|Abstract or Summary
- Napropamide [2-(alpha-naphthoxyl)-N,N-diethylpropionamide], a
preemergence herbicide, is widely used in the United States on a
variety of tree fruits, nut crops, vegetables, and mint for annual
grass and broadleaf weed control. When napropamide has been left on
the soil surface without incorporation through irrigation or mechanical means, inconsistent results have been obtained. This study was
conducted (a) to determine the extent to which napropamide photodecomposes
under field conditions, and (b) to attempt to correlate the
rate of napropamide loss with the amount of ultraviolet radiation
received at the soil surface. If successful, it would be possible
to predict napropamide photodecomposition at various locations and
times of year, and therefore take steps to avoid herbicide loss.
Studies using a spectrophotometer indicated that napropamide
absorbs light at wavelengths from 192 to 324 nm. Aqueous solutions
of napropamide were exposed to fluorescent sun lamps for different
periods of time. The ultraviolet absorption spectra of the aqueous
solutions changed rapidly with time, indicating that photodecomposition was taking place. A radiometer was used in the field studies to measure cumulative energy from the sun at wavelengths from 295 to 385 nm. At
various cumulative levels, the percentage of napropamide remaining
on the soil surface was measured chemically or by bioassay to determine the correlation between napropamide disappearance and ultra-violet radiation received. Comparing napropamide loss at equal
amounts of received radiation, demonstrated that napropamide disappeared most quickly in August, more slowly in June, and most
slowly in October, indicating a poor correlation between rate of
napropamide loss and ultraviolet radiation received.
Additional soil samples were covered with a Teflon film during
the study to prevent volatilization. Napropamide disappeared at
the same rate with or without the film, indicating that volatilization was not a factor in napropamide loss. Soil sprayed with
napropamide was placed in an oven, in the absence of light, at
temperatures of 27 C and 60 C. No loss of napropamide was measured
after 4 days, again indicating that volatilization was not a major
factor in napropamide loss.
Soil samples were also placed under a polycarbonate plastic
which absorbed ultraviolet radiation below 385 nm. Unexpectedly,
the rate of loss of napropamide under the polycarbonate plastic was
the same as on soil exposed to full sunlight.