BOUND RESIDUES OF PESTICIDES IN SOIL AND PLANT - A REVIEW

The use of C-14-labelled pesticides has shown that a sizeable portion (20-40%) of applied activity may remain on soil or plant in non-extractable or bound form (bound residues). It is presented that bound residues generally increase with time and ageing makes the binding stronger. Further, in soil, bound residues are associated with clays and humic acid and with lignin in plants. Various methods for estimation and characterization of bound residues are briefly described. The total C-14-bound residues can be easily estimated by combustion method. However, a complete characterization of bound residues has not been possible by any method because of loss or degradation during extraction and incomplete recoveries. The results of the analysis show that bound residues in soil and plant samples contain both parent compound as well as its degradation products. Significance of bound residues has been elucidated in terms of their toxicity and bioavailability. In general, the toxicity of the bound residues is much lower than in unbound or free residues. Enough evidence is available to show that only a small fraction (1-10% of the total bound residues) is bioavailable. It is discussed that soil microbes can slowly release and mineralize bound residues and therefore, accumulation of bound residues in nature is not reported. However, these studies on bioavailability or toxicity are limited to only few compounds. More research is required to draw any valid conclusion.