organic matter and clay content. Studies by Hill (32) showed that the clay content, type of 
clay, and organic matter of soils influenced the amount of monuron adsorbed. Adsorption 
increased as clay content or organic matter increased. Approximately 150 p.p.m. was 
required on a bentonite clay to give 1 p.p.m. in the soil solution whereas less than 1 
P-p-m. was required on a kaolinitic clay to give 1 p.p.m. in solution. 
The herbicidal activity of the phenylureas was correlated inversely with soil or- 
ganic matter, total clay, and cation-exchange capacity (47, 56). Multiple regression 
analyses suggested that soil organic matter was most important in toxicity reduction 
of monuron, diuron, 3-phenyl-1,1-dimethylurea [fenuron], and 3-(3,4-dichlorophenyl)-1- 
methylurea [ DMU]. 
Variations in the effective dosage ranges among several soils suggested greatest 
adsorption of diuron and DMU and least adsorption of monuron and fenuron (47). Coggins 
and Crafts (15) showed that clay suspended in solutions of the phenylurea herbicides 
reduced the toxicity to barley. The toxicity of 1l-n-butyl-3-(3,4-dichlorophenyl)-1- 
methylurea [ neburon] was altered most, and alteration of toxicities of DMU, diuron, 
monuron, and fenuron followed. In general, water solubility and adsorption were in- 
versely related. In a recent report Leopold, et al. (37) demonstrated an inverse relation 
between solubility of several chlorinated phenoxyacetic acids and their adsorption on 
charcoal. 
In an aqueous medium 2-chloro-4,6-bis(ethylamino)-s-triazine [simazine ] was ad- 
sorbed to a cation exchanger and to activated charcoal but not to an anion exchanger (48). 
Soil toxicity tests with the s-triazines suggested considerable variation in soil adsorption 
of these compounds. The effect of soil organic matter, clay content, cation-exchange 
capacity, and pH on the phytotoxicity of simazine was investigated in detail. Soil organic 
matter appeared to alter the initial toxicity of simazine in soils most. 
In soil systems adsorbed herbicides are probably gradually desorbed as leaching, 
chemical, and biological degradation; and absorption by plants reduce the concentration 
in the soil solution. However, the adsorption-desorption relations of herbicide molecules 
in soils and the importance of these phenomena in the movement of herbicides in soils 
have not been adequately investigated. 
Leaching.--The movement of herbicides in soils depends on or is influenced by 
geveral factors. Upchurch and Pierce (57, 58) indicated that at least two steps are in- 
volved in the movement of an herbicide downward in soil: (a) Entrance of the herbicide 
into solution and (b) adsorption of the herbicide to soil particles. Entrance into solution 
could occur from solid particles of the herbicide or from colloidal particles with ad- 
sorbed herbicide molecules. These two. processes, solution and adsorption, may be 
affected by several variables. 
The solubilities of herbicides and of salts of herbicides that may form in the soil 
are important properties affecting leaching (32, 41, 43). Minarik (41) discussed the 
leachability of 2,4-D and its salts. He pointed out that the calcium, magnesium, potas- 
sium, sodium, and ammonium salts of 2,4-D are more soluble in water than the acid, 
whereas salts of heavy metals such asironand copper are less soluble than the acid. The 
equilibrium status of the several forms of 2,4-D in the soil probably affects the leaching 
rate of 2,4-D. However, Smith and Ennis (53) did not measure a difference in the move- 
ment of the acid, the triethanolamine salt, and the sodium salt of 2,4-D in soils. Hill (32) 
concluded that the lower water solubility of diuron compared to monuron resulted in 
slower movement of diuron that monuron in soils. Diuron is adsorbed more strongly 
than monuron and differences in adsorption probably contribute to differential leaching 
of these two compounds. 
An herbicide that is strongly fixed in soils should leach less readily than one that is 
not so tenaciously fixed (2). The adsorptive capacity of soil is influenced by soil organic 
matter and the amounts and types of clay minerals. The adsorption process is influenced 
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