Back to the future: Understanding Biostimulants: A Comprehensive Guide

Written by Richard Boldan

As an agronomist working primarily with farmers who are looking to reduce their reliance on pesticides and fertilisers, I am no stranger to the use of that seemingly ever-increasing array of products: biostimulants. I know that in some circles they are regarded as “muck and mystery” products. Chatting to farmers or attendance at many a mainstream technical agronomy meeting confirms the cynicism amongst many growers and advisors when it is revealed that a trial of product X has shown no benefit to yield. There is often an almost palpable sense of relief from those for whom the highlight of their year is the launch of the latest fungicide product. (Who doesn’t love a free beanie/gilet/waterproofs/travel mug/pen/pad/breakfast/lunch?)

There are many scientific papers showing a wide range of responses to biostimulants in a wide range of plants. Now, I am not claiming that biostimulants are the universal cure for all evils. Many studies are carried out in controlled conditions. We know that responses to any crop input, and particularly the more “natural” ones, is more variable when exposed to the vagaries of field conditions. However, I believe they have an important role to play in helping us to implement more sustainable and environmentally friendly farming systems. The key to getting the most from biostimulants is to have some understanding of what is in each product, what it is supposed to be doing and to use it in a way that allows its effect to be realised.

The Growing Market of Biostimulants

I alluded earlier to the huge number of products available. It can seem overwhelming, especially as new products keep arriving, often from companies you have never heard of. This is only going to increase as some of the big players that you will have heard of are now moving into this market and investing large sums of money in production and marketing.

How do you get a handle on this so that you can evaluate these products and how they may have a role on your farm? The key to understanding biostimulants is that the active ingredients in them all fall into a relatively small number of categories. Some products contain one active, but many contain combinations of two or more, occasionally with added nutrients.

What is a Biostimulant?

Biostimulants are materials that improve plant growth, stress resistance, and productivity by enhancing natural processes rather than directly supplying nutrients like fertilizers do. They work indirectly by stimulating the plant’s physiological processes or improving soil conditions to enhance nutrient uptake and overall vitality. Below, I explore different types of biostimulants, how they work, and the benefits they provide. The list is not fully exhaustive but covers the majority of products used it agriculture.

Types of Biostimulants and How They Work

1. Humic and Fulvic Acids

Humic and fulvic acids come from decomposed organic matter, such as peat, compost, or ancient plant material (eg Leonardite). They can enhance soil health by improving soil structure, water retention and microbial activity. They can also improve plant health. They can be applied to soil or plants.

• Benefits:
  • Promote root growth.
  • They enhance nutrient uptake by forming complexes with nutrients, making them more accessible to plants. They have a very high cation exchange capacity so they can hold on to relatively large amounts of nutrients.
  • Improve soil’s ability to hold water and nutrients.
  • Support beneficial microbes in the soil, particularly fungi.
• How They Work: Humic acids bind nutrients, making them easier for plants to absorb. Fulvic acid has a much smaller molecular structure than humic acid so it is good at transporting nutrients across cell membranes. Broadly speaking, humic acid is targeted at soil applications and fulvic acid is used for foliar applications eg with fungicides, trace elements, glyphosate. Foliar applications of either substance can help to raise Brix levels (sap sugar content) by supplying readily available carbon compounds to the plant. This can make the plants less attractive to sucking insects e.g aphids. Something I have employed successfully against black bean aphids where the grower didn’t want to use an insecticide. The addition of humics to soil applied fertilisers can improve nutrient use efficiency by holding the nutrients in a form that is less likely to leach or volatilize or lock up.

2. Seaweed Extracts

These are one of the most widely used biostimulants. Extracted from algae like Ascophyllum nodosum, seaweed contains bioactive compounds that stimulate plant development. (Polysaccharides, plant growth regulators, betaines, sterols, flavonoids, lipids, fatty acids, proteins, peptides and amino acids.) They also contain small quantities of plant macro and micro nutrients. A.nodosum is particularly effective due to its high concentration of growth-promoting hormones, minerals, and antioxidants, which support plant resilience and productivity. There are differences in the way that the product is extracted from the seaweed. Some are simply cold pressed whilst others use heat and or chemicals to extract more from the seaweed. However, it is claimed that this can damage some of the desired chemicals, making them less effective.

• Benefits:
  • Boost root and shoot growth.
  • Elicit defence responses to pests and diseases and abiotic stress like drought and cold.
  • Enhance flowering and fruit production.
• How They Work: Seaweed extracts influence plant hormone levels and activate genes that help plants survive stress and pest and disease attack. They also affect the microbiome of both the plant and the soil. The complex compounds make them particularly favourable for the growth of soil fungi.

3. Protein Hydrolysates and Amino Acids

These are made from plant or animal proteins (e.g fish waste), broken down into smaller pieces, like peptides and amino acids, usually by chemical hydrolysis. It is possible to make fish hydrolysate on farm by sealing fish waste with some molasses in an airtight container for several weeks. The anaerobic conditions allow bacteria to break down the fish. However, this process is not for the faint hearted or those who value their sense of smell when the containers are opened!

• Benefits:
  • Improve nitrogen use and chlorophyll production.
  • Increase enzyme activity needed for growth.
  • Help plants cope with stress caused by environmental factors.
  • Help plants recover from herbicide shocks e.g ALS herbicides like Atlantis, Pacifica Plus
  • Soil applications of fish hydrolysate can stimulate soil biology. The fatty acids are especially beneficial for fungi.
• How They Work: Amino acids stabilize plant cells under stress and act as signals in growth-related pathways. The amino acids are also an efficient way to apply small quantities of nitrogen by foliar application. The plants can utilise the amino acids and peptides to build proteins rather than having to build them from simpler nitrogen compounds taken up by the roots.

