[en] Arise from the chemical and biological degradation of plant and animal residues and from the synthetic activities of microorganisms in the soil, humic substances (HS) are natural heterogeneous aromatic and organic compounds. These substances are chemically complex with no clearly defined chemical structure, although generalized models have been proposed and they can be divided into fractions of humic acids, fulvic acids and humins depending on their solubility in water as a function of the pH.
The stimulation of plant growth and development by HS are the activities that have attracted the attention of many scientists. They influence plant productivity directly by the stimulation of biochemical and metabolic processes or indirectly through the modification of soil characteristics and microflora activities. All together, these properties mainly affect root architecture. By inducing root hairs proliferation, differentiating root cells and enhancing lateral root emergence, an increase of the total root biomass is observed.
Experiments targeting the rooting stages in absence of interferences were conducted in vitro using HS extracted from landfill leachate and a stable commercial formulation (“Humifirst” from TRADECORP company: 12% humic acid and fulvic acid 3%) issued from leonardite. Shoots and leaves explants of silver birch (Betula pendula Roth) and black alder (Alnus glutinosa L. Gaertn) vitro-plants were treated with 10 ppm leachate HS and 100 ppm Humifirst HS for 5 days during the rooting induction/initiation phase. The treated explants were then transferred into elongation medium containing only nitrate calcium for 4 weeks. The results show that application of HS during the root induction/initiation phase did not significantly influence root growth of both species in comparison with control explants.
