Influence of metam sodium on the dissipation and residual biological activity of the herbicides EPTC and pebulate in surface soil under black plasticmulch

Metam sodium is a potential replacement for methyl bromide, which is used t o control soil pests. Metam sodium rapidly breaks down in the soil to form methylisothiocyanate (MITC). Dissipation of the herbicides EPTC and pebulat e in a silt loam soil under plastic mulch in the absence and presence of me tam sodium was examined in field experiments in 1998 and 1999 at Knoxville, Tennessee. EPTC half-life (DT50) was 9 d, but when applied in conjunction with metam sodium DT50 increased to 22 d. Similarly, average pebulate DT50 was 8 d and increased to 23 d when applied in conjunction with metam sodium . This increase in herbicide DT50 with the addition of metam sodium is thou ght to be due to a reduction in soil microorganisms that degrade EPTC and p ebulate. EPTC applied with metam sodium injured tomato plants and reduced t otal crop yield more than EPTC, pebulate, or pebulate with metam sodium. Th e increased tomato injury may have been related to the greater and prolonge d activity of EPTC and slower EPTC dissipation in the presence of metam sod ium or MITC.