Balancing the economics of crop input and output is an ongoing challenge for many growers. With the apparent effects of climate change affecting plant growth and crop yields, new technology becomes more of a necessity to sustain our agricultural production.

Even on a smaller scale of farming, issues regarding plant growth and crop yields are constant across growers. As a result, many farmers seek new technology to adopt, hoping to keep up with the changes in the agricultural industry. Among these modern types of horticultural technology are biostimulants.

Agricultural experts claim to see improvements in plant growth and crop yields when using biostimulants. These products reduce the need for fertilizers and pesticides while maintaining an excellent crop yield and plant quality.

To clarify, biostimulants require further studies as they are relatively new agricultural additive products. However, experts (mainly from Europe) are conducting studies that show much promise for the shifting agrarian landscape. Based on existing studies, biostimulants may be the answer to increasing farm sustainability. Still, we must first understand what these products are.

There is no official definition for biostimulants. The existing definitions come from industry experts in Europe who aim to help farmers grow plants more efficiently.

Professor Patrick du Jardin, the Director of the Botanic Garden at the University of Liège in Gembloux, Belgium, says biostimulants are “defined by what they do” instead of what they are. What biostimulants do is alter the microbial content in soil to promote plant growth. These products are initiatives to contribute to sustainable agriculture.

The European Biostimulants Industry Council (EBIC) has its definition of plant biostimulants:

Plant biostimulants means [sic] a material which contains substance(s) and/or microorganisms whose function, when applied to plants or the rhizosphere, is to stimulate natural processes to enhance/benefit nutrient uptake, nutrient efficiency, tolerance to abiotic stress, and crop quality, independent of its nutrient content.

In essence, we can compare biostimulants to fertilizers. These materials would contain substances that promote plant growth by enhancing the rhizosphere. The rhizosphere is around a plant’s roots, where microorganisms interact with soil nutrients and root secretions. Thus, biostimulants are kinds of materials that help plants grow through biostimulation.

Biostimulation is a process involving environmental modification. This process occurs when one applies biostimulants to an area. During biostimulation, the biostimulant materials raise the levels of the existing bacteria in an area. Stimulating these local bacteria will then lead to bioremediation.

Bioremediation is another process involving bacteria and microbes in a particular area. Areas that may need bioremediation are contaminated media like soil, water, and surface materials. The bioremediation process removes contaminants and pollutants by utilizing the natural bacteria present in the contaminated areas.

Experts mostly associate biostimulation with removal and remediation. Particularly, biostimulation aims to remove hydrocarbons, which naturally occur in crude oils like petroleum and natural gas. Biostimulation also aims to remediate high production volume (HPV) chemical spills.

The Environmental Protection Agency (EPA) has approved biostimulation as a process to reverse oil and gasoline spills. There areexisting cases of marine oil spills that served as testing grounds for biostimulation. Generally, the biostimulation process involves adding essential nutrients to an affected site.

The nutrients added to the marine oil spill would promote the biodegradation process. The added biostimulants enhance and stimulate the existing microbes that thrive on hydrocarbons. As the microbes degrade the hydrocarbons from the oil spill, the affected area slowly gets cleaner.

Having a solution to speed up the biodegradation process in an oil spill is promising for environmental purposes. However, it is essential to note that bioremediation is a new process that requires further studies. In addition, there are several uncontrollable variables in an oil spill that make it difficult to assess the success rate of biostimulants in such events.

The reluctance of approving biostimulants for other purposes is because of the disadvantages these have in treating marine oil spills. Bioremediation is a slow process. And, since the process involves adding foreign substances to stimulate indigenous microorganisms, there is no telling how the existing biosphere may react.

Still, ongoing research is looking into the efficacy of biostimulants while considering the different uncontrollable variables. These factors include soil composition, water characteristics of the affected site, the indigenous microorganisms present in the affected area, and the available nutrients that would benefit from the biostimulants when applied to the affected site.

Apart from being an ideal method to clean oil and gas spills, biostimulation has the potential to treat smaller-scale contaminations. Using the same principle of adding materials to stimulate the existing microbes in an affected area, biostimulants may help treat pesticide and herbicide contaminant spills.

Using microbes to degrade contaminant spills may prove helpful in areas at risk of such pollution. For example, some places that pesticides or herbicides may contaminate are open water areas or sandy plots of land. The latter case may be more apparent in the agricultural sector.

