Growth, Productivity, and Essential Oil Content of Fennel Plants Treated with Spirulina Platensis Extract and Compost Tea Under Low Nitrogen Doses

The aim of this study was to find alternatives (even partial) to mineral nitrogen fertilization to reduce its harmful accumulation in the edible parts of fennel. Fennel (Foeniculum vulgare) is a perennial plant of the Apiaceae family grown for its edible shoots, leaves, and seeds. The present work was conducted in two successive seasons, in 2018/2019 and 2019/2020, at the farm of the Agricultural Botany Department, Faculty of Agriculture, Ain Shams University, Cairo, Egypt, to investigate the efficiency of application of two biostimulants-Spirulina platensis algae extract (SP) at 10% and compost tea (CT) at 6.72 ml L-1-with partial doses (75 and 50%) of the recommended mineral nitrogen fertilizer dose on growth, photosynthetic pigments, fruit yield, and essential oil productivity. The results indicated that vegetative parameters, leaf number, branch number, shoot and bulb fresh weight, and shoot dry weight, as well as yield components such as 100-fruit weight and number of umbels per plant were significantly enhanced with application of SP as a foliar spray coupled with 75% of the recommended dose of nitrogen fertilizer (SP + 75% N) as compared to other treatments. Photosynthetic pigments and total soluble carbohydrate were improved significantly when CT was applied as soil drenching coupled with 75% of the recommended N dose (CT + 75% N). In this regard, a clear effect was noticed in terms of oil yield and fruit yield using SP and CT with a 75% N dose. It is concluded that using Spirulina platensis algae extract and compost tea as biostimulants coupled with only 75% of the recommended nitrogen fertilizer dose can be recommended for economical fennel production.