2018 ASHS Annual Conference

Silicon (Si) is a plant-beneficial element that can alleviate the effects of abiotic and biotic stress. Substrate components, substrate amendments, liquid fertilization, and foliar sprays can supply supplemental Si to plants. Substrate amendments can provide extended release of Si throughout the production cycle, but information on incorporation rates and effects on plant growth are not well known. Our objective was to evaluate substrate amendments as potential sources of Si for production of ornamentals in soilless substrates. The base substrate was an 85 : 15 sphagnum peatmoss : perlite substrate. Amendments included parboiled rice hulls at 5%, 10%, or 20% (by volume); ground parboiled rice hulls at 5%, 10%, or 20% (by volume); rice hull biochar at 1.5%, 3%, or 6% (by volume); steel slag at 2.37, 4.75, or 7.12 kg∙m^-3 (PlantTuff); hydrous potassium silicate at 0.38, 0.76, or 1.14 kg∙m^-3 (AgSil); calcium silicate at 0.39, 0.78, or 1.17 kg∙m^-3 (wollastonite); a non-amended control; and a non-amended control fertilized with 2 mM potassium silicate at each irrigation. Lime was added (1.19 to 2.97 kg∙m^-3) as needed to adjust initial substrate pH of each treatment to 6.2 – 6.5. Four-week old sunflower (Helianthus annuus L. ‘Pacino Gold’) seedlings were transplanted into 11.5-cm pots. They were arranged in a completely randomized design, with five pots per treatment, and grown for six weeks. Greenhouse conditions were 22 °C day/18 °C night, a 14 h photoperiod, and supplemental irradiance from high-pressure sodium lamps when ambient irradiance was less than 300 µmol∙m^-2∙s^-1 photosynthetic photon flux density (PPFD). Plants were fertilized with 20N-4.4P-16.6K at a concentration of 150 mg∙L^-1 N at every irrigation. Plant growth and quality (height, width, relative chlorophyll content, and dry mass) were similar to the non-amended control for all treatments except steel slag at 4.75 and 7.12 kg∙m^-3, which were greater. Leachate Si concentration ranged between 0.3 and 67.4 mg∙L^-1 (non-amended control and 2 mM liquid potassium silicate, respectively), although substrate amendment leachates ranged between 1.0 and 19.0 mg∙L^-1 Si. Foliar Si concentration ranged from 519 mg∙kg^-1 in the non-amended control to 24,671 mg∙kg^-1 in the 2 mM liquid potassium silicate treatment, but between 1900 and 12,468 mg∙kg^-1 for the substrate amendment treatments. Within each amendment, leachate Si and foliar Si increased as incorporation rate increased.