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

Metam sodium is a potential replacement for methyl bromide, which is used to control soil pests. Metam sodium rapidly breaks down in the soil to form methylisothiocyanate (MITC). Dissipation of the herbicides EPTC and pebulate in a silt loam soil under plastic mulch in the absence and presence of metam sodium was examined in field experiments in 1998 and 1999 at Knoxville, Tennessee. EPTC half-life (DT(50)) was 9 d, but when applied in conjunction with metam sodium DT(50) increased to 22 d. Similarly, average pebulate DT(50) was 8 d and increased to 23 d when applied in conjunction with metam sodium. This increase in herbicide DT(50) with the addition of metam sodium is thought to be due to a reduction in soil microorganisms that degrade EPTC and pebulate. EPTC applied with metam sodium injured tomato plants and reduced total crop yield more than EPTC, pebulate, or pebulate with metam sodium. The increased tomato injury may have been related to the greater and prolonged activity of EPTC and slower EPTC dissipation in the presence of metam sodium or MITC.