Chitin is the building block of the soil food web
Chitin has been studied by researchers and trialed by manufacturers and growers for over 40 years. Organic farmers and gardeners have long used chitin from crab meal and mushroom compost as part of their holistic approach to maintaining a balanced, healthy soil.
The benefits of chitin to crops stems from the microbes that consume or transform chitin — the chitinase enzymes that are produced by microbes — and from the direct interaction of chitin and chitosan with plant cell receptors.
There are many species of chitinolytic bacteria and fungi. These microbes produce chitinase enzymes that digest chitin; use chitin for food, energy or structural components; and split chitin into shorter-chain polymers.
Chitinolytic microbes include the following genera, some of which will surely sound familiar:
- Bacteria: Vibrio, Pseudomonas, Flavobacterium, Serratia, Photobacterium, Aeromonas, Cytophaga, Streptomyces, Photobacterium, Bacillus (e.g. subtillus and chitinosporous), Clostridium and Chromobacterium.
- Fungae: Mortierella, Deuteromycetes, Aspergillus, Verticillium, Thielavia, Trichoderma, Penicillium and Humicola.
There are hundreds of species of bacteria and fungi within these genera, as well as others not listed, that can actively consume chitin.
Chitin is a long-chain polymer similar to cellulose. Technically it’s poly-N-acetyl-D-glucosamine: a chain of glucose-like molecules with acetyl groups. It’s ubiquitous in nature — it’s the second most abundant polysaccharide after cellulose, and it forms the structural shell of crustaceans (shrimp, crab and lobster), the exoskeletons of insects and arachnids, the cell walls of fungi, the outer layer and appendages of nematodes, and also the egg casings of insects, nematodes and other invertebrates.
Microbes convert chitin into chitosan, which has many remarkable uses. It’s been developed into bioproducts that replace many chemicals: fire-retardant textile coatings, wastewater flocculants, hemostatic bandages, flocculant fining agent for winemaking, and more.
CHITIN IN CROP USE
Considering it’s so ubiquitous in nature — particularly the world of soil, insects and microbes — it makes sense that chitin, its metabolites and the chitinase enzymes have many roles in maintaining balance in soils and plants.
When chitin is digested by chitinase enzymes (of which there are several types) the long chain is broken down into shorter molecular chains. Each resulting polymer length is a different metabolite and may have distinct functions. When chitin is de-acetylated into chitosan, the acetyl groups become amino groups. Chitosan has been extensively studied and has been developed into patented production processes and blended into crop-protection products.
Chitosan also has a very important property as a chelation agent. As a positively charged polymer, it forms weak bonds with anions and can chelate micronutrients for use by plants or against pathogens. As a chelator, chitosan assists with nutrient uptake. It can also be combined with pesticides, enabling effectiveness at lower dosage. Chitosan is one of the platforms of the emerging field of nanomaterials in agriculture. For example, chitosan can deliver copper, zinc, iron and other elements in nano form, rather than as salts.
Its cationic property suggests that soils rich in chitosan would have higher anion exchange capacity and thus be more resistant to leaching of nutrients like nitrates and phosphates.
A review published in 2013 in the journal Agronomy by Russel Sharp summarizes what researchers have learned about chitin’s agricultural uses and pathways: “A number of modes of action have been proposed for how chitin and its derivatives can improve crop yield. In addition to direct effects on plant nutrition and plant growth stimulation, chitin-derived products have also been shown to be toxic to plant pests and pathogens, induce plant defenses and stimulate the growth and activity of beneficial microbes. A repeating theme of the published studies is that chitin-based treatments augment and amplify the action of beneficial chitinolytic microbes.”
Sharp grouped the modes of action into four categories:
- Direct antibiotic effects on pests and pathogens;
- Stimulation of beneficial microbes;
- Stimulation of defense mechanisms of plants to pests and pathogens; and
- Regulation of plant growth and development, nutrient uptake, and response to abiotic stresses.
Direct antibiotic effects
Chitosan, though not chitin, has been shown to have a direct antibiotic effect on pests and pathogens. This may be due to both the chitosan molecule itself and to its ability to deliver antimicrobial minerals like copper. Whether the effects occur directly, via chelation or mediated by microbes is difficult to discern in a natural setting — nature is so complex that scientists struggle to separate the causes and effects.
Teamwork with microbes
Chitin and chitosan, and presumably the medium-length polymers that come from chitin, are known to stimulate beneficial microbes, for which they are a source of food and energy. Many studies have shown the effectiveness of combining chitin (and its derivatives) with beneficial microbes. This combination effect is the basis for many commercially available products that incorporate one or more beneficial microbe species with a chitin food source. The plant-protective functions are, in many cases, provided by the enzymes secreted by the microbes: chitinase, which breaks down chitin, and chitosanase, which breaks down chitosan.
