The effect of fulvic acid treatment on the growth and development of Calendula officinalis

The study was conducted to compare the application methods and concentrations of fulvic acid (FA) solutions on the production and quality characteristics of Calendula officinalis in greenhouse conditions. The experimental design included root (2.0 ml/L) and foliar (0.5 and 1.0 ml/L) fertilization of plants with an organic biostimulant (control – no application). The experiment was carried out in 2026 in a growbox with full light culture. Nine fertilizations were carried out at 14-day intervals. Foliar application of 0.5 ml/l provided the maximum dry bud yield in the experiment – 11.0 g/plant (+37.5 % compared to the control, a total of 38 buds). Root application with a similar number of buds increased the yield by 10.0 g/plant (25 %). Foliar treatment with 1.0 ml/l resulted in decrease in the number of buds to 27 pcs. (-25 % to the control) with a simultaneous increase in their average weight to 0.30 g (+33 % to the control), which indicates a redistribution of resources in favour of bud development. All FA treatments increased the content of Ca in shoots by 23-24 %, K – by 15-22 % and sharply decreased the amount of Na by 48-62 %, compared to the control. With foliar treatments, the concentration of Fe in shoots decreased, but in buds it increased by 14 %. Root application increased the accumulation of Ca, K, Fe, P, Si, Zn in buds. Foliar treatment with 0.5 ml/l contributed to the greatest increase in the content of Cu (+35 % to the control), K, Fe and Si (+123 %), and 1.0 ml/l – a decrease in their amount in buds, except for Fe. In terms of β-carotene and vitamin C, the variant with root application of fulvic acid (+15 % β-carotene) stood out. Root application of fulvic acid resulted in the highest accumulation of β-carotene, vitamin C, and amino acids, while foliar application at a dose of 0.5 ml/l resulted in a dry flower bud yield of +37.5 %.