4. Microbial Biostimulants

Microbial biostimulants cover such a range of organisms that they are worthy of an article on their own. They include beneficial organisms such as plant growth promoting bacteria and rhizobacteria (PGPR) e.g Bacillus, Azospirillum, Pseudomonas, non-pathogenic fungi, mycorrhizal fungi, protozoa and nematodes. “Off the shelf” products are available with known contents of single species or a consortium of species. Composts, compost extracts, brews and teas owe their effectiveness to the various microbes found in them. They can be produced to favour the growth of particular types of organism aimed at different uses.

• Benefits:
  • Help plants access nutrients like nitrogen and phosphorus.
  • Improve soil health and structure.
  • Suppress harmful microbes.
• How They Work: Some of these microbes work within the plant (endophytes) and some work outside (ectophytes), either on the plants surface or in the root zone. These microbiome based biostimulants trigger plant growth through solubilization of minerals (Zn, P and K), nitrogen fixation, production of phytohormones like abscisic acid, ethylene, cytokinin, jasmonic acid, secondary metabolites, enzymes, volatile organic compounds, carbohydrates, terpenoids, nitrogen, volatile inorganic compounds and lipopolysaccharides. These components modulate root morphology, shoot length, leaf area, soil structure (nutrient, water holding capacity, porosity and water filtration), improve nutrient and mineral acquisition (N, P, Fe, Zn, Mn etc.) and photosynthetic capacity of a plant. Further, they also enhance biotic and abiotic stress tolerance by activating genes responsible for defence systems. Some microbes affect pathogenic organisms either by attacking them in some way or simply by competing for space in the biosphere.

5. Chitosan and Other Biopolymers

Chitosan is biopolymer derived from the shells of crustaceans, insect cuticles and the cell walls of some fungi. It triggers plants’ natural defences.

• Benefits:
  • Boosts plant immunity to diseases. Enhances plant growth Strengthen plant cell walls.
  • Prolong the freshness of harvested produce.
• How They Work: Chitosan encourages plants to produce protective proteins and compounds while toughening their structures to withstand stresses. It also has direct antimicrobial properties, and it has shown effectiveness against some parasitic nematodes. It boosts root development and chlorophyl content as well as aiding nutrient uptake. Applied to seeds or soil, it promotes germination and seedling growth.

6. Silicon-Based Biostimulants

Silicon isn’t considered an essential nutrient but has proven to enhance plant strength and stress tolerance.

• Benefits:
  • Protects against pests and pathogens.
  • Improves drought resistance by optimizing water use.
  • Improves resistance to other abiotic stresses
  • Improves nutrient uptake.
  • Reduces oxidative damage in cells.
  • Reduces crop lodging
• How They Work: Silicon strengthens plant tissues and regulates antioxidant activity to handle environmental challenges. The strengthened plant tissue is more resistant to grazing pests such as cabbage stem flea beetle.

7. Phosphites

Phosphites come from phosphorous acid and are usually combined with ions such as potassium, calcium, sodium or ammonium. They act as both biostimulants and protectants against certain pathogens. They are not used as fertilisers.

• Benefits:
  • Boost plant defences against diseases.
  • Enhance nutrient uptake.
  • Support root system development.
• How They Work: Phosphites trigger plant defence systems. They also upregulate the pathway used to acquire more phosphorous. Improved root systems take up nutrients more efficiently. Phosphites increase activity of the enzyme nitrate reductase which is involved in the conversion of absorbed nitrate into amino acids. They can also increase photosynthesis and stomatal conductance under heat or drought stress.

8. Botanical Extracts

These are (or originally were) extracted from plants and contain natural compounds such as antioxidants, growth regulators and elicitors.

• Benefits:
  • Protect plants from stress
  • Enhance yield and quality
  • Provide resistance to pests and diseases
• How They Work: This category covers a wide range of actives with varying modes of action. Examples include laminarin (contained in seaweed) which elicits plant defence mechanisms against diseases. Triacontanol (found in plant cuticle waxes and beeswax) regulates metabolic pathways to aid growth and protect from stresses. Salicylic acid (originally from willow bark) elicits disease protection. (Laminarin has recently been registered in the UK as a fungicide for reduction of Septoria tritici and so can no longer be sold as a biostimulant.)

The Future of Biostimulants

As the market grows, we are already seeing increasing focus on this area from researchers, scientists and manufacturers. This will bring understanding and innovation with improved products, with more targeted uses and greater integration into crop production methods. A recent development has been the identification of specific compounds that regulate various metabolic processes in plants (“signalling compounds”). Previous biostimulant use could, sometimes, be fairly accused of being a bit of a scatter gun approach. However, it is known that with some types of biostimulant (e.g seaweed extract) where the individual components have been tried alone, they are less effective than when used together. The whole is often greater than the sum of the parts.

Conclusion: Back to the Future of Agriculture

The use of biostimulants is almost as old as agriculture. Ancient farmers discovered or noticed things that improved their crops. They didn’t know how or why; they just did. It was only during the 20th century that we got clever and found we could boost yields even more with artificial inputs and formed the basis of industrial scale agriculture.

Now, pressures to reduce our reliance on the use of pesticides and fertilisers are many fold, and look set to remain with us, so the growth in interest and use of biostimulants looks set to continue. Even without those pressures they should form some part of any genuine attempt at integrated pest management. It really is time to go back to the future.