Health Food For Plants: Biostimulants

Fortunately, science is catching up to the industry’s need to better understand bio-stimulants. This starts with a look at the components of the typical biostimulant mix.

UP FROM THE SEA. Virtually all biostimulants contain a seaweed (kelp) extract which usually comes from Ascophylum nodosum, typically harvested off the northeast Atlantic coast. Seaweed extracts contain high levels of cytokinin hormones, which are substances that are synthesized in one part of the plant and translocated into another part, where it changes growth or function.

Cytokinin hormones are normally produced in plant roots and translocated to shoots. Numerous plant responses are associated with cytokinins, such as delayed leaf senescence (aging) in bermudagrass, improved cell division, seed germination and shoot initiation (Carrow, 1993).

Recent research also suggests that plants under stress have reduced metabolism due to a buildup of oxidative substances. Antioxidants can negate toxic levels of these substances, but a plant cannot always produce enough antioxidant to be beneficial, particularly in the presence of excess nitrogen. Applications of seaweed extract have been found to enhance antioxidant levels in grasses, thus improving plant metabolism (Schmidt and Zhang, 1997).

Cytokinins may also enhance photosynthesis, and subsequently the photosynthate (sugar) levels in the plant, which ultimately leads to improved rooting. This is sensible because we would expect a plant that has de-layed leaf senescence or improved shoot initiation to possess more efficient photosynthate production. Typically, the best response from seaweed extract occurs when the plant is under stress from a variety of causes (see box at left).

HUMIC ACID FACTOR. Humic acids are another component found in most biostimulants. Research has occured during the past 40 years, mostly in agriculture, but we can make some correlations to horicul-tural applications.

Most humic acid sources come from leonardite (the stage before organic substances form coal) or peat. When they are analyzed in the laboratory, they generally contain 50 percent by weight of carbon. These acids are extremely large, complex molecules which defy known methods of easy identification, but there are some common themes about their function:

1. Humic acids are known to improve nutrient availability, especially in high sand (low organic matter)and high pH soils, which typically have limited available nutrients (particularly micronutrients). One school of thought is that humic acids have a chelating effect, or serve as a transport mechanism between the soil and root hairs for micronutrients that would normally be tied up in high pH soils (Kononova, 1961).



2. Humic acids also mobilize the soil phosphate ion by competing with these ions for exchange sites on clay soils or for multivalent cations like iron, zinc, etc. This reaction can explain why humic acids enhance root growth indirectly by improving phosphorous solubility, as evidenced by high phosphorous starter fertilizers, which are commonly used at seeding (Aitken, Acock and Senn, 1964).



3. The literature also discusses humic acids as having an effect similar to the auxin hormones, which influence root, shoot and flower development and are similar to the seaweed extracts. Reports show enhanced antioxidant levels in turf under both favorable and drought-stressed conditions (Schmidt and Zhang, 1997).

VITAMINS TO GO. Biostimulants may also contain various vitamin packages. By definition, vitamins are organic compounds, which, in low concentrations, have catalytic and regulatory functions in cell metabolism. Unlike animals, plants have the ability to synthesize vitamins.

However, deficiencies can occur when the site of synthesis is disrupted via viable root loss or leaf tissue loss, (i.e. scalped turf or prolonged periods of low sunlight). Vitamin B1, for example, is a well known rooting hormone used in nursery cuttings. A decrease in root meristematic activity has been associated with vitamin B6 deficiencies where they are normally translocated from leaves, although they are found throughout the plant.

Biostimulants may also include micronutrients or NPK fertilizers. Typically, the NPK levels in biostimulants are low, and well maintained plants still require feeding with traditional fertilizer regimes.

As we gain a better understanding of the individual biostimulant materials and their functions in the plant, a common theme emerges. When events occur which disrupt normal plant metabolism, we can expect the best response from a biostimulant. These events may include:

• Mechanical root removal caused by root-feeding insects or freshly cut sod.



• Root-invading disease pathogens or nematodes.



• Transplanted trees, shrubs or bedding plants that lose root hairs in shipping and handling.



• Excessive heat or cold.



• Excessive moisture or drought stress.



• High salt soils.

APPLICATION TIMING. Biostimulant product labels should always be reviewed, but there are some recommendations regarding the best particular uses of biostimulants: