Water and Methyl Isothiocyanate Distribution in Soil after Drip Fumigation

Abstract

Methyl isothiocyanate (MITC) generators, such as metam sodium (Met-Na), are used for soil fumigation of agricultural land. The ban on the fumigant methyl bromide has resulted in greater use of MITC generators. To understand the efficacy of MITC, it is necessary to assess its generation and disappearance kinetics when Met-Na is applied to soil. This study evaluated the movement of water and distribution and dissipation of MITC in soil after application of Met-Na through surface drip irrigation systems. The effects of varying water application volume (25, 50, and 75 mm) and rate (1.9, 5.0, and 7.5 L h m) were evaluated in a sandy loam soil. Good fumigant distribution within the sandy loam soil was observed under medium water application amount (50 mm) with slow to intermediate drip application rates (1.9-5.0 L h m). Low water application amount (25 mm) or high application rate (7.5 L h m) did not provide adequate MITC distribution throughout the soil bed width and rooting depth. Dissipation patterns of MITC in soil in all water application amounts and rates followed first-order kinetics, with a rate constant of 0.025 ± 0.004 h and a half-life of 27 ± 3 h. Simulated water distribution through the soil profile using HYDRUS 2D/3D fitted measured field data well, and the model accurately simulated MITC fumigant distribution in the soil.

Recent Developments and Applications of the HYDRUS Computer Software Packages

“…This module allows users to specify additional injections of fumigants into the transport domain at a specific location at a specific time as well as to consider the presence or absence of a surface tarp, the temperature dependence of tarp properties, and the removal of tarp at a certain time. The Fumigant module has been used recently to investigate the effects of different application scenarios (such as tarped broadcast, tarped bedded shank injection, or tarped drip line‐source application) and various factors (such as initial water content or tarp permeability) on fumigant volatilization (Nelson et al, 2013; Spurlock et al, 2013a,b). Figure 3 summarizes one example. The Classic Slope Module: One frequent application of HYDRUS has been to obtain subsurface flow conditions (i.e., relative saturations and water fluxes) for subsequent slope‐stability analyses using other programs.…”

Soil Mobility of Allyl Isothiocyanate and Chloropicrin as Influenced by Surfactants and Soil Texture

“…CP controls insects and fungi, but it has less activity against nematodes and weeds (Ajwa and Trout, 2004). The most promising MeBr alternatives are chloropicrin and Met-Na (Gao et al, 2012;Jacoby, 2016;Klose et al, 2008;Yates et al, 2002); therefore, more than 7000 tons of chloropicrin were used in 2011 (Nelson et al, 2013) in California.…”

“…It is difficult to find a chemical with biological, chemical, and physical properties similar to those of MeBr that will pass regulations and that is relatively easy to transport and apply. MeBr has a boiling point of 4.5°C and a vapor pressure of 218.6 kPa; however, for MITC, CP, and AITC, the boiling points are 119, 112, and 150°C, respectively, and vapor pressures are 1.73, 2.44, and 0.49 kPa, respectively (Downing, 2016;Jacoby 2016;Nelson et al, 2013). The high boiling point and relatively low vapor pressure of MeBr alternatives do not allow these fumigants to exist in a gas phase in their original state.…”

“…The high boiling point and relatively low vapor pressure of MeBr alternatives do not allow these fumigants to exist in a gas phase in their original state. Liquid fumigants need to convert to a volatile or semi-volatile state to control soil-borne pests; therefore, the dispersal rate and the ability of a fumigant to move in soil at high concentrations will determine the effectiveness of the fumigant (Nelson et al, 2013).…”

“…In California, MeBr alternatives are usually applied to the soil through drip irrigation (also known as chemigation); however, in many other places, shank applications are still a common application method for fumigants. During drip irrigation, polyethylene plastic mulch is installed before the application of the fumigants, and this application method is gaining popularity with growers due to limited worker exposure (Jacoby, 2016;Nelson et al, 2013). Although Met-Na and other isothiocyanate generators are popular and widely used as MeBr alternatives, previous research suggested that soil-borne pest control with Met-Na is inconsistent because application methods seldom achieve optimum distribution of Met-Na (Gilreath et al, 2008;Rodriguez-Kabana, 2002).…”