Food waste liquid anaerobic digestates (FWLAD) have received much public attention for its potential as an organic fertilizer source as they are rich in mineral elements. However, FWLAD can contain high salinity and high NH4 concentration, and thus, the optimum application rates need to be determined to deliver required plant nutrients without excessive salt and NH4 level. The objective of this study was to evaluate the effects of nutrient solution concentration made from FWLAD on the growth of leafy vegetable seedlings. The seeds of lettuce (Lactuca sativa) ‘Rex’, ‘Muir’, and ‘Roxy’, Swiss chard (Beta vulgaris subsp. vulgaris) ‘Rhubarb’, bok choy (Brassica rapa subsp. chinensis) ‘Mei Qing Choi’, and kale (Brassica oleracea var. sabellica) ‘White Russian’ were sown in rockwool plug and grown at 22 °C under sole-source LED lighting with an 18-h photoperiod at a photosynthetic photon flux density of 200 µmol∙m-2∙s-1. After germination, the seedlings were sub-irrigated with nutrient solution made with either crude or processed FWLAD at electrical conductivities (ECs) of 1, 2, 3, or 4 dS·m–1. Four weeks after treatment, when using crude FWLAD, shoot fresh mass of three lettuce cultivars decreased by 76-92% as EC increased from 1 to 4 dS·m–1. In contrast, with processed FWLAD, lettuce ‘Adriana’ and ‘Roxy’ showed 68-1080% greater shoot fresh mass at an EC of 2 dS·m–1 compared to an ECs of 1, 3, or 4 dS·m–1. Shoot fresh mass of lettuce ‘Muir’ at an EC of 2 dS·m–1 was similar with that at an EC of 1 dS·m–1 but 380-516% greater than those at ECs of 3 or 4 dS·m–1. In lettuce, the EC of FWLAD had similar effects on leaf area as it did on shoot fresh mass, but it had minimal effects on leaf number, relative chlorophyll concentration, and shoot dry mass. In kale, Swiss chard, and bok choy, the EC of FWLAD had little effects on plant growth attributes. Our results suggest that leafy vegetable seedlings vary in their responses to nutrient solution concentrations derived from FWLAD, with lettuce exhibiting greater sensitivity than Swiss chard, bok choy, and kale. In lettuce, lower concentrations of FWLAD (at an EC ≤2 dS·m–1) increased shoot fresh mass.
