EFFECT OF THE NATURE OF EXOGENOUS ORGANIC-MATTER ON PESTICIDE SORPTION BY THE SOIL

A study was carried out on the sorption of two sparingly water-soluble pesticides (diazinon and linuron) by a sandy loam soil modified with different exogenous organic materials (EOMs) containing humic-like sub stances: city refuse compost (CRC), peat (P), commercial ''humic'' aci d (HA), liquid ''humic'' acid (LHA), and two (nonhumic) model compound s (surfactants), tetradecyltrimethylammonium bromide (TDTMA) and sodiu m dodecyl sulphate (SDS), before and after 2- and 8-month incubation p eriods with the soil. In all cases, the isotherms fitted the Freundlic h sorption equation (x/m = KCen), generally with r(2) values greater t han 0.99. The value of the sorption constant K for the natural soil wa s 8.81 for diazinon and 2.29 for linuron. These values increased signi ficantly for EOM modified soils with respect to natural soil, with the exception of the samples modified with SDS and LHA, in which cases th ey decreased, possibly due to the micellar properties of these compoun ds. Incubation of EOMs with soil increased their sorption capacity: th e K-oc values were increased proportionally to the incubation time for both pesticides and for all treatments carried out. Accordingly, the sorption capacity of hydrophobic pesticides increases with the degree of evolution in the soil of EOMs with ''humic''-type compounds, possib ly due, among other causes, to the increase in the EOMs' colloidal pro perties and the modifications occurring in the hydrophobic-hydrophilic characteristics of the soil surfaces. The main conclusion is that app lication to the soil of carbon-rich wastes, especially those with a hi gh degree of maturity, may offer an important strategy for reducing pe sticide leaching and for eliminating pesticide residues from soil with the use of anionic surfactants.