Background <p>Humic acids (HA) function as plant biostimulants, enhancing plant metabolism by activating the primary proton transport system, which promotes root growth. This study investigated the effects of HA on root growth, H⁺ extrusion, Ca²⁺ signaling, and reactive oxygen species (ROS) production, examining how HA might integrate nutrient sensing with growth regulation through plant hormone transport.</p> Results <p>HA rapidly increased cytosolic Ca²⁺ and ROS, and altered root architecture in <i>Arabidopsis</i> lines Columbia-0 (Col-0) and G548 TOROE (overexpressing TARGET OF RAPAMYCIN). In Col-0, HA exposure increased total, primary, and lateral root lengths, while in TOROE plants, only primary root length changed. HA also doubled the expression of transcripts in Col-0 roots, including those for PM H⁺-ATPase (<i>AHA2</i>), TOR kinase, ROS-related <i>RBOHC</i>, and auxin transporters <i>LAX3</i> and <i>PIN3</i>. Only <i>AHA2</i> and <i>RBOHC</i> were upregulated in TOROE plants. Findings indicate that HA promotes rhizosphere acidification and plasma membrane potential regulation via <i>AHA2</i> and <i>RBOHC</i>, linked to auxin transporters and calcium signaling.</p> Conclusions <p>The data suggest HA, rich in compounds like quinones and flavonoids, stimulates root development by triggering Ca²⁺ waves, NADPH oxidase and H⁺-ATPase activities. These findings advance our understanding of TOR and H⁺-ATPase roles in root architecture.</p> Graphical abstract <p></p>