Hill, et al. (33) reported that a soil bacterium of the Pseudomonas group was capable 
of oxidizing monuron particularly in the presence of yeast extract. 
One group of herbicides, the esters of chlorophenoxy alcohols, becomes herbicidally 
active only on contact with the soil. Inwarm, moist soil sodium, 2,4-dichlorophenoxyethyl 
sulfate | sesone ] is hydrolyzed to 2,4-dichlorophenoxyethanol in the presence of either 
microorganisms or acids (12, 13). The hydrolysis by microorganisms was attributed to 
acids secreted during their metabolism. The ethanol product is oxidized in the soil to 
2,4-D, the active entity. 
In experiments conducted by the senior author, 2-chloro-4,6-bis(diethylamino)-s- 
triazine [ chlorazine ] mixed in the soil became more toxic to seedling oats with time. 
This trend reversed after several months, the time depending on the soil type and con- 
centration, and thereafter the herbicidal activity of cultures containing chlorazine de- 
creased with time. The increase in toxicity could not be accounted for completely as a 
response to growing conditions in the greenhouse. Autoclaving the soil prior to treatment 
retarded the rate of onset of increased toxicity. If one ethyl group was lost from either 
or both of the amino substitutions, one or more compounds much more toxic than chlora- 
zine would be formed. Perhaps formation of one or both of these compounds did occur in 
the soil. 
The rates of inactivation of IPC, CIPC, and amitrole in the soil depended on the 
initial concentration of the herbicides; and the inactivation of these herbicides appeared 
to follow a first-order reaction (ll). The rate of disappearance of monuron from soil 
was proportional to the concentration (33). Hill, et al. (33) concluded that although soil 
moisture and temperature often altered the rates of inactivation of monuron and 3-(3,4- 
dichlorophenyl)-1,1-dimethylurea [diuron], the first-order equation was probably ap- 
plicable under usual field conditions. When monuron and diuron were applied at rates of 
1 and 2 pounds per acre in more humid regions of the United States, major parts of the 
herbicides were inactivated each year. Accumulation from applications on the same soil 
2 years in succession was negligible. Rahn and Baynard (45) found that monuron applied 
at 3.6 pounds per acre in two applications for 3 years in succession did not persist from 
one year to the next. When applied at 6.4 pounds per acre, monuron toxicity persisted 
from one year to the next, but no accumulation occurred. Research conducted in the arid 
Southwest during the last 7 years indicated that monuron and diuron did not accumulate 
Significantly from successive annual applications at rates used for selective weed con- 
trol in cotton. Some carryover often occurred, and the amount of carryover appeared 
to be related to weather conditions. 
Current research indicates that the solvent used in the application of an herbicide 
may have a profound influence on the persistence of herbicidal activity (18). Ethel 
N,N-di-n-propylthiolcarbamate [EPTC ]was applied in several solvents and incorporated 
into the soil. At weekly intervals up to 6 weeks after treatment, the soils were assayed 
by the use of oat plants as indicators of toxicity. When the commercial formulation of 
EPTC was applied in water, growth of oat plants seeded 6 weeks after treatment were 
markedly inhibited on flats which received 1 and 2 1b./A. The persistence of technical 
EPTC applied in acetone was comparable to that of the commercial formulation applied 
in water. However the rate of inactivation of technical EPTC applied in kerosene was 
much more rapid than that of the commercial formulation applied in water. Four weeks 
after treatment the 2 lb./A rate of technical EPTC applied in kerosene did not inhibit 
growth of oats. 
Persistence of several groups of herbicides in the soil is related to halogenation of 
the benzene ring. This relation was demonstrated for certain chlorinated phenoxyacetic 
acids, carbamates, and phenylureas (21, 22, 23, 50). The results of Alexander and Aleem 
(3) indicated that resistance of chlorinated phenoxyalkyl carboxylic acid herbicides or 
their derivatives to microbial degradation was governed by the position of the halogen 
rather than by the number of halogens onthe ring and that the linkage and type of aliphatic 
side chain also influenced susceptibility to microbial breakdown. 
172 
