—even nontoxic—to plant life. They are therefore very important 
in the control of insect pests. 
In general, the toxicity of an arsenical to plants depends largely on 
the percentage of “water-soluble arsenic” it contains; that is, the per¬ 
centage of arsenic which will enter solution under certain prescribed 
conditions. 3 This soluble arsenic may have its origin in impurities in 
the material, actual solubility of the material, or hydrolysis of the mate¬ 
rial by water. 
However, this is not a complete explanation of arsenical injury. Plants 
differ in susceptibility to arsenic. For instance, potato vines tolerate 
Paris green, with its relatively high soluble arsenic content, whereas bean 
plants can not be safely sprayed with any arsenical. 
Weather conditions constitute a third variable factor. Erratic results, 
believed to be due to the effect of temperature and humidity, sometimes 
1 Accepted for publication Aug. n, 1923. This paper was read at the meeting of the American Chemical 
Society at New Haven, Conn., Apr. 5, 1923. 
2 In carrying out this work the author was assisted by S. B. Hendricks, Entomological Field Assistant, 
Delta Laboratory, Bureau of Entomology, United States Department of Agriculture. 
* Association of Official Agricultural Chemists, official and tentative methods of analysis. 
As compiled by the Committee on Revision of Methods. Revised to Nov. 1, 1919 417 p., 18 fig. Wash¬ 
ington, D. C. 1920. Bibliographies at ends of chapters. 
Journal of Agricultural Research, 
Washington, D. C. 
ahc 
Vol. XXVI, No. 4 
Oct. 27, 1923. 
Key No. E-26 
