Effect of Type and Method of Application of Humic Substances on Melissa Officinalis Plants.

Melissa officinalis (lemon balm) is an important aromatic plant due to its essential oil (EO) content. Melissa officinalis plants were treated with two types of humic substances (HS): humic acid (HA) or fulvic acid (FA) at 0, 250, 500 or 750 ppm. The application was carried out three times before the 1^st cut and repeated after the 2^nd and 3^rd ones as foliar spray (FS), soil addition (SA) or (FS+SA). Generally, plants fresh and dry weights, EO yield were significantly varied among the method of application and the applied concentration. Both of method of application and the concentration had a remarkable effect on plant dry weight and EO production. Moreover, results showed a significant interaction among the investigated factors for most assessed traits. The lowest dry herb yield (397 – 415g/plant) being for the control plants however, the highest yields (479 and 467 g/plant) were assessed for plants treated with (750 ppm of HA or FA) as a FS respectively without significant difference between them. The response of EO accumulation to the HS concentration was more pronounced as the control plants had 0.403 ml/plant which increased gradually and significantly by increasing HS concentration to achieve 0.561 ml/plant with the highest. Therefore, it is recommended to treat lemon balm plants with FA at 750 ppm as SA to improve EO production. Moreover, higher concentrations of HS should be tested for improving plant growth and EO production.