Biostimulants are perhaps more of a specialty product in turfgrass management.
But, their use is increasing, particularly in the golf course industry and athletic turf. A lot of confusion exists over what these products are, what they do, and how to successfully incorporate them into a sports field management program. Even though biostimulants have been around for many years, the science behind the benefits or effects of using them is not as well understood as, for example, fertilizer nitrogen. In some cases the physiological effects of particular biostimulants are well documented. But, often there is a gap between the response of turfgrass to the application of a particular biostimulant as measured in scientific experiments and reliable management recommendations as to how to effectively use these materials in the field. Thus the aim of this month’s article is to review what biostimulants are, what the benefits of using them might be, and some comments on incorporating them into your field management program.
What are biostimulants?
Biostimulants are materials that when applied in very small quantities produce a positive impact on either the growth of the grass, the health of the grass, or its tolerance to different stresses. The type of response from the plant differs from that observed when a fertilizer nutrient is applied. They are not a substitute for fertilizer or other important agronomic practices, such as irrigation or proper mowing. That said there is a very large list of compounds that are considered biostimulants. In fact it is easier to classify a compound as a biostimulant by what it does rather than what it is. Some of the major categories of biostimulants are the plant hormones, humus and humic acids, fulvic acid, and extracts of seaweed.
Plant hormones are naturally produced to regulate growth in response to environmental signals. The major categories include auxins, cytokinins, gibberellic acid, abscisic acid, ethylene. Many other compounds are known to have hormonal effects in plants. Among these, salicylic acid has been well researched for its biostimulants effects. Plant hormones are complicated to study because they occur in very low concentrations, making plant response measurements difficult. In addition to this, hormones tend to have multiple roles in plants and different metabolic processes are often governed by more than one hormone.
Cytokinins have received a lot of attention in research because it is known that they can increase plant resistance to certain environmental stresses. They are also known to delay some of the effects of plant aging, such as leaf senescence. Gibberellic acid is the one hormone that perhaps turf managers are most familiar with because it causes cell elongation and many of the plant growth regulators on the market work by suppressing its production. In fact, the effects of an over application of trinexapac-ethyl and other similar growth regulators can be counteracted by applying gibberellic acid to the plant.
Seaweed extracts contain mineral nutrients, hormones such as cytokinins and auxins, vitamins and other compounds. Not all seaweed derived products are the same, however. There are particular species of seaweed and methods of extracting them (in cold water, for example) that result in higher concentrations of cytokinins and other materials.
In addition to biostimulants that affect the plant, others improve plant health by improving soil conditions. Humus, humic acid, and fulvic acid occur naturally in decomposing organic matter and other biological sources. They act to increase soil cation exchange capacity which reduces nutrient leeching and improves nutrient retention. Some also act as chelators, making certain micronutrients, such as iron, more readily available for root uptake. Humic acids also enhance surface water penetration of soil. Humic substances also contain auxins, which have been shown to enhance the hormonal activity of seaweed extracts.
The compounds mentioned are by no means the only ones touted as having biostimulant properties. In addition, other materials that may be formulated with biostimulants include plant nutrients (macro or micro), nitrogen fixing bacteria, and wetting agents. Many of these materials may also have a benefit for turf plant health. The problem perhaps then is elucidating which materials in the biostimulant product are having what effect on the plant either alone or in synergy with 2 or more other ingredients.
Uses of biostimulants
According to manufacturers of biostimulants, when applied to turfgrass they increase resistance to pests, improve stress tolerance, improve nutrient uptake (thus fertilizer use efficiency), improve color and quality, increase root and shoot production, increase seed germination and establishment rates, among other claims. Some of these claims have been substantiated by scientific research, others have not or have only been by testimonials of end users.
A significant number of studies have been conducted by university researchers into the effects of biostimulants. In particular, research has been done to elucidate some of the stress conditioning or stress relieving roles of cytokinins, either alone or in extracts of seaweed, or combined with humic acids, and salicylic acid.
Plants produce an adequate level of hormones for their needs so long as they are not stressed. However, when the plant is stressed, one of the deleterious effects can be decreased hormone production. When plants are stressed they have a higher level of unstable oxygen molecules, such as O2-, and hydrogen peroxide, which are referred to as reactive oxygen or free radical oxygen. These molecules can be very damaging to lipids, proteins, chlorophyll, and other parts of the plant. Antioxidants are produced by plants to eliminate these free radicals but in a stressed plant the antioxidants are not produced fast enough to deal with the free radicals. Application of a biostimulants containing cytokinin and antioxidants such as beta-carotene to the plant has been shown to improve the tolerance of the grass to stress, such as heat or drought by increasing the levels of various antioxidants in the leaves. The cytokinin application is important to the stressed plant because it acts to delay processes meditated by the plant hormone ethylene that would normally cause the plant to “shut down” under stressful conditions.