It is well known that healthy, diverse soils suppress pathological nematodes. Some have hypothesized that chitinase is the active agent that disables nematodes. Researchers are working on making this approach effective at scale. The concept is that because chitinase digests chitin, a nematode that’s exposed to it would be partially dissolved and thus disabled. There are several potential modes of action, including that its stylet loses the ability to penetrate roots, it loses the ability to move through soil, its eggs are damaged, mating is disrupted, or a wound is opened that allows bacteria to infect it.
Induced systemic response
Another proven application in farming is that chitin stimulates plants’ defensive responses to pests and pathogens. This is called Induced Systemic Response (ISR). Particular receptors have been identified that in response to chitin will initiate a cascade of activities that boost the plant’s immune response — or some other defensive action that may prevent the pest or pathogen from causing significant damage. Some plant species can produce the chitinase enzyme, which can dissolve the chitin of a potential pathogen as part of their ISR.
Plant growth promotion
The fourth category of chitin’s plant benefits is as a plant growth promoter. Increased nutrient uptake is caused by at least two modes of action: chitin or chitosan binds to specific receptors in plants and induces changes to the osmotic pressures of the plant cells, which can then better absorb nutrients and other ionic compounds.
Other plant growth promotion is accomplished in combination with microbes. For example, Bacillus subtilis is a chitinolytic bacteria that’s used for many PGP benefits, including seed germination. Some studies have shown this to be more effective when chitin is applied together with the microbe. It may be that the chitin is food that enables the microbe to thrive. Or there could be other mechanisms at work. Chitin and chitosan have also been documented to improve plant stress tolerance to heat and drought. The specific mechanism for how this benefit is derived is not well understood.
CURRENT REGULATORY ENVIRONMENT
Most of the functions described above are fertility related and are regulated by fertilizer laws. Some of the plant benefits are considered either pesticidal or bio-stimulant. Products that make claims about suppressing or controlling certain pest or pathogen problems must comply with EPA regulations in the U.S. (and DPR in California).
In the U.S., chitin and chitosan are classified as pesticides. Chitin can be registered and used as a fertilizer, but it must be represented as the source material — e.g., crustacean shell, rather than as chitin. Chitosan is currently under rulemaking review by the EPA and may be delisted as a pesticide if the current proposed rule is approved.
In Europe and Canada, neither substance is regulated as a pesticide. In the European Union, chitosan is permitted for use in organic agriculture. It’s classified as a “basic substance” that is not toxic to humans or animals, that is derived from a plentiful biological waste resource, and that is produced without deleterious environmental effects. The E.U. allows chitosan for organic use for plant protection, specifically as “an elicitor of plant resistance against pathogenic fungi and bacteria.”
PRODUCTION
The most common source of chitin and its derivatives is the shells of shrimp and crab. Crustacean shells are a rigid matrix of primarily calcium carbonate and chitin. They have nutrients in the range of 4 percent nitrogen, 4 percent phosphorus, 15 percent calcium and 25 percent chitin. Insect frass, commonly produced from black soldier fly, is another source of chitin. Its nutrient and chitin concentrations are lower than crustacean shell — typically 3 percent nitrogen, 1 percent phosphorus, 1 percent potassium, 0.1 percent calcium and <1 percent chitin.
Production of chitosan from chitin can be done by chemical means, enzymatic means or a combination of the two. For example, when crab shells are mechanically ground and exposed to acid, the calcium carbonate is dissolved, the carbonate is neutralized, and chitin and calcium become semi-soluble. To produce chitosan, the protein, calcium and other minerals are separated out. The chitin is then deacetylated in alkali conditions (e.g., sodium hydroxide). When hydrochloric acid is used, this pathway produces sodium chloride as a byproduct. Additional treatment further deacetylates the material into short-chain chitosan, which can be purified and made soluble.
This digestion and purification is accomplished in nature via enzymatic pathways. All animals and microbes that use chitin as a food source produce chitinase enzymes. For example, salmon produce enzymes that enable them to eat shrimp. Most fungi also consume chitin and build their bodies with it.
In recent years, many products with chitin, chitosan and chitinolytic microbes have been introduced and registered as agricultural input products. Applications that have been developed and proven effective over the years are now being studied with scientific rigor to understand exactly how these applications work. The insights uncovered in these investigations are leading to new and exciting products, programs and farm-scale applications.
In agriculture, as in other industries, chitin and chitosan are increasingly able to provide natural, non-toxic solutions to difficult challenges.
Warren Shoemaker is the marketing director of Pacific Gro.
