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

2,4-Dichlorophenoxyacetic acid (2,4-D) is a type of phenoxy carboxyl acid herbicide. In 2014, the US Department of Agriculture approved a new generation of genetically modified soybean and corn that are resistant to 2,4-D. With new crops available, an increase of 2,4-D usage of 200 to 600% is predicted by 2020, which means the 2,4-D drift concern will also significantly increase.

Research on 2 ,4-D volatilization was mostly conducted on the field scale. The temperature and relative humidity (RH) are very variable and therefore it is hard to accurately determine the effects of a single factor on 2,4-D volatilization. A new instrument called a vapor sorption analyzer (VSA) allows to accurately manipulate RH and temperature, and is now commercially available. For the first time, it is possible to study vaporization of herbicides as affected by temperature and RH regimes.

The objective of this research project is to determine the influence of soil surface temperature, RH as well as soil mineral composition on 2,4-D volatilization. Four minerals were used in this study as test media: smectite, Na rich montmorillonite, Ca rich montmorillonite and kaolinite. And three benchmark soil in Alabama, which are Orangeburg, Dothan, and Cecil soil, were used for additional test. The monolayer water content (ML) of the smectite is 3.37%, it can reach the ML when the RH is 35%. We applied 2,4-D amine to the smectite at a rate of 2.24 kg/ha, ran with the VSA at 25°C, 5%,15%,65% and 95% RH for 24h. 2,4-D was then extracted from the soil using methanol and analyzed on an HPLC. 44.07%,27.31%,19.33% and 27.61% of 2,4-D were volatilized at different RH. We conclude that more 2,4-D was volatilize if the soil water content is below the ML in comparison to soil water content above ML.