Questions and answers about biostimulants

Questions and answers about biostimulants

Superintendents are often in the dark about the best ways to use biostimulants and what types of results to expect. Biostimulants are organic materials that, when applied in small quantities, enhance plant growth and development. Seaweed and humic acid are the two most commonly used ingredients in biostimulants.

Several golf course superintendents have expressed some confusion about how to determine the best way to incorporate biostimulant treatments in a turfgrass nutrient program.Our conversations were reminiscent of a recent research article written by a good friend, who said that research is like a good mystery story (1). She indicated that Sherlock Holmes would have made a good research agronomist, as he would thoroughly understand the hazards of a rush to judgment. In ”A Scandal in Bohemia,” Holmes tells Dr. Watson, “It is a capital mistake to theorize before one has data. Insensibly one begins to twist facts to theories, instead of the theories to suit facts.” Some superintendents are much like Sherlock Holmes: A cultural practice becomes elementary only when the facts are realized. From the 10 years of biostimulant data that we have amassed in field, greenhouse and laboratory studies, a number of facts have emerged. A biostimulant is an organic material that, when applied in small quantities, enhances plant growth and development such that the response cannot be attributed to application of traditional plant nutrients. Biostimulants also may be referred to as “positive plant growth regulators,” or as James Beard, Ph.D., recently coined, “metabolic enhancers.”Our research has documented that applications of biostimulants have conditioned turf grasses to tolerate environmental stresses and improve grass growth, particularly root development. Biostimulants have been shown to improve turf grass photochemical activity (an estimate of photosynthetic efficiency) and overall quality when the turf is subjected to low soil moisture, dollar spot, non target pesticide applications, nematode infestation, high soil salinity, high UV light intensity and heat. How much and how often? How does a superintendent know how much of which material to use? How often should a biostimulant be applied? These questions are difficult to answer because biostimulants are manufactured from different materials and formulated at various concentrations. (Some that have low concentrations of biostimulating materials may seem less expensive, but they actually cost more in the long run because more material is required to realize stress-tolerance benefits. Economic comparisons should be done on the basis of cost per unit area.) Confusion concerning biostimulants seems to arise because many materials are classified as biostimulants but have not performed as advertised. At Virginia Tech we have shown that seaweed extracts, humic acids, triazole fungicides, amino acids, potassium silicate and, most recently, low doses of salicylic acid have demonstrated biostimulant properties. However, the two most commonly used biologically active ingredients – seaweed and humic acid – have different sources, and different procedures are used to extract these materials from their sources.


The chemical composition of seaweed is determined by the conditions under which it is grown. T.L. Senn, Ph.D., from Clemson University, has studied the influence of seaweed products on plants for many years. He has indicated that brown seaweed, Ascophyllum nodosum, prepared by alkaline hydrolysis from Norwegian waters, is a stable product when subjected to rigid quality control. We have found that seaweed extracts obtained from a similar latitude in Nova Scotian waters provide similar biological activity (12,13). Hormones such as cytokinins and auxins have been isolated and quantified from these extracts (2) and may be the active ingredients.

Humic acids

Humic substances can be extracted from soils, peat, coal (leonardite) and lignite in an alkaline solution. These extractions are separated into humic acid and fulvic acid fractions by acidification. Humic acids are precipitated under a pH of 2 or lower. Fulvic acids, soluble at all pHs,have lower molecular weights and are the most biologically active fraction. The two main aspects of these humic fractions that influence plant growth are the auxin content and the ability to chelate certain inorganic nutrients such as iron.

Dosage and frequency

Results from independent research reports should be used to verify the efficacy of commercial biostimulant materials and also to confirm that suppliers are providing formulations with proper dose and application frequency recommendations. Our experience is that frequent, low-dose applications are more beneficial than infrequent high-dose treatments. One should be aware that most of the biostimulants are hormonal in nature, and excess applications could be harmful to plants. For frequent applications (once a month or less), we obtained excellent results when 0.5 ounce of humic acid, 0.2 ounce of seaweed extract and 0.5 ounce of rriazole fungicide were applied in combination per 1,000 square feet. When these materials are applied alone, the dosage may be doubled. (Because of labeling laws, triazole fungicides will not be packaged with other biostimulants. (The superintendent will have to do the blending.) The effect of a single biostimulant application can be expected to decrease gradually with time. There are some indications that better results are obtained when sequential treatments are made, and the second year is better than the first. Monthly applications before and during the stress periods (three to six applications per year) should be programmed. Many biostimulant products list the mineral nutrient content in order to qualify in labeling. Sometimes these are listed as auxiliary to the biologically active substances (such as hormones). The “bonus” major nutrients are generally small amounts and contribute little to plant growth and development.

View the entire article as a pdf