Crop biostimulants

The desire to reduce the use of industrial fertilisers and plant protection products has led to the recent emergence of a large number of new and extremely varied alternative substances aimed at :

•

Stimulate the natural defences of plants (SDP - biostimulants).

•

Promote the absorption and use of nutrients.

•

Increase plant resistance to diseases and abiotic stresses.

•

Improve crop quality and yield.

The origin and nature of SDP and biostimulants is extremely varied; they are living micro-organisms, minerals, organic substances, and even synthetic products which are therefore not allowed in organic farming. Their mode of action is also very varied. Soil or seed germination stimulators, microflora or soil structure regulators, phosphorus availability enhancers, mineral salt absorption stimulators, are some examples of the properties of biostimulants, also called phytostimulants or biofertilisers. Some substances or preparations of substances are both plant defence stimulators and biostimulants. They are sometimes incorporated into fertilisers (such as organic fertilisers containing seaweed extracts).

For example, strains of the fungus Trichoderma have been the subject of several studies showing that they possess strong broad-spectrum antimicrobial activity. The strain GDFS1009 has a high growth rate, high sporulation capacity and potent inhibitory effects against the pathogens causing Fusarium wilt of cucumber and stem rot of maize (1).

Definition of biostimulants according to the Ministry of Agriculture and Environment (2).

Some biostimulants have been known for a long time, such as seaweed extracts, which have been used since ancient times. Algae extracts represent more than 32% of the biostimulants market (3). Others have appeared recently, such as micro-organisms and their extracts, plant extracts and biomolecules designed to stimulate plant physiology, or modulators of hormonal and enzymatic activities, etc. The mode of action of these substances has recently been studied by scientists. Scientific work is being carried out around the world to try to find out how they work.

However, care must be taken with the properties advertised. By definition, biostimulants should not directly provide nutrients to plants (such as amino acids). There is therefore a risk of confusion with certain advertised functions that are part of biocontrol or fertilisation methods. Biostimulants are sometimes incorporated into vegetable potting and mineral fertilizer or other substances. They are then found on the market under different names: agronomic additives, soil activators, biofertilisers, growth stimulators, phytostimulants, etc. These terminologies are sometimes misleading, especially as they are not always organic products. Their inclusion in conventional fertilisers makes it possible to avoid the regulations on bio-stimulants. However, controls are necessary, particularly with regard to micro-organisms, in order to determine whether they contain toxins that could have an effect on human health. It is specified in the Rural Code that anything that is not authorised is prohibited. An agronomic additive must have a MA (marketing authorisation) to be used in a mixture with a fertiliser.

In France, for the amateur gardener, apart from algae, there are few products on the market containing biostimulants. They are most often extracts of algae, mycorrhizal fungi and soils containing microbial strains.

While some are convinced of their usefulness, others see them as window dressing. Users of PDSs have often found that there is a lack of correlation between the claims made by manufacturers and the results obtained in the field. For example, with regard to certain products stimulating natural defences on organic triticale, the newsletter of 3 August 2017 from the Loiret Chamber of Agriculture shows no significant difference compared to a control group receiving no stimulant.

According to the promoters of these products, this lack of correlation between what is claimed and the results in the field is linked to the existence of a strong influence of environmental factors and other factors that are poorly known or understood by their users. Although some of these products have been the subject of scientific publications, there is still a lack of sufficient data on their concrete use in the field. Where scientific knowledge exists, it relates to specific situations that are difficult to generalise, such as the variety grown, the physical and biological properties of the soil, the cropping method and the local climatic conditions. For all these reasons, many agronomists believe that it is better to stimulate soil microbial biodiversity through organic fertilisers than to import strains specialised in the control of a pathogen or a biological function (4).