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. Three minerals were used in this study as test media: silica, montmorillonite, and kaolinite. We applied 2,4-D amine to the test medium at a rate of 2.24 kg/ha, ran with the VSA at 25°C, 3% RH for 12 hours and then increased the RH to 95% for another 12 hours. 2,4-D was then extracted from the soil using methanol and analyzed on an HPLC. 35.9%±3.2%, 34.3%±3.0%, and 77.2%±2.4% (error term equals standard error) of 2,4-D were volatilized for silica, montmorillonite, and kaolinite, respectively. This test was also conducted at 45°C and 2,4-D was not detectible anymore. We conclude that the temperature has a greater influence on 2,4-D volatilization than RH.