The Decomposition of Pentachlorophenol When Applied as a Residual Pre‐emergence Herbicide 1

TEED control by chemicals is becoming a widelyused cultural procedure. Unfortunately, such procedure often is put into use before sufficient information is available to warrant its adoption. There resulis as a consequence either poor weed control or injury to the crop in which the weeds are located. The increased use of herbicides as a cultural practice naturally raises the question as to the persistence of such chemicals in the soil and their residual effect on subsequent plantings. The persistence of herbicides in thr: soil is influenced by a number of different factors. l't has been shown by DeRose and Newman (6), Brown and Mityhell (2), and Kries (10) that raising the temperature and increasing the moisture and organic content of the soil accelerates the decomposition of 2,4--D and kindred substances. Newman (11) and Smith et al. (13) have shown that a number of herbicidal materials at the concentrations generally recommended have no harmful effect, and in some cases a slight stimulative action, on the soil microorganisms. Studies by Newman and Thomas (12) yield further evidence that the disappearance of 2,4-D from soils is due primarily to microbiological processes. Gray and Thornton (8) found that the bacteria decomposing a number of aromatic compounds, including phenol, were most often found in arable, manured soil. Evans (7) also reported that some soil bacteria will grow and completely metabolize phenol and benzoic acid. Several investigators (2, 5, 9, 8, 10) have determined that the rate of leaching of 2 ,4D from the soil depends not only on the rainfall and the character of soil, but also upon its reaction. Soils which had been limed to an alkaline reaction required a longer period than acid soils for the leachate to become free from the herbicide. Welton and Carroll (15) found that sodium chlorate when applied at the rate of 1 pound per 100 square feet on silt loam had almost completely disappeared in 12 weeks. Crafts (3) found that the toxicity of ammonium sulfamate was inversely related to soil fertility, but in the case of sodium dinitro-o-cresylate the toxicity was related more to textural grade than to fertility. Willard (16) has pointed out that soil type and rainfall have much to do with the safety of preemergence treatment. Alban and McCombs (1) state that when applied at the rate of 7.2 pounds per acre residual effects of sodium pentachlorophenate have been notled on muck soils for as long as 12 weeks. Taylor (14) reports no evidence of residual toxicity in the soil from as much as 25 pounds of sodium pentachlorophenat'e per acre 100 days after treatment. On the