“Biostimulants” are taking turfgrass and other plant disciplines by storm. Most green industry retailers sell biostimulant products with a wide range of claims. We surveyed major fertilizer companies asking, “What are the main trends you see in industry?” Without prompting or exception, they answered biostimulants are one of the top trends, with millions of dollars invested and billions in sales.

This investment wouldn’t likely occur without something valuable behind it. However, there is significant misguided and missing information. We don’t have all the answers, but the following serves as a general guide with suggested readings for those that want to explore more deeply.

What is a biostimulant?

Most major dictionaries and encyclopedic references do not define “biostimulant.” The USDA National Agricultural Library does not list it in their glossary of terms. A definition was added to the current farm bill that a biostimulant is “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.”

This definition is too vague because it fails to exclude traditional products. After reviewing the definition in various scientific papers and company web sites, we propose the following definition that biostimulants are microorganisms and/or chemical substances which enhance plant growth and quality, often due to abiotic stress tolerance; excluding traditional pesticides, fertilizers, and soil amendments, such as limestone and gypsum. Biostimulants generally fit into five categories.

Do they work?

The answer is “it depends.” There are several reviews on biostimulants listed below. Yakhin et al. (2017) shows more than 100 sources with over 300 ingredients and bioactive compounds that have been studied. Many are currently in the marketplace. The Hopkins research lab has conducted 178 field and greenhouse biostimulant trials, on a variety of plant species, over the past two decades. The average yield/quality increase was 0.9%, which was not statistically significant. When separating by type, the only category showing a significant response was the soil and geological extracts, with all of these applied as humic or fulvic acids. However all categories had at least one trial with a positive response. This shows potential for biostimulants, but also suggests caution due to so many trials not showing a positive response.

For humic and fulvic organic acids, we found they worked consistently when mixed with phosphorus fertilizer and applied to soil with low soil test phosphorus levels and generally with low organic matter and high pH. Theoretically, sites with strongly acidic soil pH would also be responsive. It is imperative to cut the fertilizer rate by a third to half to prevent toxicity. The mode-of-action we have shown for these organic acids is that phosphorus is more soluble in soil solution and, therefore, more mobile. We show the effect was likely due to this and not some other biostimulation. However, the work of Olk et al. (2018), conducted in an environment very different than ours, suggests stimulation in other ways.

In field trials with microbes (bacteria or fungi), we recognize the potential for these to be beneficial, but we didn’t measure such in our trials. The main problem was that the microbes originally in the jug were dead on arrival. A large percentage of the products we tested were packaged with fertilizer and, as such, had extreme pH and/or salt content, killing the microbes. Additionally, an important fact when considering microbial biostimulants is that soils are highly microbially active—even with applications of fertilizers and pesticides. There are more than a trillion microbes in a teaspoon of soil. Adding a few more typically doesn’t work, although we do have proven success when inoculating legume seeds with Rhizobium to facilitate nodulation and nitrogen fixation and when adding Mycorrhizae fungi to soils, especially those with low fertility and/or water supply. The potential is there, but largely unsuccessful in our trials.

In the case of silica, it is not an essential nutrient, but it is known to be beneficial to plants. However, the average soil is comprised of 28% silica. Most of this is in a solid form that is not accessible immediately by plants, but there is ample soluble silica floating in soil solution. Not surprisingly, we did not measure any plant response or increase in silica content in plant tissues in our research trials.

We tested dozens of products claiming to loosen soil compaction. None of them improved the compaction, which is not surprising. Compacted soil is a physical problem that is not likely solved with a chemical solution. Rather, aeration and topdressing continued to prove effective. 

We tested dozens of products claiming to alleviate salt stress. Again, none of them worked, although there are many reports in the literature declaring biostimulants help with drought and salt stress. In contrast, salt problems were effectively alleviated with leaching with non-saline water. Keeping the soil relatively moist, blending saline water with non-saline water, and/or using salt tolerant species/varieties are also known to be effective. We recommend these solutions as first choices before trying biostimulants.

Again, we had at least one statistically positive response for all of the biostimulant categories. And, there are many reports in the scientific literature of documented benefits. Thus, we do not want to be too pessimistic despite the majority of our studies failing.

Buyer beware

There is potential for biostimulants to be effective. In the case of organic acids, the research is a bit further advanced than the other categories. We generally know how to use organic acids in crops, although there is more to learn. Although we’ve measured many positive responses to organic acids in row crops, we have only rarely (twice out of 18 studies) measured it in turfgrass. We feel the reason for this is that the roots of turfgrasses are exceptionally efficient at finding phosphorus in the surface soil. Also, over 95% of turfgrass soil samples coming to the BYU Environmental Analytical Lab have enough phosphorus in them to last over 5 years without any additional phosphorus fertilization. According to this, adding organic acids and expecting a benefit is a bad bet if there is ample phosphorus already.

Another consideration is that informal observations suggest that biostimulants do not hold much promise for a well-managed turfgrass that is not in distress. The likelihood of biostimulants working is slim if plants have near optimal conditions, including light; water; oxygen in the rootzone; mineral nutrients; temperature; and minimal presence of toxins and pests.

Admittedly, the majority of our studies occurred under good conditions, which may be a reason for so few showing a positive benefit. More work is underway to evaluate their response when under stress.

William Edwards Deming, a major player in the Japanese post-World War II economic boom, is famous for saying “In God we trust, all others must bring data.” He attributed his success to listening to “experts,” but not just believing them blindly unless they had data to back up their claims.

In some cases, biostimulants work. In many others they do not. It is the responsibility of the companies selling these products to provide third party independent testing and reliable management guides. If they are proven, the next question is when and how do they work? Under what conditions? Stress? If a product looks promising and has this data backing it up, conduct your own trials by partnering with a scientist that can help set them up correctly. Be sure to compare it to both a negative and a positive control, as in the “Beware of false data” sidebar.

Biostimulants are an exciting trend with lots of promise. However, don’t abandon proven practices for promises that seem too good to be true. Be optimistically pessimistic. Keep an open mind, but realize that most of these won’t likely work, especially if the turf is well managed and healthy. In the meantime, independent and industry scientists will continue to search for reliable products and ways to manage them. We advise to not be on the cutting edge of the biostimulant world to the point of throwing caution to the wind and chasing every new product with miracle claims. On the other hand, don’t be so pessimistic that you miss good quality products. Remember, fertilizer used to be considered a “snake oil.”

Beware of false data

We ran across a company sharing some of our data. Unfortunately, they were only showing part of the data to shed a more positive light on their product we had evaluated. Yes, their product showed improved rooting and color in comparison to the negative control without anything added. However, it didn’t show any difference compared to the positive control. When we ran our tests we were aware that their product had fertilizer nutrients in it. Thus, we crafted a treatment that had equal fertilizer (positive control) amounts using more traditional sources. Both our version and the biostimulant we were testing performed well, although the cost of the biostimulant was exorbitantly higher. In this case, there was no measurable biostimulation. Rather, it was simply a fertilizer response.

Bryan G. Hopkins, PhD, is a Certified Professional Soil Scientist (CPSS) and a professor in the Plant and Wildlife Sciences Department in the College of Life Sciences at Brigham Young University. Elisa A. Woolley is a graduate student at BYU.

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