Evaluation of sap extraction methods for nutrient diagnostics in lettuce, cucumber, and tomato in controlled environments

Controlled Environment Agriculture (CEA) enables the cultivation of high-value crops under controlled environmental conditions, requiring precise nutrient monitoring to optimize fertigation. Standard practices to monitor plant nutrient status include nutrient leaf analysis (NLA), the reference method, but it does not reflect real-time status. Plant sap analysis offers real-time nutrient diagnostics; however, there is no standardized protocol for sap extraction methods across crops and individual nutrients. The objective of this study was to evaluate and compare multiple sap extraction methods for their reliability, practicality, and cost-effectiveness in diagnosing plant nutrient status in key CEA crops. Five sap extraction methods: ammonium acetate (AA), dry freezing (DF), extraction with ethyl ether (EE), extraction with potassium chloride (KCl), and tissue crushing (TC) were tested against NLA for assessing macro- and micronutrients in tomato (Solanum lycopersicum), cucumber (Cucumis sativus), and lettuce (Lactuca sativa) grown under CEA. For nitrate (NO₃⁻), KCL method was used as a reference and compared with EE and TC. The results indicated correlations between sap extraction methods and NLA were weak to moderate and varied across crops and nutrient types. The strongest correlations were obtained in lettuce using TC, with very high values for K (r = 0.99), Mg (r = 0.86), and Ca (r = 0.68). Other methods often yielded weak or negative correlations (e.g., tomato Ca with AA: r = - 0.36; cucumber Mg with KCl: r = - 0.15; lettuce Zn with EE: r = - 0.50). Sap NO₃⁻ correlations were variable, ranging from weakly positive (cucumber EE vs. KCl: r = 0.26) to negative (tomato TC vs. KCl: r = - 0.38), underscoring the complexity of NO₃⁻ dynamics and their dependence on crop physiology and developmental stage. A cost analysis showed that TC was the most affordable option, although it only worked for certain crops. These results indicate that, whereas some sap methods of extraction can produce indicative evidence of plant nutrient status in certain crops, none can accurately serve as a direct substitute for NLA.