The Effects of Soil Surface Temperature and Relative Humidity on 2, 4-D Volatilization

Abstract

Monolayer water content and specific surface area are two of the most significant characteristics of soil. The monolayer water content (m0) and specific surface area measurements were made by both Brunauer Emmett Teller (BET) and Guggenheim, Anderson and De Boer (GAB) method of adsorption of water molecules. Two replicate measurements of monolayer water content and specific surface area were performed for nine source clay minerals from The Clay Minerals Society and three benchmark soils from Alabama. Overall, the commonly used BET equation gives the lower monolayer water content and specific surface area values compared with GAB. The water vapor sorption isotherm (WSI) of a soil describes the relationship between the relative humidity and water content. The WSIs in this study were developed by Vapor Sorption Analyzer (VSA) with relatively high resolution and a reasonable time compared with the traditional method. Overall the sorption isotherms show the positive relationship between relative humidity and soil water content, in which the relative humidity ranges from 0.03 to 0.95. The adsorption isotherm has a lower water content than the desorption isotherm at a given relative humidity. The hysteresis phenomenon between the absorption and desorption isotherm is evident for all test media. 2,4-Dichlorophenoxyacetic acid (2,4-D) is one of the first commercially available herbicide since 1940s and it is a relatively large group of herbicides that had a revolutionary effect on agriculture. 2,4-D volatilization experiments in a lab scale were conducted for four clay minerals and three benchmark soils with four different relative humidity (RH) levels for 24 h, two lower than the m0 corresponding RH and two higher. The RH regimes were given by the VSA. Results show the volatilization rate of 2,4-D was relatively high for each test media and each RH level (range from 50-80%), the influence of RH on 2,4-D volatilization is significant (P=0.04), and the volatilization rate of 2,4-D differs among test media, the volatilization is lower when RH lower than m0 corresponding RH (P=0.01).