- The herbicides Diquat (1, 1'-ethylene-2, 2'-dipyridylium
dibromide), Paraquat (1, 1'-dimethyl-4, 4'-dipyridylium dichloride),
and Gramoxone (1, 1'-dimethyl-4, 4'-dipyridylium dimethyl sulphate), and the spreader X-77, a mixture of alkylaryl-polyethylene
glycol, free fatty acids and isopropanol, were tested by
several soil microbial methods in an attempt to find out if concentrations of these compounds at field ratio, and at higher ratio
that might result from over-application or reapplication, would
either stimulate or inhibit the microorganisms and hence affect
Three different soils were used: Chehalis silt loam, Woodburn
silt loam, and Chehalis silty clay loam. Plate counts of bacteria
and molds in the soils treated with 0.25, 0.5 and 2.5 ppm of the herbicides for periods ranging from 0 to 20 days averaged
higher than counts from untreated control samples.
The dipyridylium slightly depressed ammonification and
sulfur oxidation in Chehalis silty clay loam soil. In the two silt
loam soils minor increases resulted.
Nitrification was stimulated by the lowest rate, 0.25 ppm,
of the herbicides; the higher rates had little influence.
A respiration study using carbon dioxide evolution as an
index of the metabolic activity of microorganisms in the soil
showed that addition of the herbicides resulted in some decrease
in microbial activity. The spreader X-77, depressed CO₂ evolution only at the excessive rate of 100 ppm in one soil; otherwise
it had essentially no effect.
Four of six bacterial cultures were able to use Gramoxone
as the sole source of carbon and nitrogen in synthetic media.
Nitrogen in this herbicide promoted growth better than did the
carbon. This should be true for Paraquat also, which differs
only in the anion.
Analytical recovery of Paraquate and Gramoxone from the
different soils was correlated with soil type, more specifically
with cation exchange capacity, amount and kind of clay, and
content of organic matter. Recoveries ranged from 54 to 114
percent depending upon concentration and soil type. Recoveries generally were greater for lower concentrations. Higher concentrations of the herbicides were recovered less completely.
Gramoxone appeared to persist in soil longer than Paraquat. In
complete recoveries after incubation are attributed in part to
some microbial degradation of the herbicides, and in part to
certain soil properties responsible for losses in various stages of
The most significant conclusion from these results is that
the dipyridylium herbicides and X-77 spreader when used at
recommended rates would produce no serious changes in microbial
activities and their contributions to soil fertility.