In the agricultural sector, biostimulants are another class of products designed to increase yield and improve farm sustainability. To reiterate, biostimulants still have no legal definition. They may seem like plant fertilizers because they work toward similar goals. However, biostimulants do not directly offer increased crop quality the way fertilizers do.

Relatedly, biostimulants do not directly increase crop resistance against diseases the way pesticides do. Fertilizers and pesticides are input products that directly enhance plants. Biostimulants do not act on plants. Instead, they improve the microorganisms that affect plants.

The EPA classifies some fertilizers as plant growth regulators (PGRs), which are chemicals meant to improve plant growth. These chemicals modify plant growth in ways including, but not limited to, increasing branching, suppressing shoot growth, boosting yield, removing excess fruit, and altering fruit maturity.

Biostimulants may affect plant growth in a different way than traditional fertilizers. Biostimulants affect the plants’ vigor by changing the microbial makeup in the soil around the plants’ roots. In other words, biostimulants are like soil additives that only target the existing microorganisms in a particular area. Stimulating the natural processes without adding extra chemicals enhances the plants’ resistance against abiotic stresses like excess or deficit water and high or low temperatures.

Plants with increased tolerance against these factors may grow stress-free. When plants grow without abiotic stresses from hostile environments, they are more likely to increase yields. In contrast, fertilizers help plants grow by adding more nutrients to the soil rather than enhancing the existing microbial makeup in the ground.

The EBIC differentiates biostimulants from traditional fertilizers and pesticides in two ways:

1. Existing Nutrients: Biostimulants affect plants regardless of the presence of nutrients in the biostimulant product. Even without the extra nutritional content that traditional fertilizers supply plants with, biostimulants may help plants grow by boosting the activity of the existing nutrients in the soil.
2. Plant Vigor: Biostimulants only affect a plant’s vigour or hardiness. These products have no direct effect on the plant’s susceptibility to diseases and pests. Pesticides are chemicals that you would add to plants to protect them against pests and diseases. Biostimulants only strengthen a plant by enhancing the existing microbes with which it already works.

Since biostimulants contain extracts and compounds that should improve a plant’s growth, development, and yield, these products appear to be an exciting breakthrough in the agricultural sector’s ongoing pursuit of alternative ways for farm sustainability. Still, the lack of official definitions and standard licensing may leave many growers sceptical of biostimulants’ effects.

A study exploring biostimulants and abiotic stresses in plants highlighted the several ways crops responded to biostimulants amid different abiotic stresses. The study’s findings may provide some insight into the potential of biostimulants in keeping up with the debilitating effects of climate change on the agricultural sector.

Water as abiotic stress in plants may be excessive watering or water deficiency. The latter is a more prevalent issue, with drought conditions persisting, worsening, and dramatically expanding across vast portions of the U.S. Drought stress negatively affects a plant’s molecular, biochemical, ecological, morphological, and physiological traits. These adverse effects hinder plant growth quality and affect plant yield quantity.

The study mentioned that some plants in their early development stages had demonstrated accelerated recovery rates with biostimulants in the mix despite low water availability and unfavourable conditions. Organic biostimulants have especially shown a positive impact on a plant’s productivity and resistance to water deficiency.

Soybean plants under water stress have shown an increase in dry mass and leaf area with biostimulant application. In addition, the plants’ total chlorophyll index has increased, suggesting a greater photosynthetic efficiency, meaning the plants become more capable of converting light energy into chemical energy.

Other plants in the study exploring biostimulants and water stress were sugarcane and maize. Sugarcane with biostimulants resulted in a higher productivity and profitability index. This effect suggested that biostimulants offer an effective economical solution amid droughts.

Maize with biostimulants resulted in higher relative water content in leaves and lower leaf and air temperatures, suggesting more resilient plants despite water deficit conditions.

Plants that experience salt stress experience inhibited growth and production rates. Soil with high salinity levels reduces a plant’s ability to absorb water, leading to a decrease in growth rate. Plants also risk absorbing salt when there are excessive amounts of salt in their growing environments. This salt absorption would lead to further growth reductions because absorbed salts can injure a plant’s cells in its leaves.

Humic biostimulants are a category of biostimulants derived from decomposed material. These products have shown promising effects in salinity stress protection. Such products added to salt-affected soil have improved the affected area’s physical and chemical properties, granting the soil capacity to support plant growth. In addition, humic substance-based biostimulants showed potential in improving a plant’s water absorption despite being under salt stress.

