“Plant biostimulants contain substance(s) and/or microorganisms whose function when applied to plants or the rhizosphere is to stimulate natural processes to enhance/benefit nutrient uptake, nutrient efficiency, tolerance to abiotic stress, and crop quality.” (European Biostimulants Industry Council, EBIC)
- Biostimulants can contain traces of natural plant hormones, but their biological action should not be ascribed to them, otherwise they should be registered as plant growth regulators. Biostimulants may contain some nutrients but the mechanism of action must NOT be through provision of nutrition. Biostimulants have no direct action against pests, and therefore do not fall within the regulatory framework of pesticides.
- The term biostimulant has been applied to many different products, often with various modes of action, and vary variable composition. Often evidence of efficacy for specific products is poor quality and/or scarce; potential users should ensure they obtain reliable, good quality research data before committing to large expenditure on products of this nature.
- There is no regulation of biostimulants worldwide, however the European Biostimulants Industry Council provides a voluntary code of conduct for its members: both the code of conduct and a list of members are listed on its website.
Types of biostimulants
For the purposes of the PRG, biostimulants have been classified into three groups:
- “Organic soups” (including seaweed extracts, humic acids, fulvic acids, microbial inoculants and yeast extracts);
- Plant active compounds (including amino acids, peptides and chitin /chitosan);
- Beneficial inorganics (including phosphites, other inorganic salts, beneficial elements).
- Commercially available “organic soups” contain a wide variety of compounds which may or may not be present at concentrations which could be anticipated to be effective.
- They are usually extracts of natural products, so can vary widely between batches.
- Efficacy is likely to vary depending on the exact product and situation in which they are used.
Humic and fulvic acids
- Humic (HA) and fulvic (FA) acids are part of naturally occurring organic matter and their concentration will increase if the amount of OM in the soil is increased; application of manures and composts is likely to be more effective for improving soil health and microbial communities than HA/FA ‘out of a bottle’.
- Complex, rich organic material is likely to be best for improving HA/FA content, e.g. well-rotted manure and mature composts. Straw is very carbon rich but nitrogen poor so often takes a long time to break down, and requires an additional source of N alongside it.
- Experimental evidence for benefits to potato crops from applications of HA/FA is poor, and variability between products means robust evidence should be sought for each individual product.
Seaweed /marine algae extracts
- Similarly to HA/FA products, seaweed extracts can vary enormously depending on the source and extraction method, so evidence for individual products should be sought before use. Precise compositions and production methods are usually held as proprietary information, so it can be hard to determine exactly what a specific seaweed extract could provide for a crop.
- Extracts from seaweeds can contain reasonable concentrations of useful nutrients (e.g. Cu, Co, Zn, Mn, Fe, Ni, Mo, B), however the concentrations at which the products are used means the nutrients are so dilute that they cannot be useful for nutritional purposes. This can be checked for a given product by considering the ‘in the can’ concentration, accounting for in-field dilution (may be as much as 1:1000) and application rate (typically < 15 litres / ha); then comparing the resulting amounts to recommended application rates of that nutrient for the crop.
- It is likely that active ingredients in seaweed extracts stimulate the plant’s own hormonal responses in some way. Experiments have shown positive effects of seaweed extracts under laboratory conditions, but in-field results are variable.
Plant active compounds
- Any products used should be derived from plant, not animal, amino acids, to avoid problems with protocol restrictions.
- Response to amino acid products is particularly associated with amelioration of heat and / or drought stress.
See: Amino acids for further information.
How they work
- Phosphites are likely to have several modes of action, including:
- stimulation of stress responses
- increase in cell wall defences
- upregulation of genes for disease resistance responses
- It is likely that plant defences are only stimulated when plants are both treated and wounded, which is energetically favourable as resources are saved for when they are needed, rather than being preventatively used.
- Phosphites have been shown to successfully protect against symptoms of P. infestans in a range of potato cultivars and locations:
- application before symptoms are observed is likely to be most beneficial, for late blight on both leaves and tubers, as time is required for plant resistance responses to be activated;
- late blight protection is likely to persist for several weeks, which means it can be effective in storage.
Phosphite as nutrient?
- Phosphites SHOULD NOT be used on plants known to be deficient in phosphorus. This is because the presence of phosphite inhibits the phosphate starvation response: in other words, the plant does not “think” it needs more P, due to the presence of phosphite, but the plant cannot use phosphite as a nutrient source.
- Phosphite can convert to phosphate in the soil, however at the rates of application the amounts are unlikely to be beneficial in terms of nutrient provision. Phosphite is, however, stable and mobile in plant, so once taken up, it can benefit all parts of the plant – although application should intend to cover as much foliage as possible rather than relying on translocation.
Notes on biostimulants
Biostimulants 2016 (Summary of types of biostimulant products)
Phosphites 2015 (Effects of phosphite in disease resistance and thus suppression)
Phosphate and phosphite for potatoes 2014 (Phosphate is a nutrient; phosphite is not but can stimulate plant defence responses)
Magnesium and biostimulation (Magnesium is very important for potatoes; a biostimulant product including magnesium was investigated but not developed)
Information on biostimulant products
For information on products which have been tested by CMI see: Biostimulant products.