Seaweed, kelp and similar products are classified as biostimulants which are defined more by what they do than by what they are. As the name suggests, they stimulate growth, but they do much more.
Stress tolerance is perhaps the most important benefit of biostimulants. Biostimulants impart stress tolerance partly by stimulating root growth and partly by promoting antioxidant activity. However, we still have much to learn about how biostimulants work, so these and other functions will become better understood with additional research.
Virginia Tech evaluated a number of biostimulants for use on turfgrasses. Of the various materials examined, the most promising was seaweed extract. These products seemed to exert beneficial influences either through hormonal effects or by raising antioxidant levels in the plants.
Although biostimulants can be synthetic, naturally occurring organic materials are excellent sources of biostimulants. For example, humic acid and seaweed extract are two commonly used turf biostimulant sources.
Seaweed contains various hormones, vitamins, amino acids, mineral nutrients and other components. Thus, it may affect plants in several ways. However, its stimulating influence, particularly for turfgrasses growing under environmental stresses, has been attributed to its hormonal activity.
Under favorable growing conditions, turfgrass grows well without adding biostimulants. Beneficial effects of biostimulants may not be easy to identify based only on leaf color or other visual clues. When the turfgrasses become stressed, biostimulant treated grasses actually performed better because they had developed a better defense system, apparently due to higher levels of antioxidants created by the biostimulants.
As with mineral fertilizers, biostimulants take time to show any appreciable influence on turfgrasses. Under the university's studies, growth stimulation was not large enough to measure until 4 weeks after treatment. After 6 weeks, by contrast, they found significant differences in leaf and shoot number, as well as shoot and root weight of treated vs. non-treated Kentucky bluegrass and creeping bentgrass. These findings suggest that biostimulants yield the best results when applied several weeks before a stress occurs. So, if you believe your lawn will suffer stress in the next month, a biostimulant may prove beneficial.
Most seaweed biostimulants come from kelp that has been harvested, dried, and ground. Kelp meal is suitable for application directly to the soil, or for addition to the compost pile. It flows easily and is readily applied with most dry fertilizer applicators. It is easily mixed with other dry fertilizers and amendments.
Dried raw seaweed tends to contain about 1% nitrogen, a trace of phosphorus, and 2% potash, along with magnesium, sulfur, and numerous trace elements. Raw seaweeds are prepared by various methods and sold under a number of brand names.
More often, compounds from kelp and other seaweeds are extracted by various methods in order to concentrate both micronutrients and naturally occurring plant hormones into a soluble, easily transported form. Such kelp extracts are sometimes applied as a spray. For the most part, none of the micronutrient levels in kelp extracts is high enough to correct a deficiency, but as a "tonic" providing a broad array of micronutrients and other trace elements, seaweed extracts have won a measure of acceptance among organic farmers.