Enhanced biodegradation of soil-applied pesticides has long been correlated with soil pH above ca 6.5-7.5, but the possibility of confounding or interdependence with calcium, given that soil calcium concentration increases exponentially as pH rises above that range, has not previously been studied. Enhanced biodegradation of the broad-spectrum biocide metam-sodium was readily induced de novo in a naturally acid sandy soil (pH 4.2 measured in 0.01 M CaCl2) by multiple treatments, but only when the pH and calcium concentration were raised simultaneously using calcium carbonate (lime). Enhanced biodegradation was not induced when soil pH alone was raised with magnesium carbonate, nor when calcium alone was raised using calcium chloride. In limed sand treated monthly for 12 months, the degradation rate increased to where dissipation was complete within 24 h of application after the fifth metam-sodium treatment at pH 7.8 and after the eighth metam-sodium treatment at pH 6.8. Pesticide concentration was reduced, but not eliminated, at pH 5.8 and was unchanged at pH 4.8. When metam-sodium was applied bi- and tri-monthly, the degradation rate also increased when soil pH was raised with calcium carbonate, but to a lesser extent than with monthly applications. In an acid loam soil amended to the same pH values with calcium carbonate and treated monthly, there was no correlation between soil pH or calcium concentration and degradation. The results reveal the crucial interdependence of pH and calcium concentration in enhancement of biodegradation of soil-applied pesticides, but confirm that the phenomenon ultimately depends on interaction with soil type and frequency of application factors, all of which probably together act to affect the abundance, composition and activity of the soil microbial biomass.
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