Real-time nutrient management is crucial in controlled environment agriculture (CEA) for enhancing crop production, reducing fertilizer costs, and mitigating environmental impacts. Inadequate fertilization can reduce crop productivity and nutrient runoff. Sap-based sufficiency ranges could maintain balanced fertilization. The main objective of this study is to establish sap-based sufficiency ranges for lettuce (Lactuca sativa), tomatoes (Lycopersicon esculentum), and cucumbers (Cucumis sativus) across developmental stages and fertilizer levels in CEA. Lettuce was grown using a Randomized Complete Block Design (RCBD) with three cultivars (‘Casey,’ ‘Cherokee,’ ‘Chicarita’) and three fertilizer levels (low 50%, medium 100%, and high 200%) in a vertical farm and greenhouse. A split-plot in RCBD was used for tomatoes (‘Grandice,’ ‘Macxize,’ ‘Prodice’) and cucumbers (‘Georgia,’ ‘Verdon,’ ‘Camaro’) under the same fertilizer levels. Rockwool substrate was used for lettuce, and coco-coir for tomatoes and cucumbers. We monitored and maintained the environmental parameters: daily light integral (DLI) ranged from 17–23 mol/m²/day for tomatoes and cucumbers and 12–18 mol/m²/day for lettuce. Tomatoes received supplemental light from 1 AM to 10 AM. Temperature and relative humidity (RH) were maintained at 18–22°C and 70–80% RH for lettuce, and 22–25°C and 60–70% RH for tomatoes and cucumbers. We monitored pH and EC weekly. Sap samples were collected at half and final stages for lettuce and four stages for tomatoes and cucumbers. Chlorophyll and anthocyanin content, yield, number of fruit, soluble solids content (SSC), and titratable acidity (TA) were recorded, while fresh/dry weight, leaf area, SSC, and TA were measured for lettuce. The tissue crushing method was used to establish sap-based sufficiency ranges for nitrogen (NO₃⁻-N), phosphorus (PO₄³⁻-P), potassium (K⁺), calcium (Ca²⁺), magnesium (Mg²⁺), and sulfur (SO₄²⁻-S). Our results provide general sufficiency recommendations (in mg/L): For greenhouse lettuce, the sufficiency ranges were NO₃⁻-N (546–1027), PO₄³⁻-P (520–616), K⁺ (6250–7052), Ca²⁺ (690–899), Mg²⁺ (280–371), and SO₄²⁻-S (93–101). For lettuce in vertical farm, NO₃⁻-N (1122–1139), PO₄³⁻-P (524–629), K⁺ (5455–5672), Ca²⁺ (426–595), Mg²⁺ (173–205), and SO₄²⁻-S (102–129). For tomatoes, NO₃⁻-N (547–805), PO₄³⁻-P (730–927), K⁺ (5360–7151), Ca²⁺ (3139–3716), Mg²⁺ (1133–1427), and SO₄²⁻-S (2796–3127). For cucumbers, NO₃⁻-N (888–1081), PO₄³⁻-P (25–38), K⁺ (4291–5478), Ca²⁺ (2147–2493), Mg²⁺ (1458–1813), and SO₄²⁻-S (1615–1946). In conclusion, sap-based sufficiency ranges were established which enable real-time nutrient monitoring and support more efficient fertigation in CEA.
