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Biostimulants: What’s in the box?

When it comes to biostimulants there are plenty of questions but few clear answers

Biostimulants are one of the fastest-growing parts of the agriculture input market.

A lot of products are on the market worldwide and many are being aggressively marketed. They’re being touted for their soil health benefits and other traits, but often little evidence is offered.

Carl Rosen, a professor and extension soil scientist with the University of Minnesota, told the Manitoba Agronomists Conference earlier this winter there are three key questions worth asking.

“Do they work? Under what conditions? And are they cost effective?” he said.

Rosen defines biostimulants as substances or microbes that, when provided in minute quantities, promote plant growth. They are not pesticides, or soil conditioners, they are things that stimulate growth.

Why it matters: Biostimulants are one of the fastest-growing crop input markets but there’s little independent research to separate fact and fancy.

Biostimulants come in different forms like humic substances, amino acids and other nitrogen compounds, chitosans and seaweed extracts, which are non-biological in terms of the living part. There are also beneficial organisms that are being isolated, such as bacteria in the rhizosphere, fungi, mycorrhiza, and things like that. Sometimes a combination of organic and non-biological products will be used together.

On acid

Humic acid is one substance that’s been promoted widely in recent years.

It is the decomposition products of animals, plants and microbes that occur naturally in the soil. Some sources of commercial humic substances are peat soils with very high organic matter levels, compost and leonardite (a form of coal).

“Since it is a soluble product, it can be mixed with a liquid starter fertilizer, it can also be coated on granular fertilizer, or it can also be used as a foliar application,” said Rosen.

Research hasn’t conclusively determined how humic substances work, or even definitively if they do. There is some evidence that they can stimulate root growth and improve nutrient uptake but it seems that the response to humic substances depends to a certain extent on its source.

Humic substances that come from compost or lignite tend to perform better. It appears that the rate of application doesn’t matter as much as the crop type or growing conditions. Some studies showed a 15 to 25 per cent increase in crop growth, while others reported only five per cent.

“This high variation increases the risk to farmers,” said Rosen, who adds the other thing to note about humic substances is that the response in these studies was at very high application rates.

“It may be acting as a nutrient source when you go to very high rates, which are much higher than the amounts you’re actually applying in commercial products. So, you need to keep that in mind.”

Amino acids are components of proteins. Amino acids are used for many purposes; as a media to grow animal and plant cultures, for animal feeds, dietary supplements for humans and in crop production.

Again, how they work isn’t entirely known but they have a chelating effect, which could cause some improvement in micronutrient availability and decrease heavy metal toxicity. They also have some antioxidant activity which could provide increased stress tolerance, and if applied in a concentrated band, the amino acids may stimulate microbes in that area and increase microbial biomass and nutrient cycling.

“There are commercial products out there but you’ll get differences in response to these,” said Rosen. “You might see an effect sometimes, other times you won’t see an effect. They are a readily available source of N for microbes, but the amounts that you apply are not that much, so it’s probably not a direct source of N for plants, so any effect is probably indirect by stimulating those microbes. There have been some reports where amino acids, when applied to the leaves, have increased yield and this is mainly in horticultural crops.”

Sweet treat

Chitosans are basically sugars with some N component. They are derived from the shells of shrimp called chitin and have several uses in both plants and animals. They are used in the treatment of cancer and as a dietary supplement, with some evidence that they might help with weight loss. They are also used in winemaking and crop production.

In plants, when used as a seed treatment or foliar application chitosans have been shown to suppress pathogens, and work as a biocontrol of fungi and nematodes, and there are some products that are registered as a biopesticide in the U.S.

“There is some evidence that it stimulates growth under some conditions, increases photosynthesis and stimulates nutrient uptake,” said Rosen. “The mode of action seems to affect cell membranes. It can alter the DNA and activate defence genes.”

A meta-study on biostimulants completed about five years ago (that analyzed all the literature to date), stated that the conditions that enable chitosans to increase yield were so influenced by environment or cultural practice that in a given year, the benefits could deviate both up and down, but reported an average of 10 per cent yield increase.

“But what (the meta-study) concluded was that it reflected on the product credibility and that I think you can probably apply the same statement to a lot of the biostimulants,” said Rosen.

Finally, organic biostimulants such as seaweed extracts have been used in agriculture for thousands of years as a nutrient source in compost. In the 1950s, a process was developed to produce liquid extracts. Seaweeds are basically sugars with some other compounds like sulphur and macro- and micronutrients.

“There’s some growth regulator or hormone activity associated with these seaweeds so there’s lots of possible modes of action,” said Rosen.

In agriculture, seaweed extracts are either applied directly to the soil or to the plant as a foliar application, at really low rates of around a couple of litres per acre.

“Because you’re applying such small amounts, the rates are too low to be of direct benefit to the plant in terms of a nutrient source,” said Rosen. “There is some evidence that the foliar applications stimulate hormones so if you’re applying it directly to the leaves, this may in turn affect the growth and stress tolerance. But again, these are just some reports that are under very controlled conditions in the literature.”

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