by temperature and the nature of the solvent. The adsorptive characteristics of a com- 
pound are influenced by pH of the solution. Since these factors influence adsorption, they 
must influence movement of herbicides in soils. The organic matter content and soil 
texture are known to influence leaching (20, 24, 25, 26, 32, 35, 44, 47, 52, 53, 58). 
The leachability of DNBP appeared to be influenced by pH. Dowler, et al. (24) con- 
cluded that movement of DNBP in soils was as much a function of soil type and soil 
reacfion as of the amount of rainfall. In experiments by Upchurch and Pierce (58) soil 
temperatures of 5° to 45°C. had little effect on the monuron leached from the upper 
2-inch layer of soil columns. However, greater amounts of monuron were retained by 
the 2- to 8-inch layer at 25°, 35°, and 45°C. than at 5° and 15°C. 
That the amount of rainfall or of water applied as irrigation influences the movement 
of herbicides has been demonstrated by many research workers.Sherburne, et al. (52) 
compared the movement of monuron in soil columns to the movement of compounds in 
chromatography and concluded that the depth of the highest concentration of the herbicide 
in soil columns was a function of the amount of water added to the soil surface. Upchurch 
and Pierce (57) studied the effect of amount, intensity, and frequency of simulated rain- 
fall on the leaching of monuron. The greater the amount of simulated rain the greater 
the movement of monuron. Rainfall intensities of 1/16 to 4 inches per application had 
little influence on the amount of monuron retained in the top 2-inch layer. In the 2- to 
8-inch zone greater accumulation of monuron occurred with low intensities than with 
high. A greater movement of monuron from the upper soil layers occurred as frequency 
of rainfall increased. Approximately half of the frequency effect was attributed to evapo- 
ration of water from the surface of soil columns that received less frequent applications. 
These workers concluded that of the three variables studied, the amount of rainfall would 
be most directly correlated with the distribution of herbicides in soil profiles under field 
conditions although they maintained that intensity and frequency might also be of prac- 
tical importance. 
The effect of the amount of monuron applied on the amount moved by simulated rain- 
fall was also investigated by Upchurch and Pierce (58). Monuron was applied to surfaces 
of soil columns at rates of 0.5, 1, 2, 4, 8, 16, 32, 64, 128, and 256 1b./A, and the applica- 
tions were followed by 4 inches of simulated rainfall. The lowest percent retention (34 
percent) in the 0- to 2-inch layer was found in columns treated with 32 1b./A. Retention 
by the 2-inch layer was increasingly greater as the rate increased and decreased from 
32 lb./A. These workers suggested that the increasing percent retention when the rate 
of application was increased from 32 to 256 1b./A was attributable to the low solubility 
of monuron. 
Molecular size may also be a factor in the movement of herbicides in soils (15, 44). 
The exact pattern of movement of an herbicide in a particular soil would be im- 
possible to predict presently. However, relative rates of movement can be predicted 
for many herbicides. 
Chemical reaction.--The formation of salts of 2,4-D and DNBP and a possible reac- 
tion scheme for inactivation of 2,4-D in soils have been discussed. Relatively little is 
known of the chemical reactions that most herbicides undergo in soils. Hydrolysis, 
oxidation, and formation of complexes are known reactions for certain herbicides. 
Amitrole forms stable complexes with cobalt, copper, nickel, iron, and magnesium. 
Sund (54) suggested that complexing with metal ions in the soil solution was one mecha- 
nism by which amitrole was detoxified. 
The 2-chloroacetamides, for example 2-chloro-N,N-diallylacetamide [CDAA], can 
be hydrolyzed in the soil (31). The chlorine atom and the amide linkage are sites on the 
molecule where hydrolysis may occur. Regardless of the site where hydrolysis begins, 
the end products are glycolic acid and secondary amines. 
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