126
Floyd M. Ashton
than 1 year following the last application. This suggests that enrichment should
not reduce the efficacy of a herbicide applied to the same site only one time
each year.
Audus (1964) suggested that the mechanism of enrichment could result from
mutation or inducible enzyme formation but favored the latter hypotheses. The
induction of enzyme formation in microorganisms has been proposed for several
herbicides (Kaufman and Kearney, 1976). The proliferation of the population
of microorganisms responsible for the degradation of a given herbicide by either
mechanism is also an important aspect of enrichment.
It is well known that the adsorption of herbicides to soil components reduces
their phytotoxicy. Higher rates of application are often required to produce
adequate weed control in soils containing high amounts of clay (mainly kaolinite
and montmorillonite) or moderate amounts of organic matter. Many herbicides
are totally ineffective at reasonable rates in soils with high amounts of organic
matter, i.e., peat and muck soils. Adsorption of herbicides to soil components
can also reduce their biodegradation. Perhaps the best example of this is diquat.
It has been demonstrated that diquat is readily degraded in solution culture but
is not degraded when adsorbed to the clay montmorillonite (Weber and Coble,
1968). Diquat, a postemergence herbicide, is essentially nonphytotoxic in most
soils; some phytotoxicity can be shown in very sandy soils.
5.3.2. Nonbiodegradation
Although biodegradation is the major topic of this book, dealing briefly
with nonbiological degradation, an integral part of the persistence of herbicides
in the environment, is also appropriate. Recent books and reviews deal with the
nonbiological degradation of organic molecules (Armstrong and Konrad, 1974;
Crosby, 1976; Goring and Hamaker, 1972; Guenzi, 1974).
The nonbiological degradation of herbicides occurs in the air, water, and
soil. They are altered by both chemical and physical processes. They are subject
to oxidation, reduction, hydrolysis, epoxidation, nucleophilic displacement, and
free-radical-induced reactions as well as photodecomposition which occurs in
the air, water, and exposed surfaces, including soil and plant surfaces. Initial
degradative reactions may occur by nonbiological processes and further degradation carried out by biological means, or vice versa.
5.4. MAN'S ACTIVITIES
Man can alter the persistence and degradation of herbicides in the environment. His major methods are proper herbicide selection, correct application rate
selection, effective application, and manipulation of controllable environmental
Floyd M. Ashton
than 1 year following the last application. This suggests that enrichment should
not reduce the efficacy of a herbicide applied to the same site only one time
each year.
Audus (1964) suggested that the mechanism of enrichment could result from
mutation or inducible enzyme formation but favored the latter hypotheses. The
induction of enzyme formation in microorganisms has been proposed for several
herbicides (Kaufman and Kearney, 1976). The proliferation of the population
of microorganisms responsible for the degradation of a given herbicide by either
mechanism is also an important aspect of enrichment.
It is well known that the adsorption of herbicides to soil components reduces
their phytotoxicy. Higher rates of application are often required to produce
adequate weed control in soils containing high amounts of clay (mainly kaolinite
and montmorillonite) or moderate amounts of organic matter. Many herbicides
are totally ineffective at reasonable rates in soils with high amounts of organic
matter, i.e., peat and muck soils. Adsorption of herbicides to soil components
can also reduce their biodegradation. Perhaps the best example of this is diquat.
It has been demonstrated that diquat is readily degraded in solution culture but
is not degraded when adsorbed to the clay montmorillonite (Weber and Coble,
1968). Diquat, a postemergence herbicide, is essentially nonphytotoxic in most
soils; some phytotoxicity can be shown in very sandy soils.
5.3.2. Nonbiodegradation
Although biodegradation is the major topic of this book, dealing briefly
with nonbiological degradation, an integral part of the persistence of herbicides
in the environment, is also appropriate. Recent books and reviews deal with the
nonbiological degradation of organic molecules (Armstrong and Konrad, 1974;
Crosby, 1976; Goring and Hamaker, 1972; Guenzi, 1974).
The nonbiological degradation of herbicides occurs in the air, water, and
soil. They are altered by both chemical and physical processes. They are subject
to oxidation, reduction, hydrolysis, epoxidation, nucleophilic displacement, and
free-radical-induced reactions as well as photodecomposition which occurs in
the air, water, and exposed surfaces, including soil and plant surfaces. Initial
degradative reactions may occur by nonbiological processes and further degradation carried out by biological means, or vice versa.
5.4. MAN'S ACTIVITIES
Man can alter the persistence and degradation of herbicides in the environment. His major methods are proper herbicide selection, correct application rate
selection, effective application, and manipulation of controllable environmental