Using Humic Acid-Based Biostimulants

Technical articles - Soil and fertilization

Soil regeneration

Structure of humic acid

Humic acid is a complex organic substance derived from the decomposition of carbon-rich plant debris. It can be used as a biostimulant.

Description

Humic and fulvic substances are the main organic components of lignites, soil, and peat. Humic and fulvic acids are produced by the biodegradation of organic matter resulting in a mixture of acids containing phenolate and carboxyl groups. Humic substances include humic acids, fulvic acids (these are humic acids with a higher oxygen content and lower molecular weight), and humins.

The characteristics of these different substances vary greatly depending on the source of organic matter and the state of decomposition. Humic and fulvic acids establish numerous interactions favoring plant growth:

Scientific literature has demonstrated in numerous trials a positive impact of humic acids on various crops but it is important to note that most of these trials were conducted in growth chambers or on hydroponic crops, i.e., in highly controlled environments. Thus, while humic acids seem to have good potential for crop stimulation, field tests will allow further quantification of the effects of these substances on plants.

Interest

Trials have shown various impacts of humic substances on soil and plants:

• Local increase in water, air, and nutrient retention thanks to a "sponge" effect of humic substances.
• Phosphorus fixation.
• Stimulation of beneficial microorganism activity by providing highly assimilable carbon.
• Improvement of soil structure through the physical arrangement of humic acid particles in long carbon chains.
• Increase in stress tolerance.
• Improvement of crop quality.
• Increase in germination speed and root production.

• Increase in intracellular metabolic rates.
• Increase in cation exchange capacity (CEC) in the root zone and reduction of leaching of positively charged nutrients.
• Stabilization of ammonium form in the soil thus reducing losses by leaching and volatilization and improving plant uptake.
• Minimization of phosphate fixation (negative charges) on aluminum and iron oxides (positive charges) by competing for the same adsorption sites. Humic acids also help prevent phosphate precipitation by neutralizing aluminum and iron solubilized by acidifying fertilizers, which cause chemical precipitation of phosphorus.

Application methods

• Humic substances are preferred in sandy soils and soils with low organic matter content.
• Due to the size of their molecules, these substances should be applied to the soil.
• Fulvic acids, which have a much lower molecular weight, can be applied to leaves and absorbed directly through stomata.

Modes of action

The nature and intensity of effects observed on crops also vary depending on the species treated and the experimental conditions used. Among these positive effects, it is difficult to distinguish the direct cause among the known modes of action:

• Influence on membrane permeability,

• Stimulation of protein synthesis,
• Hormonal effect,
• Increase in photosynthesis,
• Influence on enzymatic activities,
• Indirect effects including solubilization of trace elements,
• Reduction of toxicity of certain molecules,
• Increase in fertilizer efficiency

The mode of action of humic substances is complex and still poorly understood due to the diversity of their components. Some molecules composing humic substances can be assimilated by plants and thus act directly on metabolic pathways.

It has been shown that fulvic acids and humic acids act differently on plants. Moreover, since all humic substances are complex mixtures of different molecules, what applies to one may not apply to another, making them difficult to characterize and compare. Some modes of action have been identified:

• The way humic substances promote the assimilation of ions present in the soil is quite variable and depends on their concentration in the soil and pH, but also on the ion considered. One partially elucidated mode of action is the stimulation of the expression of ion transporters in roots, although the physiological mechanism involved is not yet well understood.
• Another mode of action is the formation of complexes with ions present in free form in soils, making them more available to plants.
• Some humic substances also have a direct effect on primary metabolic pathways, particularly by improving respiration or photosynthesis of plant cells in some cases. Humic substances assimilated by plants seem to act on the expression of enzymes involved in these mechanisms.
• Hormonomimetic substances present in humic substances act directly on plant growth and development, especially root development.
• Humic substances form chelates with trace elements from soil and fertilizers and increase their availability.

La technique est complémentaire des techniques suivantes

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