The sulfonylurea herbicides comprise a group of compounds designed to control
broad-leaved weeds and some grasses in a variety of crops. The herbicides have
become popular because of their low application rates (10-40 g/ha), low
mammalian toxicity, and unprecedented herbicidal activity. We present a review
of the fate and behaviour of these herbicides in soils with particular
reference to alkaline soils of Australia. The review shows that the low
application rates of sulfonylurea herbicides continue to present an analytical
challenge, although in recent years a number of new methods capable of
detecting them at very low concentrations have been developed. A range of
analytical methods is available, including high performance liquid
chromatography, gas chromatography, immunoassay, and bioassay. However,
analytical sensitivity required to detect trace levels of these herbicides
continues to pose problems in routine detection of herbicide residues in
soils. The review reveals that there are no reports of studies of the
behaviour of sulfonylureas in soils with pH >8·2. This is of
particular significance to Australian conditions because a number of
Australian soils are even more alkaline, and the pH(water) in subsoils can be
as high as 10· 2. Sorption of sulfonylureas is pH-dependent and has a
strong negative correlation with pH. At pH >8·0 sorption is very
low. In acid soils, however, sorption of chlorsulfuron, metsulfuron-methyl,
and triasulfuron is strongly influenced by the soil temperature, clay content,
and, particularly, organic matter content. The principal modes of degradation
of the herbicides are acid hydrolysis and microbial degradation with the
latter being the only major pathway in alkaline soils. Hydrolysis of the
sulfonylureas is more rapid under acidic conditions (pH 4{7), and the
data suggest that hydrolysis is likely to be very slow in alkaline soils. Data
from other countries suggest that the half-life of chlorsulfur on increases
exponentially with pH, and that it is also influenced by variations in the
temperature and water content of the soil. Being acidic in nature, the
herbicide molecules become anionic at high pH and can move to a considerable
depth in the soil profile by leaching. Movement of the sulfonylureas in soil
is largely influenced by organic matter content and soil pH and the reviewed
data show that sulfonylureas have substantial leaching potential in the sandy
alkaline soils of Australia. This is likely to result in increased persistence
in alkaline subsoils lacking in organic matter and biological activity.
Computer models to predict the persistence and movement of the sulfonylureas
are available; however, additional input parameters are required to predict
accurately the behaviour of specific herbicides in alkaline soils under
Australian conditions. Since new herbicides with chemistry similar to existing
sulfonylureas are increasingly likely to be available for use, there is a need
to develop comprehensive understanding of their fate, behaviour, and impact on
Australian cropping and ecological systems.
COMPARATIVE ASSESSMENT OF SOME PHYSICAL PROPERTIES AND ORGANIC MATTER CONTENT OF SOILS DERIVED FROM DIFFERENT PARENT MATERIALS IN AKWA IBOM STATE, NIGERIA
