University of California

Toxicity of pentachlorophenol and its sodium salt in three Yolo soils


W. A. Harvey
A. S. Crafts

Authors Affiliations

W. A. Harvey was Associate Agriculturist, Agricultural Extension, Davis; A. S. Crafts was Professor of Botany and Botanist in the Experiment Station, Davis.

Publication Information

Hilgardia 21(16):487-498. DOI:10.3733/hilg.v21n16p487. August 1952.

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In 1940, Hance4 discovered a notable activation of herbicidal solution by using a water soluble chlorinated phenate. He found that the addition of ¼ to ½ of 1 per cent by weight of this activator and of an oxidizing substance to a herbicide solution reduced the concentration of weed killing chemical by from one half to one eighth that ordinarily used (depending on the toxic substance employed and the weeds) with equally satisfactory or even superior results. This activator has since been known as pentachlorophenol or PCP; it is available as the oil soluble parent phenol and as the water soluble sodium salt (Na PCP).

(Crafts (1944)) described studies where PCP was added to petroleum oils in order to fortify them so that the oil even when diluted or emulsified with water would remain effective. Good results were obtained by adding 0.5 per cent PCP to a 6 per cent diesel oil emulsion. Barley seems to be the only plant surviving two sprayings of this mixture. (Hance (1948)) mentions a mixture of sodium chlorate (NaClO3) (20 pounds) and water soluble Na PCP (4 pounds) in 100 gallons of water as a good contact herbicide. (Aldrich and Willard (1949)) obtained good preëmergence weed control in corn by using 8 and 12 pounds Na PCP per acre. Heavy rain fell within 30 minutes after the application.

Many other workers (Barrons, 1948); (Hance, 1948); (Shafer, 1948); and (Wilson and Hall, 1948) report encouraging results with PCP or with its Na salt, mostly as preëmergence treatments. This chemical is now being widely used, principally as a fortifying agent and in preëmergence applications. Sugar cane and pineapple are quite resistant to PCP; when the chemical is correctly applied, good results are obtained in preëmergence treatments.

According to (Barrons (1948)), selectivity is based on depth protection. The top growth pushing through the surface layer of soil containing the active chemical toxicant apparently does not absorb lethal amounts because the waxy stems or leaf surfaces do not permit the entry of such ionic materials as the phenolic salts. Roots that lack a waxy covering do absorb such salts, and germinating seeds are thus killed. These chemicals apparently have only an acute and local toxicity. The occasional injury resulting to top growth is, according to Barrons, noticeable only on cotyledons and primary leaves and does not affect subsequent growth.

The increasingly widespread use of PCP and its Na salt has created a need to determine their toxicity and their rate of breakdown in some California soils.

Literature Cited

Aldrich R. J., Willard C. J. Effects of five chemicals applied as preëmergence on corn 1949. p.203. Research Report, 1949, Annual North Central Weed Control Conference

Barrons K. C. Preëmergence weed control. Down to Earth. 1948. 4(3):2-4.

Crafts A. S. The toxicity of sodium arsenite and sodium chlorate in four California soils. Hilgardia. 1935. 9(9):481-98. DOI: 10.3733/hilg.v09n09p459 [CrossRef]

Crafts A. S. Experiments on general contact herbicides. Botany Division. 1944. College of Agriculture, University of California. p. 1-6. (Mimeo.)

Hance F. E. The factor of synergism. Hawaiian Planters Record. 1940. XLIV(4):263-72.

Hance F. E. Weed control on Hawaiian sugar cane lands—contact herbicides. Hawaiian Planters Record. 1948. LII(2):93-112.

Hance F. E. Weed control on Hawaiian sugar cane lands—developments in use of 2,4-D. Hawaiian Planters Record. 1948. LIII(2):93-105.

Shafer N. E. Effect of four contact herbicides on several species of annual weedy grasses 1948. Research Report, 1948, Annual North Central Weed Control Conference

Wilson J. D., Hall W. E. Preëmergence weed control in vegetables 1948. Research Report, 1948, Annual North Central Weed Control Conference

Zobell C. E. Action of microörganisms on hydrocarbons. Bact. Rev. 1946. 10:1-49.

Harvey W, Crafts A. 1952. Toxicity of pentachlorophenol and its sodium salt in three Yolo soils. Hilgardia 21(16):487-498. DOI:10.3733/hilg.v21n16p487
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