Íslenskar landbúnaðarrannsóknir - 01.09.1982, Page 15

RESIDUE OF LINURON IN SOILS 13
source (Hill et al. 1955) The rate of
microbial degradation is dependant on
several soil factors - organic matter,
moisture and temperature are all posi-
tively correlated with microbial activity.
iii) Decomposition by light. This can
amount to about a 30% loss oflinuron from
an aqueous solution during the summer
months (Plimmer 1970).
Overall this could indicate that under
Icelandic conditions where soil tempera-
tures are low and precipitation high,
degradation and leaching losses could be
higher in sandy soil thus restricting linuron
accumulation in this soil type. The loamy
soils, however, not only receive a larger
amount of linuron each spring, but are also
subject to less leaching and bacterial
decomposition than are the sand soils
because of lower soil temperature and
greater adsorption in loam soils.
An average linuron residue of 1.9 ppm
for the 12 loamy soils in this investigation,
12 months after the previous application,
can be considered high in comparison to
the results of other workers. In numerous
experiments, soil linuron levels do not
exceed 0.5 ppm after 3.5 — 5 months
following application of orthodox doses of
the herbicide (0.75 - 1.5 kg/ha) (Maier-
Bode and Hártel 1981).
The residue level in the 20 sandy soils of
this investigation does not exceed 0.6 ppm
after only 3 months following application
and cannot therefore be considered high
relative to foreign results.
This investigation does not give any
indication as to whether there is clinical or
subclinical damage to the crop at these
levels of soil linuron. The literature pro-
vides few results of linuron bioassays using
potatoes. In one such paper (Caverley
1978) the damage levels for potato crops
are quoted as follows.
Direct comparison with the icelandic
results, however, is difficult due to inevit-
able differences in soil characteristics.
There is no obvious difference in residue
level between potatoes grown in the loamy
soils and those from the sand type soils
(table 5). The levels are comparable with
those found in the literature for potato
tubers (Maier - Bode and Hártel 1981)
i.e.<0.1 ppm. These results give no indi-
cation as to whether the linuron residue
levels have had an effect on the crop since
TABLE 6.
Results of a bioassay using potato as test plant (Caverley 1978).
6. TAFLA.
Niðurstöðurfrá athugun, þar sem kannað var samband milli linurons íjarðvegi og skaðaeinkenna á kartóflum (Ca verley
1978).
Soil type Fine sandy loam Silt loam
Jarðvegsgerð Leir Fínsendin mold Méla
Organic matter (%) Lífnent efni (%) 5.6 3.1 3.2
Soil residue causing visible damage (mg linuron/kg undried soil) Leifar í jarðvegi er valda sjáanlegri skemmd (mg linuron/kg af óþurrkuðumjarðvegi) 0.85 0.36 0.80