2017 ASHS Annual Conference

The ornamental and greenhouse plant production industries rely on soilless growing media (SGM) as a substrate. These substrates are generally lightweight and have high porosity. Little is known about the retention capacity (or sorption) of pesticides to SGM. The objectives of this study were to determine the sorption capacity of selected pesticides by SGM components based on the recommended application rates for ornamentals, and to investigate the use of Al-water treatment residuals (Al-WTRs) as an amendment to reduce potential leaching from pots or containers. SGM components were obtained from SUNGRO, Orlando. Al-WTRs were provided by a water treatment plant in Bradenton. SGM components were extracted with 2.5 mM CaCl₂ using a mass:solution ratio of 1:5 (v/v). pH and electrical conductivity (EC) were kept within the recommended values for ornamental plant production (pH 5.5-6.5, EC 0.8-1.6 dS m^-1). The extracts were used for calibration and validation of analytical methods using a Waters/Quattro-Ultima HPLC-MS system. Two insecticides, imidacloprid (neonicotinoid) and acephate (organophosphate) were selected for batch sorption equilibria experiments (24-hr.) that were conducted in the same mass to solution ratio with the following SGM components: peat, pine bark, coir, sand, perlite, and vermiculite. The initial solution concentration of the pesticides was 4 μg mL^-1. Acephate sorption was relatively low with most SGM components (26 ± 4%, average ± st.dev.). Six percent of the acephate was sorbed to sand (control). Al-WTRs sorbed 63% of the applied acephate. Imidacloprid showed similar retention percentages with Al-WTRs, peat, pine bark, and coir (84 ± 7%); and lower retention in the inorganic SGM components (9% in both sand and perlite; 43% in vermiculite). Sorption isotherm data for imidacloprid (concentration range from 1 to 40 μg mL^-1) showed Freundlich characteristics in the Al-WTRs (K_f = 1038 mL g^-1, N=0.55) and linear sorption on peat (K_D=237 mL g^-1). Both pesticides showed different retention values in the SGM. The differences are related to the unique interactions between the organic molecule and SGM sorption surfaces. Managers should pay special attention when applying pesticides to SGM with low sorption capacity (low K_f -K_D) since leaching and pollution of soil and water resources can occur. Al-WTRs show promise as an amendment or filter to avoid leaching of pesticides from SGM in ornamentals production settings.