Effect of soil moisture on the persistence of soil applied tebuthiuron

The effect of three moisture levels viz., saturation, field capacity and 50% field capacity was studied on the rate of disappearance of methanol extractable tebuthiuron applied to an Alfisol and Vertisol from Hyderabad. Technical grade Tebuthiuron (>98.5% purity) was applied at the rate of 20μG/g of soil and incubated at constant temperature. The samples were extracted by shaking with methanol for 30 minutes, and assayed spectrophotometrically after correcting for soil blanks, at different time intervals ranging from 0–160 days. The concentration‐time curve showed that there were two distinct pathways for disappearance, a faster one and a much slower one. First order kinetics was applied to the disappearance and the rates were resolved graphically, from a semi‐log plot of concentration vs. time. The disappearance pattern is interpreted in terms of a rapid partioning of a herbicide into a “non‐solvent extractable bound phase and a solvent extractable labile phase”; which undergoes degradation. Specific rates for the two events and their implication on the bioavaila‐bility are discussed.

Key words:

• Tebuthiuron
• soil persistence
• bioavailability