Geoscience Reference
In-Depth Information
The eventual removal of herbicides from soil is, of course, a vital prerequisite for the
continued use of soil for growing plants. The same applies to the residues of any oth-
er potent synthetic pesticides. Some of these chemicals, including formaldehyde, are
removed or degraded by various processes such as volatilization, leaching by rain,
photochemical oxidation, hydrolysis and other chemical means, but most important
of all are the activities of the soil microorganisms. Fortunately, soils harbour a most
abundant microflora and fauna. Although their numbers and diversity may fluctuate
in response to constantly varying conditions, there is a continuing turnover of organ-
ic material involving the breakdown of many complex substances including synthetic
compounds applied by man.
The first synthetic weedkillers were the phenoxyacetic acid compounds 2,4-D
and MCPA, known as growth hormone herbicides from their mode of action. Even
before their practical use, the possibility of their decomposition by microorganisms
had already been recognized. The half lives of these two substances in soil rarely ex-
ceed four to six weeks under normal conditions and with recommended dosage. The
related herbicide 2,4,5-T is much more stable; it is a useful agent for dealing with
woody plants, but its persistence in soils is a disadvantage. (It is no longer used in Bri-
tain because of the danger to spray operators from impurities which could be present)
The microbial breakdown of the phenoxyacetic acid herbicides has been the
most investigated, and a number of bacterial species and some fungi have been shown
to be responsible. The interactions between soil microorganisms and these herbi-
cides are interesting because they illustrate the effects of different factors on pesticide
breakdown in soils. When 2,4-D, for example, is sprayed on the soil for the first time,
there is generally a lag during which the concentration of herbicide in the soil solution
remains more or less unchanged. This period may last for a few weeks, depending on
temperature and soil conditions. Then follows a shorter period, perhaps a few days,
during which the herbicide concentration steadily declines until little if any remains.
During the lag period it is generally accepted that potential microbial decomposers
are in some way adapting themselves, i.e. developing suitable enzyme systems, to
metabolize the unfamiliar chemical (
see here
). Such adapted microorganisms can use
the herbicide molecules as nutrients and so have an advantage over other organisms.
A population of adapted organisms is then produced corresponding to the period of
herbicide decline.
Addition of more herbicide allows further growth of the population of adapted
organisms without any lag period until all herbicide is consumed. Without any further
addition of herbicide, the 2,4-D-decomposing microbial population can remain in the
soil for a long time, declining only very slowly. When grass turf was treated monthly
with large doses of 2,4-D for two years, one of us (N.W.) detected the presence of
