Biofertilisers: rhizobia, Azotobacter, PSB and mycorrhiza
The microbes that supply or mobilise plant nutrients, how each works, how they are applied and stored, what they realistically add, and how to judge them.
On this page
Biofertilisers are products containing living microorganisms that, applied to seed, roots or soil, increase the supply or availability of nutrients to a crop. The best known are the rhizobia that form nitrogen-fixing nodules on legume roots; others fix nitrogen in the root zone of cereals, dissolve bound phosphate, or extend the root system.
They are cheap, low-risk and widely promoted, especially for smallholders and organic systems. Exams ask for the organisms, the crops they suit, their mechanisms and the methods of application. A clear-eyed view of how much each actually contributes is just as important.
The main groups
Nitrogen fixers
Rhizobia (Rhizobium, Bradyrhizobium, Mesorhizobium, Sinorhizobium and others) infect legume root hairs and form nodules, where they fix atmospheric nitrogen using the enzyme nitrogenase, protected by leghaemoglobin (which makes active nodules pink inside). The partnership is specific: each legume nodulates effectively only with certain rhizobia, historically described as cross-inoculation groups (for example soybean with Bradyrhizobium japonicum, chickpea with Mesorhizobium ciceri, lucerne with Sinorhizobium meliloti). Inoculation gives the largest benefits where the legume is new to the field or soils lack effective strains.
Free-living and associative fixers:
- Azotobacter: aerobic, free-living soil bacteria; also produce growth-promoting substances.
- Azospirillum: associates with roots of cereals and grasses, fixing some nitrogen and promoting root growth.
- Gluconacetobacter diazotrophicus: an endophyte of sugarcane.
- Clostridium and other anaerobes in soil.
Their nitrogen contribution is usually modest compared with legume nodules; much of their benefit comes from root growth promotion.
Cyanobacteria and Azolla in flooded rice: free-living blue-green algae (such as Anabaena, Nostoc) and the small water fern Azolla, which houses the cyanobacterium Anabaena azollae in its leaves. Azolla grown in paddies as a green manure or dual crop has been used for centuries in East and South-East Asia, adding nitrogen and organic matter when incorporated.
Phosphorus mobilisers
Phosphate-solubilising bacteria and fungi (species of Bacillus, Pseudomonas, Aspergillus, Penicillium and others) release organic acids and enzymes that dissolve phosphate bound to calcium (in alkaline soils) or iron and aluminium (in acid soils), and mineralise organic phosphorus.
Arbuscular mycorrhizal fungi (AMF), such as Rhizophagus (formerly part of Glomus), form a symbiosis with the roots of most crop plants. Their hyphae extend far beyond the root's depletion zone and take up phosphorus, zinc, copper and water, exchanging them for the plant's sugars. Crucifers (cabbage family) and beets are notable non-hosts. Long fallows and intensive tillage reduce mycorrhizal populations, which is part of why some crops after a long fallow struggle for phosphorus.
Other mobilisers and promoters
Potassium-, zinc- and sulphur-mobilising microbes, and plant-growth-promoting rhizobacteria that act through hormones, siderophores (iron chelators) and suppression of pathogens. Several products combine organisms; in some regulations they are classed as biostimulants rather than biofertilisers.
How biofertilisers are made and applied
Formulation: the microbes are grown in fermenters and carried in a carrier: peat (the classic carrier for rhizobia), lignite, charcoal and other powders, granules, or increasingly liquid formulations, which have longer shelf life and easier use. Quality standards in many countries specify a minimum viable count (colony-forming units, CFU, per gram or millilitre) at manufacture and until expiry, freedom from contaminants, and the correct organism.
Application methods:
- Seed treatment: the commonest. Seeds are moistened with a sticker solution (for example a little sugar or gum), mixed with inoculant, dried in the shade and sown promptly.
- Seedling root dip: transplants (rice, vegetables) are dipped in an inoculant suspension before planting.
- Soil application: inoculant mixed with compost or manure and applied in furrows or around plants.
- Sett treatment in sugarcane and tuber treatment in potato.
Handling rules: they are living products.
- Store cool and away from sunlight; observe the expiry date.
- Do not mix with fungicides or insecticides that kill the organism; if seed must be treated with chemicals, apply them first and the inoculant just before sowing, or use compatible products.
- Use the right organism for the crop, particularly the right rhizobium for the legume.
- Avoid high doses of nitrogen fertiliser on inoculated legumes: soil nitrate suppresses nodulation.
Is there enough rhizobia on each seed?