The study mentioned the effects of adding humic acids to common beans and rice. Applying humic acids to common beans under high salinity stresses reduced the plants’ membrane leakage rate and increased endogenous proline (an amino acid) levels.

A boost in the inner specific amino acid levels and reduced membrane leakage are indicators of a plant’s adaptation to environments with high salinity levels. The effects of humic acids suggest the potential of humic substance-based biostimulants in improving a plant’s resistance to high salinity levels.

Meanwhile, other biostimulants applied to plants, as mentioned in the study, include algae and AMF-based products. Algal extracts have shown alleviation in salt stress among Kentucky bluegrass based on turfgrass experiments. Likewise, arbuscular mycorrhizal fungi (AMF) have suggested protective effects for tomato plants against salt stress.

There are three classifications of temperature stress in plants: high, chilling, and freezing temperatures. These abiotic stresses reduce a plant’s ability to photosynthesize, which would lead to plant growth retardation and reduced germination rates. Furthermore, the effects would then affect a plant’s yield.

Plants have natural mechanisms to cope with temperature stress. Their biological processes involve molecular mechanisms composed of natural protectants, including antioxidants, membrane lipids, metabolites, and proteins. Biostimulants target these natural processes to help plants cope better under temperature stresses.

Biostimulants have shown promising effects on melon plants under high-temperature conditions. The study highlighted an experiment subjecting melon plants to temperatures of 25°C and 40°C (77°F and 104°C). The experiment showed that adding biostimulants to melon plants as a temperature stress reliever improved germination and initial growth in both temperature levels. These findings suggest that biostimulants have the potential to enhance melon’s initial development in high temperatures, which may prove beneficial in regions experiencing rising temperature levels.

On intense cold temperature stress (including chilling and freezing categories), porcine haemoglobin hydrolysate (PHH; proteins derived from pig, having undergone water treatments or hydrolysis) used as biostimulants for strawberry plants have promising effects on increasing plant biomass. Researchers subjected strawberry plants to intense cold temperatures, applied PHH biostimulants, and then observed early flowering. This finding may open possibilities of improving yields during winter.

Since biostimulants still have no legal definition, categorizing plant biostimulant products remains an ongoing effort. Still, scientists, regulators, and stakeholders in the biostimulant movement widely recognize some major biostimulant categories, which mainly include substances and microorganisms.

Volume 196 (2015) of Scientia Horticulturae, a scientific journal, highlighted the following seven main categories of biostimulants:

Humic substances (HS) are organic compounds from the plant, animal, and microbial decomposition. HS also forms when soil microbes metabolize the residue from decomposing matter.

Experts have long recognized HS as an essential contributor to soil fertility. These compounds affect soil properties on physical, chemical, and biological levels. HS biostimulants’ effects mainly involve root nutrition improvement by increasing a plant’s uptake of macro and micronutrients.

This category of biostimulants contains amino acids that undergo hydrolysis. These proteins come from plant and animal matter. These products may also have other compounds with nitrogen molecules like betaines and polyamines.

Biostimulants based on protein hydrolysates have shown promising effects in modulating a plant’s nitrogen uptake and assimilation, essentially affecting a plant’s growth. Some products have also variably demonstrated significant improvement in yields.

Biostimulants based on seaweed are still new, but this organic matter has a history of fertilizer usage. Existing reports on biostimulants with seaweed extracts highlight the substance’s effects on improving a plant’s tolerance to abiotic stress.

Chitosan is a deacetylated chitin derivative. Deacetylation is a process that removes chitin (a biopolymer or fibrous substance that forms fungal cell walls) acetyl groups and substitutes them with reactive amino groups.

This process improves a plant’s stress responses, particularly against fungal pathogens, consequently inducing significant physiological changes. In addition, biostimulants based on chitosan have demonstrated the capacity to enhance a plant’s self-defence against environmental stress.

Inorganic compounds are traditional fungicides. These compounds include cobalt, silica, and selenium. As biostimulants, they improve plant tolerance against abiotic stress and act on nutrient efficiency to promote plant growth.

Some experts may classify biostimulants containing beneficial fungi as biopesticides. This inconsistency goes back to the fact that biostimulants remain unofficially categorized with no standard definition. Still, some products containing beneficial fungi may promote crop yield by affecting the microbial activity in the rhizosphere, which may classify these as biostimulants.

Like products containing beneficial fungi, biostimulant products with beneficial bacteria target the rhizobacteria that improve a plant’s nutrient efficiency. In addition, these products offer benefits in disease resistance and abiotic stress tolerance through the rhizosphere.

