BBESG define a biostimulant as:

“A non-fertiliser material containing substance(s) and/or microorganisms applied to plants or the rhizosphere whose function is to stimulate and benefit natural plant development processes embracing nutrient uptake, nutrient efficiency, crop quality, crop vigour, resistance to or tolerance of external environmental stresses.”

Biostimulants can be split into 8 sub categories and definitions as defined by Source: Pr. P. du Jardin (2012), “The Science of Plant Biostimulants – A Bibliographic Analysis”, European Commission (Contract 30-CE0455515/00-96).

Humic Substances and organic acids

Humic substances (HS) are natural substances belonging to the soil organic matter and resulting from the decomposition of dead cell materials and from the metabolic activity of soil microbes using these substrates. HS are collections of heterogeneous compounds, originally classified according to their molecular weights and solubility into humins, humic acids and fulvic acids, but with loosely defined boundaries and complex molecular constituents. This area is expanding all the time with new organic acids being isolated and used for different purposes from cation exchange capacity increase, foliar nutrient delivery and even complexed nutrient chelation and ISR/ASR stimulated crop response. READ MORE..

Seaweed extracts

Seaweeds now constitute a vast group of species (over10,000) which are classified into different phylum’s, including brown, red and green macroalgae. Molecular systematics has demonstrated their early divergence in the evolution of photosynthetic organisms. They should be regarded as separate taxonomic entities and this should be borne in mind when attempting a general description of their biological, biochemical and functional characteristics. Apart from manures, seaweed extracts have the longest historical use as biofertilisers, soil conditioners and biostimulants and their biochemical expression can be very diverse, from the same species grown in different locations or extracted using different methods to differing species with unique biological expression and blends of several species.

Seaweed supplies macro- and micronutrients, has liming properties and increases phosphorus availability. The phycocolloids within seaweed improve soil structure, increase the water retention and cation-exchange capacities of soils. They are also used binds metals, boost biological activity, aid foliar nutrient application and reduce susceptibility to both biotic and abiotic stress factors.

Complex organic materials

Complex organic materials have been used in farming forever and are now obtained from several types of media from composts, manure, sewage sludge extracts, agro-industrial and urban waste products. They are mainly applied to soils with the aim to increase soil organic matter and base nutrient levels however mainly with no real idea of what the exact affect will be. More work is now being done in higher value crops to improve the physico-chemical characteristics of soils, to provide macro- and micro-nutrients after analysis of the applied media and to promote rhizobacterial activity. This has been found to aid nutrient cycling and nutrient use efficiency, suppression of soil-borne pathogens, increase the speed the degradation of pesticide residues and xenobiotics. The promotion of plant growth parameters and of crop yield in defined conditions of use explains why the term biostimulants is now used to refer to these organic materials and why their use may ultimately be regulated.

Beneficial nutrient elements

These are nutrient elements that promote growth and may be essential to particular taxa, but are not required by all plants. The four main beneficial elements are Silicon (Si), Cobalt (Co), Sodium (Na), and Selenium (Se). The definition of beneficial nutritional elements is not limited to their elemental natures but must also refer to the positive effects on plant growth and stress response they may support.

Inorganic salts & Phosphites

This term encompasses many inorganic anion salts, including phosphites, phosphates, and bicarbonates, sulphates, nitrates. It also cover specific cations such as copper, manganese and zinc and calcium. Used in targeted applications and specific forms they can support reduced susceptibility to pathogenic pressure and may involve direct interaction with fungi and bacteria or indirect protection by stimulating/supporting plant defence mechanisms. Their action on the physiology of the plant, on stress response and on yield explains why these inorganic compound salts are sometimes referred to as biostimulants.

The term phosphite is used to refer to the salts of phosphorous acid (HP3PO3) whilst phosphite esters are designated as phosphonates, which include ethyl-and methyl phosphonates. In practice, the term phosphite is sometimes used for designating both phosphite salts and phosphite esters. They are clearly distinct from phosphates, which are the salts of phosphoric acid (HP3PO4), however they do constitute as an indirect source of phosphorus from P) used by plant nutrition. They are currently restricted in use due to the enforced MRL (maximum residue level) from the EU prompted by extensive lobbying from the agrochemical industry and pharmaceutical industries wishing to both control the sales of these compounds and the more recent development of phosphonates for the treatment of cancer and osteoporosis.

Chitin& Chitosan derivatives

Chitin and its deacetylated forms, chitosan, are bioactive polymers from which many derivatives are produced by hydrolysis and chemical modification, for increasing their water solubility and bioactivity. Used in agriculture for over 200 years, chitin is a large, structural polysaccharide made from chains of modified glucose. Chitin is found in the exoskeletons of insects, the cell walls of fungi, and certain hard structures in invertebrates, crustaceans and fish. In terms of abundance, chitin is second to only cellulose. In the biosphere, over 1 billion tons of chitin are synthesized each year by living organisms. This extremely versatile molecule can form solid structures on its own as in insect wings or can combine with other components like calcium carbonate to make even stronger substances like the shell of a clam. In agricultural use, chitin and chitosan are extremely versatile. Chitin is known to have antibacterial and antifungal properties so is used as a natural barrier to infection. It is also used as an elicitor to stimulate the SAR response in many plant species. It is known growth promoter, enhancer of secondary metabolite production, and is regularly used across the world for soil correction.

