The dithiocarbamates, for example 2-chloroallyl diethyldithiocarbamate [CDEC], 
can be broken down in the soil by oxidation and hydrolysis (31). If hydrolysis of CDEC 
precedes oxidation, allyl alcohol is an intermediate; whereas if oxidation precedes hy- 
drolysis, 2-(diethyldithiocarbamyl)acetic acid is an intermediate. The two reactions can 
occur separately or concurrently in the soil. End products of breakdown are formic acid, 
glycolic acid, carbon disulfide, and secondary amines for both reaction pathways. The 
breakdown products may undergo further reaction in the soil. 
Freed, et al. (29) suggested that EPTC was hydrolyzed in water; and according to 
the reaction scheme which they proposed, a secondary amine, carbon dioxide, and ethyl- 
mercaptan were end products. 
Soil treated with 3,5-dimethyltetrahydro-1,3,5,2 H-thiadiazine-2-thione [DMTT] 
evolves formaldehyde, which is thought to be the first product of DMTT breakdown in 
the soil (40). Methylaminomethyldithiocarbamate forms next and activation continues by 
forming monoethylamine, methyl isothiocyanate, and hydrogen sulfide. Monoethylamine 
and hydrogen sulfide react with formaldehyde and form methylaminoethanol, dimethyl- 
aminomethane, and 1,3,5-trithiocyclohexane. Eventually this reaction proceeds to carbon 
dioxide, ammonia, sulfur dioxide, and water. The methyl isothiocyanate and water react 
to give carbon dioxide, bydrogen sulfide, and methylamine; and the methylamine degrades 
into carbon dioxide and ammonia. 
Monuron is thought to be hydrolyzed slowly in the soil to p-chloroamiline (45). 
In the presence of moisture tris-(2,4-dichlorophenoxyethyl)phosphite [ 2,4-DEP ] is 
slowly hydrolyzed step-wise to form one mole of phosphorous acid and three moles of 
2,4-dichlorophenoxyethanol (28). 
The reactions which herbicides and agricultural pesticides in general undergo in 
soils and the products formed are important with respect to residues in soils. Weed 
research scientists should emphasize this phase of herbicide research. 
Photodecomposition.--Less is known about the direct effect of light on the breakdown 
of herbicides than other factors suspected of being involved. However, photodecomposi- 
tion of monuron was demonstrated by Hill, et al. (33). When a solution containing 88.3 
P-pem- of monuron sealed in quartz tubes was exposed for 48 days to sunlight, an 83- 
percent loss of monuron occurred. Hill, et al. (33) concluded that in dry areas of the 
Western United States monuron may be inactivated by ultraviolet irradiation. They sug- 
gested that this factor would account for disappearance of only a small part of the herbi- 
cide in humid regions where frequent rains move it into soils. 
Neburon, diuron, monuron, fenuron, and DMU were applied as alcohol solutions to 
filter paper (14). After the paper dried, it was exposed to ultraviolet light for several 
hours. The herbicides were not visible prior to exposure, but in white light they were 
readily visible after exposure as light tan spots. The compounds were apparently changed 
during exposure. 
The effects of shade, moisture, and position in the soil on the residual activity of 
monuron, diuron, and simazine were investigated in cooperation with the California 
Agricultural Experiment Station. The activity of monuron and diuron disappeared more 
rapidly from soil exposed to the sun than from shaded soil. The activity of monuron and 
simazine disappeared more rapidly from moist soil than from dry soil. Monuron, diuron, 
and simazine were not affected to the same degree by these variables. Soil temperature 
was measured but not controlled in this experiment, and soil temperatures varied con- 
siderably among the treatments during the day. Temperature markedly influences vapor 
pressure and chemical reactions. Therefore the difference in the rate of disappearance 
in shaded soil and soil exposed to the sun cannot be attributed unquestionably to light 
inactivation. 
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