At the same time, biostimulant products containing beneficial bacteria closely resemble rhizospheric PGPRs (plant growth-promoting rhizobacteria), which are under a different product class.

The main benefits of biostimulants align with the product’s intended use, based on the EBIC’s definition of biostimulants. These products boost a plant’s nutrient efficiency for more effective growth.

These products also increase its tolerance to abiotic stresses like changes in temperature, fluctuation in water intake, high salinity levels, and ultraviolet radiation.

This product class has demonstrated abilities to overcome farmers’ expensive investments in other methods to improve their harvestable yields. High-quality and carefully researched biostimulants have supposedly enhanced the sustainability of a grower’s land by reducing the need for inputs like fertilizer and pesticides.

Biostimulants’ potential to reduce the need for fertilizers and pesticides owes to the products’ unique mechanism of targeting the natural microbes existing in the soil. Traditionally, farmers would add fertilizers or pesticides to impact a plant’s growth directly. Instead, now, with biostimulants, farmers add substances to the earth to activate the natural micro-bacterial activity in the soil.

An extra benefit that may come with the reduced dependence on chemical additives like fertilizers and pesticides may be the reduced risk of chemical runoff. If farmers use less of these inputs in their agricultural practices, there will be lesser chances of the same chemicals leaching into other sources.

Biostimulants have also allegedly made agricultural land more sustainable by decreasing nutrient leaching, which is detrimental to plant growth and crop yield. In addition, experts have seen increases in crop value through improvements in fruit quality and shelf-life extensions.

Other potential benefits biostimulants carry are improvements in biofortification. This process may involve agronomic practices and conventional plant breeding, but biofortification involves modern biotechnology in the context of biostimulants. Biostimulants can improve food crop quality through biofortification, enhancing the plant’s nutritional composition, such as their sugar and protein content.

Biostimulants are products that contain substances or microorganisms intended to improve plant growth and boost crop yield. Regardless of the products’ nutritional content, biostimulants boost plant traits, improve their nutrition efficiency, and increase natural tolerance to stressful environments.

No, biostimulants and fertilizers are different horticultural product classes. Biostimulants are comparable to fertilizers in terms of their goal to enhance farming sustainability, but biostimulants operate differently.

Biostimulants alter the microbial content in the soil. These products stimulate the micro-bacterial activity in the ground by providing nutrients to which existing bacteria respond. When these naturally occurring microorganisms work double-time, natural plant processes will likewise increase.

With increased natural plant process activity, plant growth will improve and crop yield will increase. The biological processes also improve a plant’s hardiness, making it more tolerant to abiotic stresses like low or high temperature and deficient or excessive water levels.

Yes, biostimulants can improve plant growth. These products contain substances that are vital to a plant’s natural processes that influence its development. Biostimulants are applicable to plant seeds, shoots, and the surrounding soil. The products directly affect the bacteria that promote plant growth instead of directly affecting the plant.

Yes, biostimulants can increase plant yield. Correctly applying biostimulants to a crop will directly affect the bacteria that promote plant growth, development, and resistance to abiotic stresses. By stimulating growth and mitigating plant stress, crop yield is likely to increase.

Biostimulants can be natural or synthetic products. These products may contain substances composed of plant hormones or their precursors to benefit the physiological processes the plant goes through.

Biostimulants may be the answer to the pressing need for new technology for sustainable agriculture. Plant biostimulants contain substances that increase a plant’s natural ability to resist stressful environments, thus contributing to overall plant growth and development. This resistance also boosts a plant’s quality and yield.

Official definitions for biostimulants continue to evolve. This lack of common identity indicates that biostimulants are comparable to many input materials such as fertilizer, despite being different product classes altogether. Still, European experts dedicated to studying biostimulants agree on a definition that revolves around its function.

According to the NBIC’s accepted definition, biostimulants are products with substances that incite natural processes to enhance nutrient uptake, nutrient efficiency, tolerance to abiotic stress, and crop quality, independent of its nutrient content. Simply put, these products have the ingredients to improve plant resilience for better growth and development, regardless of the product’s nutritional content.

Despite the promising effects of biostimulants in improving agricultural sustainability, longitudinal studies into the underlying mechanisms of the products’ effects remain ongoing. In addition, there are several accepted categories of biostimulants, and we still know little about each of their specific actions on certain plant species. The possibility of various effects on different species is also a concern, suggesting that biostimulants may not be an all-encompassing product for enhanced plant growth and development.