Plant sap analysis is a technique for monitoring plant nutrient status in real-time, enabling precise nutrient management to enhance growth and yield in controlled environment agriculture (CEA). Comprehensive sampling techniques are vital for accurate determination of nutrient concentrations, considering the variability of nutrients across different developmental phases of plants. However, questions remain regarding the selection of the appropriate plant tissues, including the number of leaves collected, sampling time, type and age of plant tissue, and frequency. Different crops need specific sampling procedures due to their unique leaf morphology, growth habits, and physiology. Many commercial laboratories only distinguish between new and old leaves. In this series of studies, we determined the most effective sampling method including the number of leaves, the type and age of tissue, as well as the timing and frequency of the collection. Optimal sampling techniques were identified for lettuce and tomato by conducting five different experiments across three cultivars. These experiments varied the number of leaves sampled (10, 20, 30 per sample with three replicates), types of tissue (leaves for lettuce with three replicates, and petioles and leaves for tomatoes with 20 each per sample), age of tissue (new vs. old with 20 leaves per sample and three replicates), time of collection (6, 8, 10 am with three replicates). For lettuce, two developmental stages (half and final harvest maturity), while for tomatoes, sampling frequency at four different growth stages was investigated (first fully expanded leaves, 1/3 and 2/3 of crop development, and final harvest). The results indicate that collecting 20 fully expanded leaves at 8 am, particularly at the final harvest, was considered the best sampling technique for nutrient analysis for both lettuce and tomatoes, providing the most effective sampling technique for optimizing nutrient management.