Application of chitin can be done by different ways:

1. a) Seed coating

One of the most important bioactivity of chitin and chitosan on plants is stimulation of seed germination. Chitosan coating alters permeability of the seed plasma membrane, increasing not only the concentrations of sugars and proline, but also enzymes activities. Germination rates increases significantly and seedlings germinate quicker, better and vigorously.

1. b) Plant growth promoter

Chitosan application has shown significant effects on the growth and yield of many crops such as potato, cabbage, soybean, rice, tomato, lettuce, and glasshouse salads to name but a few. There is a significant amount of data to support its positive effects on growth rates of roots, shoots and the flowering/fruiting process.

1. c) Soil applied Nematode suppression

Chitosan application in soil is effective to reducing levels of predatory nematodes. It acts by supporting the proliferation of chitinolytic microorganisms which feed on the cuticle of nematodes and their eggs.

1. d) Fertiliser

Chitosan is used as a slow-release fertiliser in many parts of the world due to its high concentration of natural nitrogen which can be between 5-8%. It also has shown the capability to accelerate the breakdown of soil applied chemistries and to reduce high levels of individual trace elements. As a foliar spray chitosan is a known anti-transpirant at higher level application. It will also stimulate plant defence mechanisms which can stimulate the production of phenolic compounds and flavonoids which are known to be beneficial to many crops used for processing.

Free Amino Acids & Nitrogenous Substances

This is a term which has been coined to indicate the overall effect on the plant, the chemical compounds and underlying mechanisms can be, and in fact are, very diverse. Some of the compounds have physical effects at the surface and/or within the plant organs, others are regulators of the leaf’s openings diffusing water vapour, called stomata. Free Amino Acids and Other N-containing Substances Organic nitrogenous compounds animal and plant based amino acids (or protein and non-protein origins), peptides (or protein and non-protein origins), polyamines, betaines and related substances.

Amino acid fertilisers are manufactured either by hydrolysis or enzymatic treatment of animal or plant protein feedstock. They are readily absorbed, transported, and utilised as a source of nitrogen and carbon for plants. They are regularly used to reduce the energy expended by the plant to reduce organic matter, synthetic nitrates and ammonia into amino acids. Some amino acids are efficient metal ion chelators which can help with metal ion nutrient uptake and help protect plants from toxic levels of metal ions.

Amino acids also function as biostimulants for plants. As biostimulants, amino acids can play an important role in enhancing plant productivity, especially when a crop is facing abiotic and biotic stress conditions.  This is an important distinction as they can act palliative manner to support and maintain plant process and function until the stress factor is reduced. These functions, combined with the ability to be approved for organic use, makes sustainably produced amino acids a regular choice for growers across the world.

Although primarily serving as nutrients for the plant, specific amino acids can play an additional role as a process specific biostimulants. For example, L-Proline can reduce the effect and speed up recovery time from abiotic stress by strengthening cell walls. L-Glycine and L-Glutamic Acid are key components for chlorophyll production. They can chelate metal ion nutrients and facilitate plant uptake and movement into cells. L-Tryptophan is a precursor in auxin synthesis which is used for root growth and development. L-Methionine is a precursor to ethylene and when used timely will aid ripening.

L-Arginine is a precursor to cytokinin production involved in cell growth, axillary bud growth and leaf senescence.  Various amino acids when used at higher rates are known to support higher rates of pollination and fruit formation. L-Histidine is known to aid ripening; L-Proline increases pollen fertility; L-Lysine, L-Methionine and L-Glutamic Acid increase germination; L-Alanine, L-Valine, and L-Leucine and used to improve fruit/grain quality.

Crop specific extracts

A relatively new sector for biostimulants is the area of crop specific extracts. For many years the use of garlic and neem have been used to deter insects from predating on crops either by direct or indirect contact however, an increasing number of extracts are now being used to support crops. Cinnamon extracts and Citrus extracts are used as predation barriers for insect predation across the world and plant extracts of Mimosa tenuiflora and Quercus robur have promising potential to be used ad foliar barrier sprays to reduce the incidence of a wide range of phytopathogenic fungi due to the antimicrobial compounds they contain. Thyme, tagetes and wintergreen oil have been noted as having potential as environmentally friendly insect repellent sprays. Willow extract (salicin/salicylic acid) is used to boost plant immune system response, to stimulate the SAR response. Salicylic acid acts as a stimulant or transmitter of cells to withstand abiotic stress conditions such as drought, cold, heat, stress of heavy elements, and conditions of ammonia tension and also increases the plant’s ability to withstand salt stress salt in particularly sodium chloride.

It also has the ability to bind conjugate with some amino acids such as proline and arginine, which increase the plant’s effectiveness in resisting environmental stresses however the important effect of salicylic acid is that it is known to stimulate the production high level of antioxidants.