Graduate Thesis Or Dissertation
 

The persistence of five herbicides in Hawaiian soil

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  • The persistence of DCPA, trifluralin, diphenamid, diuron, and prometryne was studied in three soils which were located at the Kula, Poamoho, and Waimanalo Experiment Stations. The order of decreasing persistency was diuron, prometryne, trifluralin, DCPA, and diphenamid. Diuron and prometryne activities were recorded for at least six months and diuron phytotoxicity was noted in the Poamoho plot, one year after application. Degradation was faster when the herbicides were applied the second time in the same plots. For example, DCPA was inactivated between 60-80 days after application in the Poamoho soil, in the second application, inactivation took place in 40-60 days. This indicated that herbicide accumulation or build up was not an important factor in successive croppings. Tilling the soil did not seem to affect inactivation with the exception of prometryne. The bio-assay plants in the prometryne plots showed less phytotoxic symptoms than those grown in soils from untilled areas. Inactivation was most evident in the Waimanalo soil which has montmorillonite as one of the predominant clays. Adsorption and micro-degradation appeared to be the dominant factors in degradation. Inactivation was not related to herbicide solubility, even under high rainfall conditions. Photo-decomposition and volatilization were minimized by rainy periods soon after application. The data indicated that sensitive crops like lettuce could be resown in soil treated with diphenamid, DCPA, and trifluralin at 20 to 40, 20 to 60, and 40 to 80 days, respectively. A phytotoxicity study at Poamoho and Waimanalo indicated that more variations existed in toxicity symptoms between lettuce and cucumber than between soil types of the two locations. Subjective rating of the bio-assay crops (lettuce and cucumber) appeared to be a satisfactory method of evaluating phytotoxicity.
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