A soybean field is sown at 100 kg of seed per hectare; seeds weigh about 0.15 g each. The inoculant contains 1 × 108 viable rhizobia per gram, applied at 200 g per 10 kg of seed.
- Seeds in 10 kg: 10,000 g ÷ 0.15 g ≈ 66,700 seeds.
- Rhizobia applied: 200 g × 108 = 2 × 1010 cells.
- Per seed: 2 × 1010 ÷ 66,700 ≈ 3 × 105 rhizobia per seed, assuming none die.
Inoculation guidelines commonly aim for at least about 105 viable rhizobia per seed for large-seeded legumes such as soybean. If the inoculant had been stored warm and lost 90 per cent of its count, each seed would receive only about 3 × 104, and nodulation could fail in a field with no native soybean rhizobia. Seeds sown hours after inoculation in hot, dry soil also lose many cells.
What to expect
- Legume inoculation is the best-documented biological input in agriculture. Where effective rhizobia are absent, it can determine whether the crop fixes most of its own nitrogen (often tens to a couple of hundred kilograms per hectare) or yields poorly. Where native rhizobia are abundant and effective, responses are small.
- Azolla and cyanobacteria can add useful nitrogen to flooded rice when well managed, with contributions often cited in the range of a few tens of kilograms of nitrogen per hectare per crop.
- Azotobacter, Azospirillum and phosphate solubilisers give variable responses: positive in many trials, negligible in others, depending on soil, crop, product quality and the nutrient status of the field. They complement fertilisers and organic manures rather than replacing them.
- Mycorrhizae matter most in soils low in available phosphorus and for crops strongly dependent on them; commercial inoculation is most reliable in sterilised or disturbed media, nurseries and degraded soils.
Biofertilisers are best seen as part of integrated nutrient management, combining organic manures, fertilisers and biological inputs according to soil tests.
Key points
- Biofertilisers are living microbial products that supply or mobilise nutrients.
- Rhizobia fix nitrogen in legume nodules; the strain must match the legume; inoculation pays most where the legume is new to a field.
- Azotobacter, Azospirillum and Gluconacetobacter fix modest nitrogen in the root zone and promote growth; cyanobacteria and Azolla serve flooded rice.
- Phosphate-solubilising microbes release bound phosphate; arbuscular mycorrhizal fungi extend roots for phosphorus, zinc and water.
- Apply by seed treatment, root dip or soil application; store cool, check expiry and viable counts, and avoid contact with harmful pesticides.
- Responses are reliable for legume inoculation and variable for many others; use them within integrated nutrient management.
What the exams ask
- School-leaving and entrance papers: examples of biofertilisers and the crops they suit, and the role of rhizobia and Azolla.
- Degree and research-fellowship entrances: mechanisms of nitrogen fixation, cross-inoculation groups, phosphate solubilisation, mycorrhizal symbiosis, carriers and quality standards, and methods of application.
- Extension and professional examinations: recommending biofertilisers within integrated nutrient management, handling and storage, compatibility with seed treatments, and judging product quality.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Can biofertilisers replace chemical fertilisers?
Mostly not on their own. Legume inoculation can replace most nitrogen fertiliser for the legume itself. Other biofertilisers usually supplement fertilisers and manures, and their effects vary. Soil testing and integrated nutrient management give the best results.
Why did my inoculated legume not nodulate?
Common causes are a dead or low-count inoculant (old or stored hot), the wrong rhizobium strain for the crop, contact with a fungicide seed treatment, very acid soil, drought at sowing, or high soil nitrate.
Are biofertilisers safe?
The organisms used are generally soil microbes with no harm to people, animals or the environment. Quality-controlled products should be free of contaminants; products of unknown quality can contain the wrong organisms.
Sources
- Herridge, D. F., Peoples, M. B. and Boddey, R. M. "Global inputs of biological nitrogen fixation in agricultural systems." Plant and Soil 311, 1–18 (2008).
- Howieson, J. G. and Dilworth, M. J. (eds) Working with Rhizobia. Australian Centre for International Agricultural Research, 2016.
- Smith, S. E. and Read, D. J. Mycorrhizal Symbiosis, 3rd edition. Academic Press, 2008.
- Richardson, A. E. and Simpson, R. J. "Soil microorganisms mediating phosphorus availability." Plant Physiology 156, 989–996 (2011).
- FAO. Plant nutrition for food security: a guide for integrated nutrient management. FAO Fertilizer and Plant Nutrition Bulletin 16, 2006.
More in soil science
Discussion
Questions, corrections and notes from readers on this article. Every comment is checked before it appears: keep it on the topic, and do not share phone numbers, links or contact details.