Influence of N, K, and Seaweed Extract Fertilization on Biomass, Photosynthetic Pigments, and Essential Oil of Thymus vulgaris: Optimization Study by Response Surface Methodology

Nutrient management has a decisive impact on the biomass and essential oil yield of medicinal plants. This study aimed to determine the optimal levels of nitrogen, potassium, and seaweed extract fertilizers to maximize the yield and quality of thyme using the response surface methodology (RSM). The experiment was laid out as a Box-Behnken design with three replications and three experimental factors, including nitrogen (urea) (0, 200, and 400 kg ha−1), and foliar application of potassium (Flourish Sulfopotash) (0, 6, and 12 kg ha−1) and seaweed extract (0, 3, and 6 L ha−1). The generated models were statistically significant for all measured traits except for γ-terpinene and p-cymene. While the influence of N on the amount of photosynthetic pigments followed a quadratic trend, the response of total chlorophyll and carotenoids to increasing potassium was linear. The response of biomass yield to N and seaweed was quadratic and linear, respectively. Potassium application had no significant influence on biomass. Essential oil yield reached its peak value (12 kg ha−1) when N and seaweed were applied at their intermediate levels and with the maximum application rate of potassium. Thymol was identified as the highest essential oil component (46.1%), followed by γ-terpinene (19.2%), p-cymene (14.1%), and carvacrol (5.6%). The optimization results suggested that the application of 162 kg ha−1 urea, 12 kg ha−1 Flourish Sulfopotash, and 4 L ha−1 seaweed extract was sufficient to produce the maximum dry matter (1247 kg ha−1), and more than 11 kg ha−1 of essential oil, with a concentration of 1%. Through optimization, the amounts of thymol and carvacrol were estimated to be as much as 44.2% and 6.2%, respectively. The results of the study suggested that resource optimization through RSM can be used as an efficient method to manage the consumption of fertilizers in thyme production.