Graduate Thesis Or Dissertation
 

Studies of napropamide decomposition on the soil surface

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  • 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.
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