Fulvic Acid, Chitosan, and Amino Acids Improve Productivity and Bioactive Composition of Hydroponically Grown Parsley

Biostimulants are increasingly recognized in modern agriculture as eco-friendly inputs that enhance plant growth, improve stress tolerance, and promote product quality. This study investigated the effects of fulvic acid, amino acids, and chitosan on the growth, yield, and nutritional quality of parsley grown under hydroponic greenhouse conditions. The research was conducted in two stages. In the first stage, different doses of fulvic acid (80–120 ppm), amino acids (40–80 ppm), and chitosan (0.3–0.6 mL L−1) were evaluated. In the second stage, the most effective treatments were tested in combination. The results showed that all biostimulants positively influenced plant growth, productivity, and nutritional parameters. In the first experiment, the highest total yield was obtained with chitosan at 0.3 mL L−1 (2068 g m−2; +30.1%). The greatest increase in total phenolic content was observed with AA 40 (391.1 mg GA 100 g−1 FW; +64%), while the strongest nitrate reduction occurred with FA 120, reducing nitrate levels from 1048 to 405 mg kg−1 (−61%). In the second experiment, the FA 80 + C 0.3 combination was the most effective treatment, increasing total yield from 493 to 856 g m−2 (+73.7%) and reducing nitrate content from 937 to 460 mg kg−1 (−50.9%). These findings suggest that fulvic acid and chitosan, applied individually and particularly in combination, may serve as effective biostimulant strategies for improving yield and nutritional quality while reducing nitrate accumulation in hydroponically grown parsley.