AMMONIUM THIOSULFATE EFFECT ON HERBICIDE LONGEVITY IN SOIL

Soil-applied herbicides often are mixed with liquid fertilizers to reduce application cost and increase time efficiency. Ammonium thiosulfate (ATS, 12-0-0-26S), a common liquid S fertilizer, can reduce microbial reaction rates in soil. The purpose of this study was to determine the influence of ATS on duration of effectiveness of soil-applied, microbially-degraded herbicides. Greenhouse and growth-chamber experiments were conducted using a coarse-loamy Pachic Udic Haploboroll and a coarse-silty Aeric Calciaquoll. The ATS at 0.3 to 0.38 mL kg-1 soil extended the longevity of weed control by 2,4-D [(2,4-dichlorophenoxy)acetic acid], alachlor [2-chloro-N-(2,6,-diethylphenyl)-N-(methoxymethyl)acetamide], EPTC (S-ethyl dipropylcarbamothioate), cinmethylin [exo-1-methyl-4-(1-methylethyl)-2-[(2-methylphenyl)methoxy]-7-oxabicyclo[2.2.1]heptane], and triallate [S-(2,3,3-trichloro-2-propenyl)bis(1-methylethyl)carbamothioate]. The ATS had little effect on longevity of weed control by butylate [S-ethyl bis(2-methylpropyl)carbamothioate] or metolachlor [2-chloro-N-(2-methoxy-1-methylethyl)acetamide]. The lowest rate of ATS needed to extend 2,4-D and EPTC efficacy was 0.15 mL kg-1 soil (about 8 gal A-1 or 26 kg S ha-1 assuming a tilled soil bulk density of 1.0 and a 5-cm incorporation depth). This rate of ATS may be too costly for current use, but the concept deserves further evaluation because of the wide use of ATS and the need to extend the longevity of certain soil-applied herbicides.