Biostimulants are natural and environmentally friendly substances with positive effects on plant growth enhancing plant metabolism and physiology when applied in small doses. The most common biostimulants’ categories are described below.

SEAWEED EXTRACTS

Seaweed form an integral part of marine costal ecosystem and it has been estimated that there are about 9000 species. In agriculture, the type most commonly used is ascophylum Nodosum, Fucus spp, Laminaria spp, Sargassum spp, and Turbinaria spp. Seaweed extracts act as biostimulants mainly due to the presence of plant hormones. Main phytohormones identified in seaweed extracts are: auxins, cytokinins, gibberelins, abscisic acid and ethylene. On the market, there is variety in production of seaweed extract. These extractions provide processed products that are either solid and or liquid depending on the purpose of the applications. Thus we find products that more or less transformed - from simply crushed seaweed to extractions of active molecules.

PEPTIDES AND AMINO ACIDS

Peptides and amino acids are important molecules in plant mechanisms because they are the basis of proteins. Some bioactive peptides are able to modulate important physiological functions of plants because they bind to the cell receptors that regulate specific metabolic processes. There are two sources, an animal source which is obtained by a hydrolysis of the collagen at high temperature, and vegetal hydrolysates which are obtained by enzymatic hydrolysis at low temperatures.

Some amino-acids like Hydroxyproline and Hydroxylysine are only present in hydrolysed collagen (leather). Some organic regulations limit the use of animal, to the non-edible part of the crop.

HUMIC SUBSTANCES

Humic substances are natural constituents of the soil organic matter resulting from the decomposition of plant, animal and microbial residues, but also from the metabolic activity of soil microbes using these substrates. Humic substances are collections of heterogeneous compounds, originally categorized according to their molecular weights and solubility into humins, humic acids and fulvic acids. Humic substances have long been recognized as essential contributors to soil fertility, acting on physical, physico-chemical, chemical and biological properties of the soil. Most biostimulant effects refer to the amelioration of root nutrition, via different mechanisms.

PLANT EXTRACTS

Inside plants, they are a lot of useful substances which act as biostimulants. Each plant extract is unique due to the origin of the vegetal and the process of extraction. So it’s difficult to highlight specifics. However, plant extracts can act directly on the crop without demanding a lot of energy.

BENEFICIAL MICROORGANISMS

There are two large families of microorganisims -  beneficial fungi and beneficial bacteria. The first category is strongly represented by Micorrhizal fungi. These fungi work in a mutualistic symbiose, between plant roots and some fungi of the soil. Mycorrhizal fungi colonize the plant roots and provide them mineral elements and water they extract from soil through an external net of hyphae, whilst the plant supplies the micro-organism with radical organic compounds and sugar. Thanks to mycorrhizae the plant increases its root surface from 100 TO 1000 TIMES, and consequently the absorption capacity of nutrients and water.

Mycorrhizae are produced by in vivo or in vitro techniques. The first one ensures that the fungi are grown as in natural conditions and by consequence are more resistant on the soil conditions.

Some other fungi (like some strains of Trichoderma) stimulate plant development and increase plant productivity. They are also able to induce the formation of rootlets and stimulate the colonization of the rhizosphere and root by other beneficial micro-organisms.

Beneficial bacteria settle by association within the rhizosphere. They will act mainly on the nutrition functions of plants - in particular on the increase in the absorption of minerals, and better resistance to abiotic stress. The main products offered on the market are rhizobium and plants growth–promoting rhizobacteria.

（sourcing from biostimulant.com）