European Biostimulant Industry Council uses a functional definition to describe biostimulants that was developed over the course of a year-long consultation process with stakeholders including researchers, regulators and related industry sectors: “Plant biostimulant means a material which contains substance(s) and/or microorganisms whose function when applied to plants or the rhizosphere is to stimulate natural processes to benefit nutrient uptake, nutrient efficiency, tolerance to abiotic stress, and/or crop quality, independently of its nutrient content.”
Types of biostimulants
Next, we are going to see in more detail the different types of biostimulants that we can find in the market and that fall within the definition we have just seen above. Among them, within the types we talk about acid-based biostimulants, algae and plant extracts, microbial biostimulants and inorganic compounds.
In each case, these types of biostimulants present their own benefits in terms of fertility improvement, vigor enhancement, plant health advantages and increased quality of agricultural crops.
Humic substances, which include humic and fulvic acids, are among the most common organic substances on earth, making up much of the organic matter in the world’s soils. Humic and fulvic acids are complex organic molecules of diverse structure and composition that form in the soil as byproducts of the decomposition and microbial metabolism of plant and animal residues. Most biostimulant effects of humic substances refer to the amelioration of root nutrition, via different mechanisms. They can be applied in various ways, including direct application to the soil, foliar application, incorporation into fertilizer and other products, and through irrigation water.
Amino acid products (along with other protein-derived biostimulants, based on peptides or protein hydrolysates) can be derived from the chemical or enzymatic hydrolysis of animal, plant, or microbial protein. Direct effects on plants include modulation of N uptake and assimilation, by the regulation of enzymes involved in N assimilation and of their structural genes, and by acting on the signalling pathway of N acquisition in roots. There is strong evidence for a variety of benefits, including improved soil fertility, better plant health and vigor, enhanced crop yields and quality, and improved stress tolerance.
Seaweed and Plant Extracts
The use of seaweed in agriculture dates to ancient times, when it was often used to improve fertility by augmenting soil organic content. They have been shown to improve soil properties (to improve soil structure, water retention, and aeration), as well as helping to fix or chelate nutrients and improve CEC. Seaweed extracts have also been shown to aid in the functioning of beneficial soil microorganisms, and to improve the provisioning, uptake, and utilization of plant nutrients. Improved stress tolerance effects have also been broadly reported.
Plant extracts (or “botanicals”) are perhaps less well studied but represent a fast-growing category of biostimulant materials. The use of allelochemicals — active plant compounds that can be extracted and concentrated — is an especially promising and innovative area of both industry and academic investigation.
Beneficial fungi and bacteria represent the core of the focus within the smaller but rapidly growing category of microbial biostimulants. A wide variety of microbial products are sold as biofertilizers, plant inoculants (to aid primarily in nutrient processing), soil amendments, and other beneficial additives. Microbial products can include “pure strain” fermentation solutions, based on individual isolates; consortia of admixed or co-fermented isolates; or much more complex “natural” communities derived from organic matter processing. Microbial products have been shown to enhance plant growth through various direct and indirect mechanisms; and to help with nutrient availability and uptake, improving soil condition, helping plants tolerate abiotic stress, and enhancing overall crop quality attributes.
Chemical elements that promote plant growth and may be essential to particular taxa but are not required by all plants are called beneficial elements. The five main beneficial elements are Al, Co, Na, Se and Si, present in soils and in plants as different inorganic salts. These beneficial functions can be constitutive, like the strengthening of cell walls by silica deposits, or expressed in defined environmental conditions, like pathogen attack for selenium and osmotic stress for sodium.
Definition of beneficial elements is thus not limited to their chemical natures but must also refer to the special contexts where the positive effects on plant growth and stress response may be observed. Many effects of beneficial elements are reported by the scientific literature, which promote plant growth, the quality of plant products and tolerance to abiotic stress.
This includes cell wall rigidification, osmoregulation, reduced transpiration by crystal deposits, thermal regulation via radiation reflection, enzyme activity by co-factors, plant nutrition via interactions with other elements during uptake and mobility, antioxidant protection, interactions with symbionts, pathogen and herbivore response, protection against heavy metals toxicity, plant hormone synthesis and signaling.
Biostimulants in integrated pest management (IPM)
Although they may not immediately spring to mind, biostimulants, which are designed to improve yield and overall crop health, can also provide secondary benefits in pest management. The impact of biostimulants on pest management and plant health is not direct, so they are not pesticides, but indirect, acting on plants and with plants to resist pest attacks. Biostimulants contribute to IPM strategies through several mechanisms, such as inducing the plant immune system, building better soil structure, increasing organic matter which in turn promotes better soil microbial activity, competition in access to resources, or regulation of biological processes in the plant.
Nine different categories of plant biostimulants (mycorrhizal fungi, plant growth-promoting rhizobacteria, protein hydrolysates, humic substances, algal and botanical extracts, silicates and phosphates, and chitosan) have recently been studied and their indirect impact on arthropod pests, plant pathogens, and plant parasitic nematodes has been validated. Each has been shown to act differently but have a proven role in IPM approaches. If the overriding principle of IPM is to manage the health of a crop holistically, rather than focusing on individual pests and diseases and treating them with individual tools in isolation, we can say that biostimulants are the perfect tool in the holistic IPM toolkit.
Biostimulants and Soil health
Soil health is attributed to multiple parameters, many of which are biologically mediated and, as such, may be influenced by biostimulants application. Is seems clear that the application of biostimulants augment soil enzymatic activities, induce changes in microbial community, and support soil protection against erosion and contributed to its restoration. Moreover, it seems that biostimulants can increase the activity of rhizosphere microbes and soil enzymes, and the production of soil growth regulators.
Seaweed based biostimulants for example, contain large amounts of polysaccharides such as alginates and fucoidans, which bond with the metallic ions in the soil to produce a gel that helps hold water and maintain an aggregate structure. This helps the plant grow a robust root system, which in turn can increase nutrient uptake.
With fertilizer prices at all-time highs, we need to harness the ability of soil microbial communities to solubilize nutrients stored in the soil in inorganic forms. Using biostimulants can help reduce fertilizer applications, while also increasing soil health and functionality.