Growth of soybeans under the influence of biostimulant doses

Soybeans are crucial to Brazilian agriculture, with the country being a world leader in production and export. Biostimulants based on humic substances (HSs) alter plant morphology and physiology, increasing nutrient use efficiency and stress resistance. However, the use of HSs to stimulate soybean plant growth is still underexplored, and there is a lack of studies addressing this practice specifically through foliar application in soybean crops. Given this knowledge gap, the objective of this study was to investigate the effects of foliar application of HSs on the morphology and physiology of soybean plants, specifically the cultivar OLIMPO 80I82RSF IPRO. For this purpose, five different doses of HSs (0, 1, 2, 3, and 4 L ha⁻¹) were applied foliarly. The experiment was conducted in a completely randomized design with five replicates. The HSs were extracted from ruminal compost using KOH and applied directly to the leaves at the V4 vegetative stage. The crop was monitored until the V7 stage, and various variables related to plant morphology, biomass, and physiology were measured and evaluated through analysis of variance (p ≤ 0.05), with the models fitted by polynomial regression. The results showed that most evaluated characteristics followed a linear adjustment with a significant difference at the 4 L ha⁻¹ dose, especially in relation to aerial dry mass, root dry mass, root volume, CO₂ assimilation rate, stomatal conductance, and total chlorophyll index, when compared to the zero dose.