Food waste liquid anaerobic digestate (FWLAD) contains higher salt concentrations than synthetic fertilizers at comparable nutrient levels. As a result, plants may experience salt stress at optimal nutrient concentrations or nutrient deficiencies when electrical conductivity (EC) is reduced. Silicon (Si) has been shown to alleviate salt stress and promote plant growth under nutrient-limited conditions. This study investigated whether Si supplementation can mitigate salt stress under high EC and enhance growth under low EC when FWLAD is used to fertilize lettuce (Lactuca sativa ‘Muir’) transplants. Lettuce seeds were sown in either rockwool plugs or a 128-cell plug tray filled with a peat-based growing medium. One week after sowing, seedlings were sub-irrigated with one of nine nutrient solutions prepared from FWLAD. These solutions were adjusted to one of three target EC levels (1, 2, or 3 dS·m⁻¹) and amended with one of three Si treatments: (1) no Si (control, supplemented with 1.5 mM potassium sulfate to match potassium levels), (2) 1.5 mM lab-grade potassium silicate (prepared from silicic acid and potassium hydroxide), or (3) 1.5 mM commercial-grade potassium silicate (AGSIL16H, a commercially available product). Seedlings were grown indoors at 22 °C, with a photosynthetic photon flux density of 200 µmol·m⁻²·s⁻¹ and an 18-hour photoperiod, for a total of three weeks after sowing. For the control treatment without Si, leaf number, total leaf area, and shoot fresh mass were highest at EC 2, indicating that moderate EC promoted optimal growth. Compared to the control, supplementation with lab-grade potassium silicate increased leaf number, total leaf area, and shoot fresh mass by 16%, 63%, and 56% at EC 2, and by 69%, 423%, and 255% at EC 3 in rockwool, and by 10%, 10%, and 11% at EC 2, and by 19%, 42%, and 63% at EC 3 in growing medium. At EC 1, lab-grade potassium silicate had little to no effect on leaf area and shoot fresh mass in either substrate. Commercial-grade potassium silicate did not affect leaf number, leaf area, or shoot fresh mass at any EC. These results suggest that Si supplementation using lab-grade potassium silicate can improve lettuce seedling growth under high-EC conditions when fertilized with FWLAD, with more pronounced effects observed in rockwool compared to the growing medium.
