Biostumulants

(Largely from https://www.fbn.com/community/blog/what-is-an-inoculant-and-how-to-apply-it)

A biostumulant is a substance or microorganism that, when applied to seeds, plants, or the rhizosphere, stimulates natural processes to enhance or benefit nutrient uptake, nutrient efficiency, tolerance to abiotic stress, or crop quality and yield. They may include fertilizers, soil amendments, inoculants etc.

Many types of biostimulants are used in crop production, and each has different benefits. Common biostimulant effects on plants include:

Increased stress tolerance and recovery
Improved plant water and nutrient uptake, translocation, and use
More robust root structure and growth
Better disease tolerance
Improved plant metabolism for enhanced yield and crop quality
Enhanced quality characteristics of produce, including sugar content, color, fruit seeding, etc

Biostimulants are often associated with organic production systems, but in the past decade, their popularity has grown among conventional growers.

While the crop health benefits of biostimulants have been well documented, the mode of action of these products is not always well understood. Generally, biostimulants boost a plant's vigor, making it more tolerant to abiotic and biotic stress. However, biostimulants do not have a direct mode of action against weeds, diseases, or insects like herbicides, fungicides, and insecticides do.

Types of Biostimulants

There are many biostimulant classes and formulations that may stimulate various plant responses. A specific product's efficacy may be affected by its raw ingredients and how it is manufactured, stored, and applied.

Here are the most common biostimulants used in agricultural production.

Humic and Fulvic Acids

Humic and fulvic acids are the largest segment of the biostimulant market. They are organic acids that occur naturally in soil, resulting from the decomposition of plant, animal, and microbial residues. These acids can also come from soil microbe activity. Humic acids may be derived from non-renewable (mineral deposits like leonardite and soft coal) or renewable (compost or vermicompost) sources.

Benefits of humic acids in agricultural production include:

Improving soil physiochemical properties
Increasing root nutrient uptake
Expanding lateral root development

Seaweed Extracts

Seaweed extracts are another popular biostimulant class with a long history in agriculture. Farmers have used seaweed extracts to fertilize the soil and improve its structure for hundreds of years. However, the biostimulant effects of seaweed extracts are a relatively new development.

Brown seaweeds, including species of the genera Ascophyllum, Fucus, and Laminari, are the most commonly used seaweeds in agricultural production. Different extraction processes are used to produce most seaweed biostimulant products and can affect overall product efficacy.

Seaweed extracts are usually packaged as a soluble powder or liquid formulation. Beneficial polysaccharides account for 30-40% of the dry weight of seaweed extracts and are known to elicit plant defense responses against bacterial and fungal pathogens.

Other benefits of seaweed extracts include:

Improved plant growth and development from phenolic-rich compounds
Improved nutrient uptake and utilization
Soil conditioning and metal-chelating properties
Increased water retention capacity in plants

Other plant extracts (botanicals) are increasingly being studied and used for their biostimulant effects.

Beneficial Bacteria

Plant-growth-promoting bacteria (PGPBs) are well-studied species of beneficial bacteria, including free-living bacteria that inhabit a plant's root zone, bacteria that colonize the root surface, and bacteria that live within plant roots.

Bacillus, Rhizobium, Pseudomonas, Azospirillum, and Azotobacter bacteria are popular in the biostimulant market. They offer many plant benefits; the most well-known is rhizobacteria's nitrogen-fixing capabilities in legume crops. Beneficial bacteria may be inoculated on the seed or applied directly to the soil.

Other benefits of PGPBs include:

Improved water and nutrient uptake
Increased nutrient use efficiency
Plant hormone stimulation and regulation
Resistance to insects and non-beneficial bacterial pathogens

While the modes of action and benefits of PGPBs are well understood and documented, they can be challenging to work with because they are living organisms sensitive to handling and extreme temperatures. PGPB biostimulants may not mix well with other crop protection products and often have limited shelf lives.

Beneficial Fungi

It's well-known that mycorrhizal fungi are vital to soil health and crop production. These fungi live symbiotically with plant roots to increase root mass and nutrient and water uptake.

Other benefits of symbiotic fungi products include:

Increased drought stress tolerance
Improved phosphorus uptake in phosphorus-deficient soils
Beneficial antifungal properties against plant fungal diseases

As living organisms, beneficial fungi are sensitive to crop management practices, including soil tillage, fallow periods, and high rates of fertilizer and fungicides.

Chitosan & Other Biopolymers

Chitosans are derived from a naturally occurring biological building block, chitin, that supports fungal cell walls and the exoskeletons of insects. Chitosans can induce plant-defense responses, making plants more tolerant to abiotic and biotic stresses. Although these products' exact modes of action have yet to be fully understood, more research is being done to support further product development.

Benefits of chitosans and similar biopolymers include:

Antibacterial, antifungal, and antiviral properties
Soil amendment characteristics to reduce Fusarium wilt and other soilborne pathogen populations

Chitosan efficacy can vary greatly depending on how and when it is applied and the quality of raw materials and manufacturing processes.

Inorganic Compounds

Inorganic compounds include minerals such as silica, selenium, cobalt, and others, which promote plant growth, the quality of plant products, and abiotic stress tolerance.

Some benefits of silicon, specifically, include:

Increased plant resistance to diseases
Increased photosynthesis efficiency
Improved water and nutrient translocation
Immobilization of toxic metals in the soil and plant tissues
Delayed plant senescence

Protein Hydrolysates

Protein hydrolysates are amino-acid and peptide mixtures obtained by chemical and enzymatic protein hydrolysis from both plant sources and animal wastes. Crop residues and by-products and animal industrial by-products, including leather, collagen and epithelial tissues, are typical sources.

The plant-based peptides, particularly, are the most interesting of the biostimulants due to their multifunctional activity. Protein hydrolysates are known to affect plant hormonal activities and metabolism.

Benefits of protein hydrolysates may include:

Increased soil fertility
Improved soil microbial activity
Chelating properties to protect against heavy-metal soils
Increased micronutrient uptake and translocation

Biostimulant Application Best Practices

Choosing the Right Product: ...
When/Where/How to Apply a Biostimulant: ...