Archive for the ‘Mycorrhizae’ Category

This post is a bit longer and more technical than usual.  I originally wrote it for a different audience and this is my attempt at making it user friendly for most people.  Like many things in nature it’s complex.

Healthy soils teem with microbes. Inoculating soil with with microbes to boost crop production is all the rage at the moment with companies sprouting – well, like microbes! Usually they are being sold to the commercial grower but more and more they are also being marketed into the home garden market where cost is less of an issue (ie home gardeners don’t have to make a profit).

Whether you can parachute in foreign organisms and expect them to live and prosper is really open to debate.  In most studies it doesn’t happen.  And in sandy soils with virtually no organic matter there is nothing there for them to feed on so it is even less likely.  Sure you can add composts and manures but these don’t tend to hang around much in sands, they burn off very quickly unless some clay is also added.  And large amounts of compost and manure provide phosphorus far in excess of plant needs which means it leaches down into the groundwater, potentially getting into waterways and causing algal blooms.

I have seen some papers showing how mycorrhizae have increased plant growth, even without fertiliser being added.  Those papers carefully omitted to cite a soil analysis!  One thing to bear in mind – I certainly don’t dispute that some substances can increase plant growth but they can’t do it forever without, at some stage, the food supply having to be replenished.  What you are doing is mining the soil.  It might work once or twice but then you run out of food.  Plants can’t feed on fresh air!  OK I can hear you saying they can fix nitrogen from the air – yes – but not phosphorus or potassium!

Mycorrhizae may alter root architecture – ie the way roots branch and proliferate.  That can make plants take up nutrients more efficiently by exploring more of the soil.  So you might see a growth spurt – but again, that nutrient will have to be replenished if that response is to be repeated and maintained.

So lets look at microbes.

A changing population

Populations of fungi are not constant. They change frequently in response to a whole range of factors. Even those associated with a single plant, change with the growth stage of that plant. Figure 1 shows the relative amounts of different fungal species over time for a pea plant. In the vegetative stage, Fusarium is most common but diminishes over time in contrast to Heliotales which increases as the plant matures.

The relative amounts of each fungus also change with fertility (Figure 2) and crop health (Figure 3).

                     Vegetative                                                    Flowering





Figure 1.  Change in microbial populations with growth stage in the rhizosphere of pea. (Note the rhizosphere is that thin coating of soil left on the roots after the rest of the soil is shaken off).

OM rates

Figure 2  Relative amounts of fungal species in pea roots grown with either nil or three levels of organic fertiliser (OF)


Figure 3.  Relative abundance of fungal species associated with the rhizosphere of healthy and diseased pea plants.

You will note that in all the examples above there is a mixture of ‘good’ (e.g. Glomus sp.) and ‘bad’ species of fungi (e.g. Olpidium sp.).  This is normal and disease only occurs when this balance is disrupted in favour of the pathogen and other conditions in the environment and host are right for infection and disease development.

Mycorrhizae are one group of fungi known to have beneficial effects on plant growth in some circumstances, but this is highly variable. The term mycorrhiza covers a large number of genera. Glomus species are some of the most prevalent. Examples of their effects on plant growth are provided below.

Highly specific effects

Example – Three mycorrhizal species were studied on basil.  None affected plant phosphorus level.  Only one significantly affected plant growth and increased the amount of one essential oil produced while the other two increased the amount of a second oil and decreased that of a third.

Microbes can also have adverse effects such as growth suppression

Example ‑ A trial on onion and plantago showed varying effects of mycorrhizae on plant nutrient levels.  The authors expected the mycorrhizae to increase host nutrient levels, however in some cases they reduced them.

Example – Twenty-three different mycorrhizal strains were evaluated for their symbiotic response with Piper longum (long pepper).  Almost all resulted in increased plant growth, biomass and nutrient content (nitrogen and potassium) over the control, however six species depressed growth.

It’s Complicated!

Example – A comparison of mycorrhizal versus non-mycorrhizal roots showed phosphorus uptake doubled and was independent how much was in the soil. There was no additional benefit of the mycorrhizae on plant growth other than that due to increased P uptake.

Example – The effects of a mycorrhiza on growth and photosynthesis of cucumber were studied using different rates of nutrient supply, phosphorus ratio and different forms of nitrogen.


Plants inoculated and given full-strength nutrient solution showed a 19 per cent reduction in total biomass compared with mycorrhizal (AM) plants (Figure 5).

The highest percentage of mycorrhizal infection in cucumber was found at the low P treatment, however a 90 per cent reduction in total nutrient supply almost totally counteracted the potential positive impact of a low concentration of P on mycorrhizal infection.

The extent of mycorrhizal infection in cucumber was correlated with a low root P concentration, which agrees with other studies that plant P status influences mycorrhizal infection.

Not all plants are mycorrhizal

Some plants do not form relationships with mycorrhizae even when inoculated. Brassicas, beetroot and spinach are among those.

Local species usually prevail

This is probably the most important consideration of all.  Any microbe placed in a field situation will face competition from local species and may eventually be displaced.

Example.  A Turkish study took 70 soil samples from 25 different plant varieties grown in local fields.  Arbuscular mycorrhizal fungi (AMF) were found in 59 soil samples; 58 of these were identified as Glomus and one as Gigaspora.

The effect of the local Glomus sp. was compared to a commercial preparation on tomato and cucumber plants.

The local Glomus species increased cucumber and tomato plant growth, but the commercial mix did nothing.  The local Glomus species colonised plant roots at almost twice the rate of the commercial one.

The relationships are very specific

Each plant species, and even variety, tends to have a preference for certain mycorrhizal species. It is difficult to manipulate relationships and so the effect of inoculating a soil with mycorrhizae depends on many factors.  If there are already successful mycorrhizal associations present, those existing association may be stronger and the introduced species may fail to displace those already present, or it may displace a proportion of the existing associations.  Inoculation with mycorrhizae does not necessarily mean more root colonisation in terms of either numbers or species.

And lastly – Beware trial results!

Many research results, when you read the fine print, are from work that has trialled mycorrhizae under unnatural situations, whether using sterile media or in a laboratory. Those results are not often reproducible in field situations.  They work because the introduced (foreign) microbes have no competition.  A better approach and one more likely to succeed, would be to use local species, bulk them up and inoculate back.

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