Íslenskar landbúnaðarrannsóknir - 01.09.1982, Page 14

12 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR
DISCUSSION
It is evident from both the spring and
autumn results that linuron residues are
noticeably higher in the loamy soils than in
the sandy or sandy loam samples. The
average content of linuron (on dry matter
basis) in the spring loamy samples is 1.9
ppm whereas in the spring sandy soils, it is
only 0.2 ppm. In the autumn samples the
average linuron content of the loamy soils
is 2.4 ppm and of the sandy soils, 0.3 ppm.
The level of linuron in soil depends on 3
factors:
a) quantityofherbicide applied to the soil.
b) rate of loss by degradation, evapora-
tion, leaching and photodecomposi-
tion.
c) rate of uptake by plants.
It can be seen from table 1 that on
average about 1.25 kg of linuron is applied
to each hectare of loamy soil each year,
whereas only about 0.5 kg is applied to each
hectare of sandy soil. This is entirely in
accordance with the manufacturer’s
instructions and must be a dominating
factor determining linuron residue level.
This diíference in recommended
application levels allows for the fact that
the organic components in a soil are known
to adsorb linuron thus making it tempor-
arily unavailable for plant uptake (Hance
1965). Soils with a high organic matter
content (e.g. loam) require therefore
higher herbicide doses than soils with less
organic matter (e.g. sand) to have the same
effect. Adsorption of herbicides to soil is a
reversible process. Adynamic equilibrium
exists between adsorption and desorption,
such that removal of herbicide from soil
solution by plant uptake or degradation
results in desorption of more herbicide
from the soil particle surface into the soil
water phase. Adsorption to soil particles is
equivalent therefore to a temporary
inactivation of the herbicide.
The method of extraction used in the
present study does not distinguish between
adsorbed and unadsorbed linuron, but is a
measure of the total potentially available
linuron in the soil. The higher residue
levels in the loamy soils are therefore not
directly comparable to the low levels in the
sand because the organic matter contents
of these two soil types are very diíferent
(table 3).
A comparison of spring results with
autumn results shows higher residue levels
in the latter - on average an increase of0.5
ppm in the loam samples but only 0.1 ppm
in the sandy soils. It can be concluded that
3 months after application to the soil, there
has not been a complete disappearance of
the linuron added that spring. This is in
agreement with other workers results
which show, on average, a 75% loss of
linuron over three months (Boerner 1965).
The principal paths of degradation of a
herbicide such as linuron in the soil can be
summarized as follows:
i) Leaching — since linuron is relatively
insoluble in water it is leached slowly into
lower soil layers. Numerous investigators
have shown that linuron is almost always
retained in the top 10 cm of a loam type soil
but there is a tendency for it to be leached
intodeeper layers ofasandy soil (Homburg
and Smit 1964, Lode 1967).
ii) Degradation by microorganisms. It
has been shown that a combination of
several soil organisms (bacteria, fungi,
algae) can utilize linuron as a nitrogen