Fulvic acid promotes cucumber (Cucumis sativus L.) growth and nitrogen use efficiency under moderate nitrogen deficit in soilless culture

Over-fertilisation is common in horticulture, leading to low crop nutrient use efficiency and risk of releasing reactive nitrogen in the environment. Fulvic acid can improve plant biomass and nutrient use efficiency, especially under limited nitrogen supply. Two experiments were performed to investigate cucumber plant responses to the combined effects nitrogen and fulvic acid. Exp. 1 in soilless culture with growing substrate combined factorially two N rates (6 mM and 12 mM) and 4 rates of fulvic acid (0, 3, 15 and 30 mg plant -1 ) in season 1 and 3 rates of fulvic acid (0, 30 and 60 mg plant -1 ) in season 2. Exp. 2 in hydroponics combined two N rates (6 mM and 12 mM) and two rates of fulvic acid (0 and 30 mg plant -1 ). In both experiments, 30 mg FA plant -1 consistently alleviated low-N stress. In exp. 1, FA increased fruit yield up to 43 %, average fruit weight, and fruit number. Allometric analysis demonstrated that FA shifted dry-matter partitioning toward fruit and leaf under N limitation. Under 6 mM N, FA improved nitrogen use efficiency by more than 60% and phosphorus use efficiency by more than 28 %. In exp. 2, FA increased total root length, surface area, volume, and tip number under both nitrogen rates. We conclude that supplementing nitrogen-stressed plants with fulvic acid could improve root development, reproductive partitioning, fruit yield, and nutrient use efficiency in soilless culture. • Fulvic acid alleviates low-nitrogen stress and increases fruit yield by up to 43%. • Fulvic acid improves nitrogen use efficiency (>60%) and phosphorus use efficiency (>28%) under low nitrogen supply. • Fulvic acid enhances root system architecture and shifts biomass partitioning to fruit.