Mycorrhiza and friendly bacteria: VAM with PSB, Azotobacter and Rhizobium
Mycorrhizal fungi feed, attract and even carry helpful soil bacteria. What happens when VAM is combined with bacterial biofertilizers, what Indian field trials found, and how to combine them without wasting money.
In brief
- PGPR are helpful root-zone bacteria, sold in India as biofertilizers: Rhizobium, Azotobacter, Azospirillum, PSB and others. Trichoderma is a fungus, not one of them.
- Mycorrhizal threads have their own crowd of bacteria. In lab studies the fungus fed a phosphate-solubilising bacterium and even carried bacteria along its threads to locked phosphorus, then took up the phosphorus they freed.
- At seven Indian sites, wheat with mycorrhiza plus Pseudomonas yielded 41% more, against 29% and 31% with each alone.
- Combinations often add up rather than multiply, and some pairs, such as mycorrhiza and Trichoderma, can hold each other back.
- Match the partner to the crop, use tested combinations, keep phosphorus moderate, and test any fertiliser cut on a strip first.
What PGPR are
PGPR, plant growth-promoting rhizobacteria, are bacteria that live on and around roots and help the crop: some fix nitrogen from the air, some free phosphorus or potassium locked in the soil, some make growth hormones or help protect roots. In India the main ones are sold as biofertilizers under the Fertiliser (Control) Order, which lists, among others, Rhizobium, Azotobacter, Azospirillum, phosphate-solubilising bacteria (PSB), potassium-mobilising and zinc-solubilising bacteria, mixed consortia and mycorrhizal biofertilizer (VAM).2
Trichoderma, often sold alongside them, is not a bacterium but a fungus, used mainly to protect roots from disease.3 The difference matters, as we will see.
Mycorrhizal threads have their own bacteria
A mycorrhizal fungus is fed by the plant: in a study on cucumber, about a fifth of the carbon the plant fixed was used by the mycorrhiza.4 Part of that carbon leaks out of the fungal threads into the soil, and bacteria gather to use it.
The thin layer of soil around the threads, called the hyphosphere, holds its own set of bacteria. In one study the same kinds of bacteria gathered on the threads within 14 days, in different soils and with two different fungi.5 Substances released by the fungus speed up bacterial growth and change which bacteria thrive.6 Roots that carry mycorrhiza also draw more of some helpful bacteria: fluorescent Pseudomonas and Azotobacter moved towards mycorrhizal tomato roots more strongly than towards roots without it.7
A trade: sugar for phosphorus
The clearest example of teamwork involves phosphorus. Much of the soil's phosphorus is locked in organic forms, such as phytate, that neither the plant nor the fungus can use directly. Some phosphate-solubilising bacteria can release it.
In laboratory studies, a mycorrhizal fungus fed a phosphate-solubilising bacterium with carbon, the bacterium released phosphorus from organic compounds, and the fungus took up the phosphorus the bacterium released.8 A sugar released by the fungus, fructose, even switched on the bacterium's phosphorus-releasing enzymes.9 And the fungal threads carried bacteria along their surface to patches of organic phosphorus out of the roots' reach, like a highway.10 Only bacteria that can swim use this highway.11
These results come from laboratory and pot systems. They explain why the pairing can work; how much it adds in a field is a separate question.
| Partner | What it does | With mycorrhiza |
|---|---|---|
| Rhizobium (pulses) | Fixes nitrogen in root nodules | Often more growth than either alone; across 88 studies the gain was additive in annual legumes such as most pulses, and greater than additive only in perennial legumes |
| PSB | Releases locked phosphorus | Lab: fungus feeds the bacterium, bacterium frees phosphorus. Dryland chickpea: more pods per plant with both than with either alone |
| Fluorescent Pseudomonas | Growth promotion; some protect roots | Wheat at seven Indian sites: +41% yield with both, +29% and +31% with each alone |
| Azotobacter, Azospirillum | Fix nitrogen near the root | Azotobacter is drawn to mycorrhizal roots; we found no published Indian field trial measuring the combination |
| Trichoderma (a fungus) | Protects roots from disease | Can reduce mycorrhiza and attack its threads; some strain pairs work, others do not |
What the research shows
In Indian fields. At seven sites from the Himalayan foothills to the Indo-Gangetic plain, wheat inoculated with a mix of mycorrhizal fungi and two fluorescent Pseudomonas strains yielded 41% more grain than uninoculated wheat; mycorrhiza alone gave 29% more and Pseudomonas alone 31%. Gains were largest where yields had been low.12 In dryland chickpea, PSB and VAM together gave more pods per plant than either alone.13
Often additive, not multiplied. A review of co-inoculation studies found that bacteria and mycorrhizal fungi together often add their effects rather than multiply them.14 With legumes, rhizobia and mycorrhiza helped each other most in perennial legumes; in annual legumes the effects simply added up.15 In one detailed study the two partners boosted a legume's growth together, but rhizobia reduced the mycorrhiza and the benefit depended on soil phosphorus.16
Some pairs work against each other. Trichoderma harzianum reduced root colonisation by a mycorrhizal fungus, while the mycorrhizal fungus held back the Trichoderma,17 and Trichoderma can attack mycorrhizal threads and enter roots through them.18 More on mycorrhiza and root diseases: can mycorrhiza protect crops from root rot, wilt and nematodes?
Results depend on place. Across 171 published studies, biofertilizers raised yields by about 8.5% on average in continental climates and about 20% in dry climates, and mycorrhiza worked best in soils low in organic matter with a near-neutral pH.19
Cutting fertiliser. In a greenhouse, tomato inoculated with PGPR and mycorrhiza at 75% of the recommended fertiliser did as well as plants on the full dose.20 That is promising, but it is one greenhouse study; field results vary, so claims of much larger fertiliser savings should be tested before you rely on them.
Helper bacteria. Some bacteria help mycorrhiza form or work better; scientists call them mycorrhiza helper bacteria. Most of that research is on the mycorrhiza of trees,21 and products built around such helpers for field crops are still new.
How to use mycorrhiza with bacterial biofertilizers
- Match the partner to the crop. Use the Rhizobium strain made for your pulse, PSB on most crops, and VAM on any crop except the mustard, cabbage and beet families (see types of mycorrhiza).
- Buy registered, fresh products. Check the FCO registration, the live count and the expiry date: for VAM, look for infective propagules per gram (see how to judge a VAM product).
- Follow the label on mixing. Use combinations that have been tested together. Do not mix bacteria or VAM with chemical fungicides, or with Trichoderma in the same treatment, unless the label says they are compatible.
- Keep phosphorus moderate. Apply it according to a soil test; across field studies, phosphorus fertiliser reduced mycorrhiza by 32% on average.22
- Test before you cut. If you want to reduce fertiliser, try it on a strip of the field first and compare the yield.
For a tested combination of friendly microbes against a crop disease, see biochar and friendly microbes against brinjal blight.
Editor's note: this article is based on a technical note by Dr. Abhishek Mathur. NanoKheti's editors rewrote it in plain language, checked each statement against the published research listed under Sources, and added the Indian field results, the summary, table, questions and links.
