REACTIONS OF COPPER-SULFATE WITH WETLAND-TARO SOILS IN HAWAII

The environmental impact of copper sulfate (CuSO4 . 5H(2)O) must be ev aluated before the chemical can be registered as a pesticide to contro l the apple snail (Pomacea canaliculata) in Hawaii's wetlands. To help achieve this goal, we investigated the sorption-desorption reactions of CuSO4 . 5H(2)O with six wetland-tare soils (Tropaquepts) of Hawaii. Our results indicated that: (i) copper (Cu) was sorbed rapidly: 98.0- 99.9% of the added Cu was removed from solution within one hour when t he loading rate was less than or equal to 300 mg Cu kg(-1) [initial Cu concentration = 30.0 mg Cu L-1 or 12 kg (ha.cm)(-1) as CuSO4 . 5H(2)O which is 10 times the maximum recommended rate of pesticide applicati ons, (ii) Cu sorption increased as soil pH increased from 5.0 to 8.0, and (iii) sorption capacity varied from 210 mg Cu kg(-1) in a Tropaque pt from Kauai Island to 500 mg Cu kg(-1) in another Tropaquept from Ma ul Island, after seven days of incubation at soil-solution pH 6.0 and total solution Cu concentration of 0.10 mg Cu L-1, a Cu level deemed t oxic to some living organisms. It appears that more Cu was sorbed (les s Cu remained in solution) if the soil contained high organic carbon ( C) and low indigenous Cu. also, there was an inverse relationship betw een Cu sorption and desorption by the soils tested: the more Cu a soil can sorb, the tighter it holds Cu, and the less Cu it releases. Since soil pH increases by 1 to 1.5 units upon flooding and Cu sorption inc reases with increasing pH, the recommended practice of flooding the so il for at least 48 hours between CuSO4 . 5H(2)O application and crop p lanting should be followed.