One of the best ways to increase the citrus yield and quality is proper fertilization. In the present research, the effects of foliar application of seaweed extract (0.5%), fulvic acid (0.5%), and potassium silicate (0.3%) on fruit yield and quality of mandarin cv. ‘Page’ were evaluated. Leaves of trees sprayed with seaweed extract, fulvic acid, and potassium silicate accumulated greater concentrations of N (15.09–22.16%), P (20.00–120.00%), K (17.02–46.80%), Ca (1.69–32.76%), Mg (26.31–84.21%), Zn (199.87–261.76%), Mn (79.92–139.04%), and Fe (59.12–148.65%) than those unsprayed. Mandarin trees sprayed with the nutrient solutions, especially seaweed extract, had higher chlorophyll a (up to 29.20%) and total chlorophyll (up to 24.37%) contents. Applying the nutrient solutions, especially seaweed extract, increased yield (up to 48.87%), fruit diameter (up to 5.37%), fruit juice (up to 16.35%), and firmness (up to 19.58%), but reduced the pulp of fruits (up to 39.64%). Moreover, fruits of mandarin trees sprayed with the nutrient solutions and especially seaweed extract showed a greater level of TSS (total soluble solids) (12.73–14.30%), TSS/TA (total soluble solids/titratable acidity) (9.98–11.32), color index L* (36.10–39.03), color index a* (5.10–6.65) and color index b* (23.99–29.30), vitamin C (27.08–35.49 mg 100 mL–1), total phenol (0.9–1.17 mg mL–1), carotenoid (0.3–0.36 mg g–1 FW), antioxidant activity (88.13–96.06%), as well as the activity of GPX (guaiacol peroxidase) (2.70–3.48 unit mg−1 protein), CAT (catalase) (1.65–2.00 unit mg−1 protein), APX (ascorbate peroxidase) (5.85–6.69 unit mg−1 protein), and SOD (superoxide dismutase) (1.77–2.20 unit mg−1 protein) enzymes. The study findings evidenced that foliar application of seaweed extract, fulvic acid, and potassium silicate has a high potential for improving fruit yield and quality of mandarin.
