Biostimulant products have been considered innovative agronomic tools as demonstrated by the increase of scientific publications and by the constant expansion of their market. The global biostimulants market is one of the fast-growing crop protection markets worldwide. France, Italy, and Spain are the leading EU countries in the production of biostimulants.

Plant biostimulants are defined as as any natural substance, microorganisms or any combination that when applied stimulate plants natural processes benefiting plant physiological processes, nutrient use efficiency and/or tolerance to abiotic stress.

High-throughput phenotyping and -omic technologies seem to be useful approaches to understand biostimulants activity and hypothesize a mode of action. They can act directly on plant physiology and metabolism by improving soil conditions. They are able to modify some molecular processes that allow to improve water and nutrient use efficiency of crops, stimulate plant development, and counteract abiotic stresses by enhancing primary and secondary metabolism.

Biostimulants are classified as these major groups:

Humic substances:
They include humic acids, fulvic acids and humins. Humic substances are natural constituents of soil organic matter, resulting from the decomposition processes of plants, animals, and microbial residues, but also from the metabolic activity of soil microbes.

Seaweed extracts:
Seaweeds are a vast group of macroscopic, multicelular marine algae that can be brown, red, and green.

Hydrolysed proteins and amino acids containing products:
Hydrolysed proteins are a mixture of amino acids, peptides, polypeptides and denatured proteins that can be obtained by chemical, enzymatic and thermal hydrolysis of proteins (or by combining these different hydrolysis types) from both plant and animal sources.

This group includes bacteria, yeast, filamentous fungi, and micro-algae. They are isolated from soil, plants, water, and composted manures or other organic materials.

Another category of biostimulants includes those derived from extracts of food waste or industrial waste streams, composts and compost extracts, manures, vermicompost, aquaculture residues and waste streams, and sewage treatments among others.

A new category of biostimulant products, including nanoparticles and nanomaterials, has been recently proposed. Nanoparticles and nanomaterials are usually defined as particles with dimensions between about 1 nm and 100 nm that show properties that are not found in their bulk form. They are able to modify the quality of the production and the tolerance to abiotic stresses when applied in small quantities as foliar spray or in nutrient solution, also in vegetable crops.