Hypothetical framework integrating the main mechanisms involved in the promoting action of rhizospheric humic substances on plant root- and shoot- growth

•Critical integration of the mechanisms of action of humic substances on plant growth.•What are proven facts and what are hypothetical?.•The complexing pathway and the biochemical pathway in HS effects on plant nutrition.•Relevance of the nutritional and stimulant effects of humic substances in plants.•Crosstalk between HS-mediated effects on root functionality and shoot growth.