Inter-Day Instability in Plant Sap Composition Undermines Single-Day Diagnostics

Plant sap analysis has emerged as a practical, real-time tool for nutrient monitoring, complementing conventional tissue tests. As adoption expands, standardization has become a priority; yet current research emphasizes tissue selection and analytical procedures, whereas short-term variation in sap composition has been largely overlooked. Consequently, a single-day diagnostic paradigm has prevailed, resting on the unverified premise that inter-day shifts lack practical relevance. To test this, we conducted a five-day field trial in broccoli, quantifying sap pH, EC, K+, NO3-, and °Bx daily under five extraction methods. Extraction method had a negligible effect on inter-day shifts (%Δ), whereas chemical parameters differed markedly, ranking pH (0.8%) < °Bx (3.7%) < K+ (5.9%) < EC (6.2%) < NO3- (8.2%). The largest observed changes across successive days were 0.13 pH units, 0.49 °Bx, 6.94 mmol/L K+ (271 mg/L), 1.17 dS/m EC, and 11.98 mmol/L NO3- (743 mg/L). These results show that inter-day instability can bias single-day diagnostics, motivating adoption of new sampling strategies. We urge researchers and advisors to account for inter-day variation and incorporate multi-day sampling into experimental design and decision-making until predictive frameworks are available. Recognizing and addressing this dimension will improve sap diagnostic reliability and support more sustainable, science-based nutrient management.