Humic Acid Improves Plant Yield, Antimicrobial Activity and Essential Oil Composition of Oregano (<i>Origanum vulgare</i> L. subsp. <i>hirtum</i> (Link.) Ietswaart)

The effects of humic acid on plant yield, essential oil content, the composition of essential oil and the antimicrobial activity of <i>Origanum vulgare</i> L. subsp. <i>hirtum</i> (Link.) (cv. Tinmaz) cultivated in 2017 and 2018 under Eskisehir ecological conditions were evaluated. Three humic acid (HA) doses in response to 50.0 L ha⁻¹ (HA 50), 30.0 L ha⁻¹ (HA 30) and 0.0 L ha⁻¹ (HA 0, as control) were applied to soil at the vegetative stage and beginning of the blooming stage of the plant throughout each harvest in both years. Essential oil composition was determined using GC-FID/GC-MS. The antibacterial and antifungal activity were determined by the well-diffusion method. Fresh herb yield, dry herb yield and dry leaf yield were highest at HA 50 both years, although essential oil content increased in 2017 at both HA 50 and HA 30, but was greatest at HA 50 in the second year. The essential oil content differences between the control dose and 50.0 L HA ha⁻¹ were 0.46% and 0.42% in 2017 and 2018, respectively. The antimicrobial activity of the essential oil samples against two bacteria species (<i>Enterococcus faecalis</i> and <i>Staphylococcus aureus</i>) and two yeast species (<i>Candida albicans</i> and <i>Candida parapisilosis</i>) was generally higher than that of the control drugs, and the activity increased with increasing HA doses. Analysis of the essential oil components showed that the carvacrol and γ-Terpinene ratios generally increased as the HA doses increased to 50.0 L HA ha⁻¹. Soil HA applications could be recommended for higher quality, plant yield and antimicrobial activity of <i>Origanum vulgare</i> L. subsp. <i>hirtum</i>.