The adsorption and degradation of glyphosate in five Hawaiian sugarcane soils*

Abstract

Summary: The rate of aerobic evolution of 14 CO 2 from 14 C‐glyphosate labelled in the methylphosphonyl carbon, varied 100‐fold within a group of five Hawaiian sugarcane soils. The rate depended inversely on the degree of soil binding, probably associated with the phosphonic acid moiety, and to a less certain extent on soil pH and soil organic matter. After an initial rapid degradation, the rate of 14 CO 2 evolution in three soils reached a constant at 16–21 days which continued to the 60‐day termination. The other two soils showed a continually decreasing rate throughout. Two soils released over 50% of the labelled carbon in 60 days, a third released 35%, while the remaining soils released 1.2 and 0.8% respectively. Labelled carbon in the soils after 60 days consisted of glyphosate and one metabolite, aminomethyl‐phosphonic acid, with glyphosate predominating in high fixing soils. The 14 C could be extracted almost completely with NaOH solution, and remained mainly in solution after acidification.