VOLATILIZATION AND DISTRIBUTION OF METHYL-IODIDE AND METHYL-BROMIDE AFTER SUBSOIL APPLICATION

Methyl iodide (MeI) was recently proposed as a potential replacement f or methyl bromide (MeBr) as a soil fumigant, but little is known about its fate in the environment. Volatilization of MeI was measured and c ompared with that of MeBr in packed soil columns with different soils and under various soil surface conditions. Under the same conditions, MeI volatilization loss was greater than that of MeBr, and the enhance d volatilization was attributed to its slower degradation in soil. In a Greenfield sandy loam (coarse-loamy, mixed, thermic Typic Haploxeral fs), the greatest loss occurred in a nontarped application (78% for Me I and 62% for MeBr), with the smallest loss in a high-barrier plastic- tarped treatment (28% for MeI and 24% for MeBr). Covering the soil sur face with polyethylene film was ineffective in preventing MeI or MeBr volatilization. Volatilization losses of MeI and MeBr were also influe nced by soil type, and were significantly less from soils that were hi gh in organic matter content and capable of rapidly degrading the fumi gants. Ten days after a point application (30 cm) in field plots cover ed with polyethylene film, higher MeI concentrations were detected at most depths. Because of its Longer persistence in soil, MeI may have a greater tendency ten reach groundwater than MeBr.