Plant Biostimulants, What Are They?

Interest in plant biostimulants has exploded in recent years. Their ability to sustainably improve soil health, plant growth, water retention, nutrient efficiency, and overall crop quality is hard to ignore, and growers are paying attention.

But what exactly are biostimulants, and how do they work? This article aims to answer those questions and more. We review how biostimulants are used by growers today, and look ahead to the role they may play in shaping our agricultural systems of the future.

What is a Plant Biostimulant?

Plant biostimulants are substances or organisms that can enhance growth and overall plant health. The 2018 Farm Bill perhaps comes closest in offering an official definition, categorizing biostimulants as:

"A substance or micro-organism 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."

What they do, rather than what they are made of, is the common thread that unites biostimulants. Whether it is a living bacteria, a plant extract, or a chemical element, if it stimulates natural processes to a plant’s benefit, it is a biostimulant.

Many biostimulant products are relatively new to the market, and for some, their efficacy requires further testing. But the growing interest in these substances represents an important shift. Conventional agricultural inputs implicitly value yield above all else, often at the expense of the soil, water, and climate. Plant biostimulants are one frame of a bigger-picture approach to agriculture, one that seeks to balance sustained yields without sacrificing soil health, freshwater supplies, and the long-term resiliency of our agricultural systems.

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Is Fertilizer a Plant Biostimulant?

When most people think of plant growth, they think of the fundamental macronutrients: nitrogen, phosphorus, and potassium—or NPK.

All plants require these three nutrients to thrive and grow, so does that mean NPK fertilizers are plant biostimulants? Not exactly. By definition, biostimulants work by modifying the microbial environment of the phyllosphere (plant foliage) and of the rhizosphere—where the plant’s roots meet and interact with the surrounding soil. Biostimulants stimulate desirable biological processes. Fertilizers simply provide the nutrients that enable growth. 

Some biostimulants can have fertilizing properties, but those nutrients must be derived from an organic source. Likewise, fertilizer products can include synergistic biostimulants that aim to reduce total fertilizer inputs by improving the nutrient uptake of crops.

Compost has long been used for both functions. It provides some NPK and other necessary micronutrients, along with a host of beneficial fungi and bacteria—important plant biostimulants. But this is not a requirement. Some products, like microbial inoculants, can have no nutritional content and still enhance plant growth.

Ultimately, plant biostimulants and fertilizers are two different categories of agricultural inputs and work through different mechanisms.

Understanding Different Plant Biostimulant Sources

The category of plant biostimulants has long evaded an official definition, largely because of the vast diversity and number of potential substances and microorganisms. The list of potential sources is long and ever-expanding, but here is a breakdown of some of the most commonly used and well-researched biostimulants.

Beneficial Microorganisms

Soil may look inert and lifeless, but that is mere deception. Beyond the power of the human eye hides the most diverse, dense, and vibrant habitat on the planet.

This is the world of microorganisms, many of which are beneficial for plant growth. These beneficial microbes partner with plants to improve nutrient uptake and tolerance to environmental stressors, like drought and extreme heat. Often called “microbial inoculants”, they can be in the form of bacteria, fungi, or mycorrhizae. Some have been studied for over a century and their ability to promote plant nutrient availability is well-established.

Beneficial microbes are found in organic inputs like compost and worm castings. They are also increasingly grown in highly controlled fermentation vessels. Here, researchers can target certain microbial functions for use in an array of growing mediums, including hydroponics. Unlike many conventional inputs, the supply of beneficial microbes is theoretically infinite and can be produced with a relatively small environmental impact.

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Humic and Fulvic Acids

Humic and fulvic acids occur naturally in soils through the breakdown of organic material. They can also be added as amendments via any aged organic matter, or as concentrates in granular and liquid form. Together, these acids can help reduce fertilizer application rates, enhance nutrient efficiency, and increase water stress tolerance in cropping systems.

Humic acids promote the growth of beneficial microbes and help stimulate chemical processes that “unlock” nutrients in the soil, making them available to crops. If you think of humic acids as a bridge connecting plants to previously unavailable soil nutrients, then fulvic acids are the trucks that move those nutrients. They help efficiently transport them through the plant cell membrane.

Seaweed Extracts

Seaweed is rich in biostimulatory compounds like amino acids and phytohormones. They have been used by organic farmers for decades.

Most seaweed extracts for use in agriculture are created from brown algae (Phaeophyceae). The algae go through heat or chemical-based extraction processes and are typically applied as a liquid spray to both soil and foliage. Studies have shown that seaweed extract treatments can significantly alter rhizosphere and phyllosphere microbiomes and reduce major incidences of pests and diseases, improving total marketable yields of a variety of crops.

Protein Hydrolysates

Protein Hydrolysates (PHs) contain carbon, peptides, and amino acids—biostimulants that encourage microbial growth, increase nutrient availability, and help plants withstand biotic and abiotic stress.

PHs are produced through enzymatic or chemical hydrolysis of animal waste and plant biomass. Common sources include blood meal, fish byproducts, casein, legume seeds, alfalfa hay, corn wet-milling byproducts, and other vegetable byproducts. Most PH products are currently animal-derived, but there are concerns about the safety of using animal byproducts on food crops, spurring growing interest in plant-derived products. Plant biomass PHs also provide a sustainable use for agricultural byproducts that are often wasted.

