Microalgae associated to humic acid as a novel biostimulant improving onion growth and yield

Abstract The search for new natural sources for plant growth promotion and yield gains can contribute to ensuring safe and sustainable vegetable production. The potential of microalgae as a plant biostimulant source has been investigated in recent years, while the use of humic substances is well established; however the study of the combination of both is novel, and is the aim of this work. A step-by-step approach was adopted, conducting three experiments using the microalgae Scenedesmus subspicatus combined with humic acid: i) rooting bioassays in a growth chamber to identify bioactivity of microalgae, humic acid and their mixtures using Vigna radiata as a model plant. ii) immersion treatments of two onion cultivar seedlings at transplantation which were then grown in pots in a greenhouse, iii) immersion treatments of two onion cultivar (open-pollinated and hybrid) seedlings at transplantation which were then grown at field in organic system. Growth and biochemical variables, yield and nutrient content of onion leaves and bulbs were evaluated. The bioactivity of microalgae biomass, of humic acid and their mixtures were identified on bioassays, and synergy was found. The immersion of onion seedlings in microalgae plus humic acid solutions showed growth promoting effects at the early stages, improved bulb caliber and yield of the hybrid cultivar, and incremented sugars and proteins content in bulbs. The yield gain was not related to nutrients uptake stimuli.