PHYTOTOXICITY OF SOME CHLOROANILINES AND CHLOROPHENOLS, IN RELATION TO BIOAVAILABILITY IN SOIL

Soil adsorption and the effect of four chlorophenols and three chloroa nilines on the growth of lettuce (Lactuca sativa) were determined in t wo soil types differing in organic matter content and pH. Adsorption i ncreased with increasing organic matter content of the soils. Phytotox icity, based on dosed amounts, was significantly higher in the soil wi th the low level of organic matter. This difference could be reduced b y recalculating the EC(50) values for the effect of the test substance s on plant growth in mg kg(-1) dry soil towards concentrations in mg L (-1) pore water using data from soil adsorption experiments. For penta chlorophenol only this recalculation increased rather than decreased t he difference between the two soils, however, when the EC(50) values f or pentachlorophenol were corrected for the difference in soil pH, alm ost the same values resulted for both soils. Calculated EC(50) values on the basis of pore water concentrations appeared to be in good agree ment with values determined in nutrient solution tests. These results indicate that, for plants, the toxicity and therefore the bioavailabil ity of organic chemicals in soil mainly depend on the concentration in the soil solution, and can be predicted on the basis of sorption data . Attempts to develop QSARs relating log EC(50) values in mu mol L(-1) pore water with lipophilicity (expressed as the octanol/water partiti on coefficient: log K-ow) of the test substances resulted in a statist ically significant relationship. This relationship was further improve d by correcting the chlorophenol data for dissociation effects.