Researchers found that, along with cytokinins, pre-stress applications of salicylic acid increased chlorophyll and carotenoid concentration in heat stressed plants due to the increased antioxidant activity. And, it’s not just heat and drought stress. The plant responds similarly to stress induced by saline conditions (irrigating with reclaimed water), high UV light, cold, and even misapplications of herbicides. Thus biostimulants may increase tolerance to a wide range of environmental stresses.
There is also published research that documents enhanced shoot and root development following application of seaweed extract. Early research showed that seaweed extract was more effective than an application of a synthetic cytokinin for enhancing plant growth. It was found that seaweed extracts contain auxins, vitamins and other substances that resulted in a more positive plant response compared to cytokinins applied alone. Other biostimulant products have been shown to increase shoot density. Applications of certain amino acids (which are building blocks for proteins) has been shown to improve photosynthesis and increase root mass.
Potential stress conditioning effects of biostimulants are well documented. Other research into the use of biostimulants has found that humic acid and seaweed extract applications were associated with reduced incidence of dollar spot on creeping bentgrass. Thus biostimulants may help to reduce disease incidence. Visual quality improvements when using biostimulants in addition to fertilizer have been documented. Research has also found that biostimulants applications acted as safeners for preemergence and other herbicides.
For athletic field management, it is important to be able to establish turf as rapidly as possible. Use of biostimulants has been investigated for this purpose and this is just one example of where potential benefits of use become less clear. A recent research paper did not report that the addition of a biostimulants would increase Kentucky bluegrass establishment on a sand based field. However, the author is aware of a sod farmer with a very large operation who claims to have reduced sod production time at his farm by 33%. Why the difference? It could be different products and biostimulants applied, or different soil types and environmental conditions. Or, it could be something else that we are not aware of.
Biostimulants on athletic fields
University research has, in some cases confirmed the potential for positive agronomic benefits through application of biostimulants. But the recommendations for how to effectively utilize particular end user biostimulants products are still emerging for turfgrass management. To put it another way, how biostimulants affect turfgrass managed as an athletic field is not as well known. A complicating factor, for example, is that the concentration of cytokinins and other biostimulants varies considerably among manufacturers which make it difficult to determine how much of a biostimulant is causing what effect. Products sold as biostimulants tend to be a mixture of ingredients derived from several different sources. Management recommendations for the use of fertilizers and growth regulators have existed for years, such that when you apply a particular product at a given rate, you can expect a particular range of increase or decrease in plant growth. In contrast, while end users are provided with application rate recommendations for a particular product, it is not easy to determine what amount of a particular biostimulant is being applied and at that rate, what range of response from the turfgrass might be expected. Management recommendations concerning how and when to use certain biostimulants or mixtures of different biostimulants in order to achieve a certain desired result are also under-developed.
We can generalize and make some management recommendations for biostimulants use. For example, products that contain plant hormones are more likely to produce a plant response. Turfgrass that is not under stress at time of application might not show much response to a biostimulant. However, applications of biostimulants prior to the onset of stress have been shown to improve the turfgrass plants response when the stress occurs. Applying frequently at lower does tend to produce more desirable effects. Since many of these products contain plant hormones, they can be over-applied and result in harmful effects to the turf.
When applied to turfgrass, biostimulants can…
- Increase resistance to pests
- Improve stress tolerance
- Improve nutrient uptake (thus fertilizer use efficiency)
- Improve color and quality
- Increase root and shoot production
- Increase seed germination and establishment rates
*ACCORDING TO MANUFACTURERS
Biostimulants are an interesting, diverse, and complicated group of compounds which when applied to turf may produce a favorable growth response. This response might be an increase in shoot or root development or an increase in tolerance to certain environmental stresses, such as heat and drought. You should first establish what deficiency in your field management program you intend to correct with a biostimulant. Or, you should determine what performance enhancement goal you hope to achieve. Remember that biostimulants are not meant as a substitute for proper agronomic practices, such as irrigation and fertilization. In fact, some claims regarding benefits of biostimulants such as reducing requirements for pesticides and fertilizers are not often supported by university research.
In conclusion, research continues to investigate potential uses of biostimulants for turfgrass management. When selecting a biostimulant for use on athletic turf, the value of testimonials from other end users (field managers) should not be totally discounted. But, when available, you should consult university testing data or data gathered from other reputable sources. Some companies are long established and have research data to support use of their products. Others formulate complicated products with little or no research.
Be wary of product claims that sound too good to be true. Also, when using a biostimulant on your fields for the first time, leave a check plot or two so that you have a direct comparison between treated and untreated turfgrass. Another thing to consider is that, in many cases, biostimulants require many applications over a period of time before benefits may appear. Thus, you may need to test a product over a growing season or two in order to really determine if the benefit exists with that product on your field. At a minimum, those products that might improve stress tolerance should be applied beginning several weeks before the anticipated stress occurs.