“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
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
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.
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
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.