Some herbicides have been extracted from soils and their concentrations determined 
by physical or chemical methods. Methods are available for monuron, amitrole, CIPC, and 
pentachlorophenol [PCP] (10, 11, 30,62). Whiteside and Alexander (61) followed the break- 
down of several chlorinated phenoxy aliphatic acid herbicides in solutions inoculated with 
soil by the disappearance of the specific ultraviolet absorption. 
A physical or chemical assay may be most suitable in one situation and a biological 
assay in another. Both types of analyses are useful in some cases. Rahn and Baynard (45) 
reported that the chemical method for the determination of monuron in soils was accurate 
if the assay was made within a few weeks after application. If soils were chemically 
assayed more than 1 month after treatment, values for monuron concentration were 
greater than those obtained by bioassay. Rahn and Baynard (45) suggested that this ap- 
parent disagreement could be explained since the chemical assay for monuron was based 
on p-chloroanaline, a nonphytotoxic, hydrolytic product of monuron. 
FACTORS ALTERING HERBICIDES IN SOILS 
Factors affecting the movement and persistence of herbicides in soils have been re- 
viewed by several workers (1, 8, 25, 26, 32, 41, 43). Leaching, fixation by soil colloids, 
chemical and microbial decomposition, and volatilization were stressed in one or more 
of these papers. In this discussion microbial action, volatilization, adsorption, leaching, 
chemical reaction, photodecomposition, and absorption by plants will be considered for 
their significance in the performance and fate of soil-applied herbicides. 
Microbial action.--Most organic herbicides subjected to appropriate tests have been 
inactivated more rapidly in soil under conditions favoring growth and proliferation of 
soil microorganisms. Absorption by microorganisms is one of the major pathways by 
which organic herbicides are detoxified. Perhaps 2-(2,4-dichlorophenoxy)propionic acid 
[2-(2,4-DP)], 2,4,5-trichlorophenoxyacetic acid [2,4,5-T], 2-(2,4,5-trichlorophenoxy) 
propionic acid [silvex], and 4-(2,4,5-trichlorophenoxy)butyric acid [4-(2,4,5-TB)] are 
exceptions (7, 61). Optimum oxygen, moisture, temperature, and nutrients favor micro- 
bial activity and also herbicidal detoxication. Numbers of soil microorganisms capable 
of inactivating 2,4-dichlorophenoxyacetic acid [2,4-D] apparently increase when 2,4-D is 
present in the soil (8, 42, 43, 61). Thus repeat applications of 2,4-D were less persistent 
in soil and therefore may be less effective herbicidally than the initial application. With 
the phenylureas and s-triazines such an increase in microbial activity apparently does 
not occur, because soils appear to exhibit about the same capacity for inactivation for 
long periods. Therefore it seems that with the phenylureas and the s-triazines the action 
of soil microorganisms is passive; organisms utilize them but not selectively or pref- 
erentially. Another explanation of this effect is that inactivation of these chemicals is 
catalyzed by heat-sensitive substances occurring in the soil as products of microbial 
activity and that the herbicides are not utilized directly by microorganisms as energy 
sources. 
Bacteria, Bacterium globiforme and Flaviobacterium aquatile, which were capable 
of inactivating 2,4-D were isolated from soil and grown in pure culture (6, 8, 36). Evans 
and Smith (27) isolated a small, Gram-negative, motile soil organism which grew freely 
in a mineral-salt medium containing p-chlorophenoxyacetic acid as the only organic- 
carbon source. They separated 2-hydroxy-4-chlorophenoxyacetic acid and 4-chloro- 
catechol from the culture. The same investigators isolated a Gram-negative, motile rod 
which grew on a mineral-salt, 2,4-D medium. From this culture they separated a pheno- 
lic acid and presented evidence to suggest that the compound was 6-hydroxy-2,4- 
dichlorophenoxy-acetic acid. They hypothesized that hydroxylation of the ring was 
followed by ring cleavage. 
Whiteside and Alexander (61) presented evidence suggesting that 4-(2,4-dichloro- 
phenoxy)butyric acid [4-(2,4-DB)] was converted to 2,4-D by microorganisms in the soil 
and that a microflora capable of quickly inactivating both 2,4-D and 4-(2,4-DB) was 
present in soils which had received 4-(2,4-DB) previously. 
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