Biomass Yield and Essential Oil Chemical Composition of Mentha x piperita as Affected by Amino Acids and Different Fertilizer Resources

The aim of this study was to answer the question of whether foliar application of amino acids along with chemical and organic fertilizers can be effective for peppermint growth characteristics, nutrient concentration, and essential oil content and composition. A split-plot experiment based on the randomized complete block design with three replications was used. The main factors included the following treatments: no fertilization (control), amino acid (AA), chemical fertilizer (CF), AA + CF, vermicompost (V), V + AA, farmyard manure (FYM), and FYM + AA. The sub-factor was represented by the first (H1) and second harvest (H2). The morphological traits and chlorophyll index values increased in peppermint after application of CF + AA and V + AA. The highest dry matter yield (274.3 gm−2), essential oil (EO) content (2.56%), and essential oil yield (EO) (7.04 gm−2) were obtained with V + AA at the first harvest. The highest concentrations of N, P, and Fe were observed with V + AA; those of K and Zn with FYM + AA; that of Mn with CF + AA at the second harvest. At both harvests, menthol, menthone, 1,8-cineole, neo-menthol, and δ-terpineol were the major components of peppermint essential oil. Menthol showed the highest value at H1 with FYM + AA (32.0%) and at the H2 with V + AA (38.4%). The correlation between the menthol content and the essential oil (EO) percentage and yield was significantly positive. The application of organic fertilizers such as vermicompost and farmyard manure integrated with amino acids can be a valuable alternative to chemical inputs and suitable treatment for improving the essential oil quantity and quality in peppermint.