PH products are available as liquid extracts and soluble powders. They are commonly used in both soil and foliar applications.


Biopolymers are materials produced through the natural processes of plants, animals, bacteria, and fungi. Common biopolymers, like collagen, gelatin, and plant starches, are used in countless industrial and medical applications.

Some biopolymers have certain biostimulatory characteristics. Chitosan, derived from the shells of crustaceans, can protect plants against fungal pathogens and may improve plant tolerance for drought, salinity, and cold.

Other biopolymers, like sodium alginate, may have positive effects on fertilizer efficiency. When used in controlled-release fertilizer products, they can improve nutrient availability and uptake, limiting over-application and pollution resulting from fertilizer runoff.

Inorganic Compounds

Some chemical elements, like NPK, are required by all plants. Other elements, while not universally essential, still provide biostimulatory effects. 

The most important of these “beneficial elements” are:

  • Aluminum
  • Cobalt
  • Sodium
  • Selenium
  • Silicon

Many of the beneficial effects of more complex biostimulants, like seaweed extracts, can likely be in part attributed to the physiological effects these elements can have on plant growth.

Plant Biostimulants and the Future

Climatic changes are already impacting plant growth around the world. Drought and heat waves are increasing in intensity and frequency, affecting yields and the nutritional content of essential food crops. These changes are partly driven by agriculture itself, through the overuse of synthetic nitrogen  fertilizers and widespread soil degradation.

These enormous challenges are compounded by the fact that yields must go up in coming decades as the planet adds another 2+ billion people. Plant biostimulants offer one potential solution—a way to sustain yields while using fewer resources. They can reduce fertilizer use by improving nutrient bioavailability, save water by promoting water retention and root growth, and give plants added resiliency against the increasingly severe abiotic stresses of heat and drought.

Important questions about biostimulants still demand more answers. Much remains unstudied regarding proper application rates, the molecular modes of action, not to mention the countless beneficial microbes still unknown to science. But this much is clear: plant biostimulants are poised to play a necessary role in building a more sustainable and resilient agricultural future.

For further reading on plant biostimulants see the references used to write this blog:

Ali, O., Ramsubhag, A., & Jayaraman, J. (2021). Biostimulant Properties of Seaweed Extracts in Plants: Implications towards Sustainable Crop Production. Plants (Basel, Switzerland), 10(3), 531.

Backer, R., Rokem, J. S., Ilangumaran, G., Lamont, J., Praslickova, D., Ricci, E., Subramanian, S., & Smith, D. L. (2018, October 23). Plant Growth-Promoting Rhizobacteria: Context, Mechanisms of Action, and Roadmap to Commercialization of Biostimulants for Sustainable Agriculture. Frontiers in Plant Science, 9.

Borrelli, P., Robinson, D. A., Panagos, P., Lugato, E., Yang, J. E., Alewell, C., Wuepper, D., Montanarella, L., & Ballabio, C. (2020, August 24). Land use and climate change impacts on global soil erosion by water (2015-2070). Proceedings of the National Academy of Sciences, 117(36), 21994–22001.

Canellas, L. P., Olivares, F. L., Aguiar, N. O., Jones, D. L., Nebbioso, A., Mazzei, P., & Piccolo, A. (2015, November). Humic and fulvic acids as biostimulants in horticulture. Scientia Horticulturae, 196, 15–27.

Colla, G., Hoagland, L., Ruzzi, M., Cardarelli, M., Bonini, P., Canaguier, R., & Rouphael, Y. (2017, December 22). Biostimulant Action of Protein Hydrolysates: Unraveling Their Effects on Plant Physiology and Microbiome. Frontiers in Plant Science, 8.

Pilon-Smits, E. A., Quinn, C. F., Tapken, W., Malagoli, M., & Schiavon, M. (2009). Physiological functions of beneficial elements. Current opinion in plant biology, 12(3), 267–274.

Rouphael, Y., & Colla, G. (2020, February 4). Editorial: Biostimulants in Agriculture. Frontiers in Plant Science, 11.

Rouphael, Y., & Colla, G. (2018, November 13). Synergistic Biostimulatory Action: Designing the Next Generation of Plant Biostimulants for Sustainable Agriculture. Frontiers in Plant Science, 9.

Saberi-Riseh, R., Moradi-Pour, M., Mohammadinejad, R., & Thakur, V. K. (2021). Biopolymers for Biological Control of Plant Pathogens: Advances in Microencapsulation of Beneficial Microorganisms. Polymers, 13(12), 1938.

Thies, Janice & Grossman, Julie. (2006). The Soil Habitat and Soil Ecology.

Thompson, R. L., Lassaletta, L., Patra, P. K., Wilson, C., Wells, K. C., Gressent, A., Koffi, E. N., Chipperfield, M. P., Winiwarter, W., Davidson, E. A., Tian, H., & Canadell, J. G. (2019, November 18). Acceleration of global N2O emissions seen from two decades of atmospheric inversion. Nature Climate Change, 9(12), 993–998.

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