Thursday 23 May 2013

Reasons not to dig 7. You will destroy mycorrhizal networks

Fifty years ago my first encounter with mycorrhiza was in the Duneman seed bed. Foresters would sow their trees in a seedbed of composted deciduous and coniferous leaf litter. Because of the natural symbiotic association between soil-living fungus and plant, mimicking the forest floor, the technique produced very fine sturdy plants. Now, when I pop down the road and see my friend Peter Williams’ strong seed-raised cut leaf maples I think I see the same thing. He sows and raises his fine plants in his home made mix of well composted leaf litter and mown grass.

Ninety five percent of plants in the wild form mycorrhizal associations. This mutual arrangement enables them to thrive and in some cases mycorrhiza are needed for the plants to survive. All the bluebells in a natural wood are not only connected, one with another by fungal strands, without mycorrhiza they would die.
In the past farmers and growers have neglected the  potential of mycorrhiza and generally have achieved  quality and yield without their help. Agricultural practices such as soil cultivation and heavy use of fertilizers hugely inhibit this beneficial fungus ‘infection’. Minimum cultivation is now seen to be a route to enjoy nature’s help. 

Imagine a wood covered with bluebells. Without mycorrhiza the wood would have no bluebells at all. 

Long recognised benefits of mycorrhiza

  • The botanical details of different symbiotic associations between plants and a myriad of different fungi vary, but all involve fungal mycelium ‘invading’ plant tissue and at the same time penetrating the soil. In return for the ‘services’ it renders, mycorrhizal fungi receive a rich supply of sugars from plants.
  • Fungal hyphae (strands)  penetrate deeply and intimately into the soil creating a huge surface area of contact which vastly increases the absorptive ability of roots.
  • The nutrient phosphate is frequently locked up in insoluble form in the soil. Mycorrhiza enable the plant to extract it. 
  • Other nutrients, including nitrogen and trace elements are extracted from the soil and sometimes come from unexpected sources such as decaying woody vegetation and harmful soil organisms killed by a fungus!
  • Soluble organic materials can be exchanged between fungus and plant.
  • Some fungal diseases are inhibited and harmful soil nematodes may be killed.
  • Nitrogen fixation can be enhanced.    
  • Better plant growth on soils contaminated with toxic heavy metals.
  • Nutrients are preserved from leeching within the combined fungus/plant system.

In recent years there has been an explosion in our knowledge of mycorrhizae and neglected research and new knowledge has now reached the public and farming domain.

Modern developments 

Scientists now understand how plants are connected one with another in the wild. This may involve interconnections between many different species of plant and fungus. Fungal networks exchange sugar, nutrients and organic materials. Plants such as trees contribute disproportional amounts of sugar and have a profound effect on surrounding soil ecology. Young plants sometimes receive sustenance from their parents. Plants chemically communicate information about invading pest and disease. It is now recognised that mycorrhiza help plants withstand drought.

Only recently has the full significance been given to how mycorrhiza and associated fungi improve soil. In 1996 the fungal protein glomalin was discovered. Apart from ‘true’ humus it is probably the most long lasting organic constituent of soil. It binds together soil particles and is a powerful beneficial influence on soil structure. Recent research shows how the  strong penetrative forces of fungal mycelium can even penetrate rock! Our children will probably learn in future geography lessons how mycorrhiza helps to form soils!  

Most growing media provide natural access to mycorrhizal propagules and spores. Many man made environments however, are deficient, and a new industry has emerged that sells fungus inoculum. Such products have an important role in ‘seeding’ fungal-deprived media. I have no idea whatsoever how great a contribution these much hyped products can make to everyday garden use. I  am sceptical, but wait to be convinced! I would love to hear from readers who have used these products.

Benefits of zero cultivations
You are not chopping up roots and mycorrhiza. The marriage between plant and fungus is maintained. Every time cultivations take place the plant and fungus have to start their relationship again!  Please excuse my exaggeration, in established plantings, deep root/fungus connections will survive cultivation. 

My cemetery gardens never receive fertilizer and the soil is not cultivated at all. My weed control is with glyphosate and the soil is undisturbed. With zero cultivation virtually all of my plants will benefit from rich fungal associations.

I was interested to read this website which describes how when weeds are killed by cultivation their mycorrhizal contribution to the soil is lost. It’s the same when they are killed with herbicide, except that the beneficial fungus has time to lay down spores and propagules before it dies.

Integral to no dig gardening is to grow plenty of plants. I now realize that this post on green manuring fails to recognize its mycorrhizal contribution.

Brassicas are one of the few plants that do not form mycorrhizal associations. I wonder if this is why with no mycorrhiza to help them they have evolved such wonderful deep penetrating roots. In the words of one blog follower the brassica, daikon radish, ‘busts open’ clay soil.

UPDATE 1st Oct 2013
I have often wondered that whilst mycorrhiza are so common one rarely sees them with the naked eye. My microbiologist friend Peter Williams has just sent me this picture of mycorrhiza on the roots of Sorbus aucuparia that he had dug up in his woodland garden. He reminded me that they have lovely characteristic fresh fungus smell. When exposed to the naked air within a few minutes their delicate membranes disintegrate and you can no longer see them.


  1. This is a very interesting article.

    I heard that there are two schools of gardening: one says - to dig, and the other says not to dig. I dig mine soil, because I used to have a heavy clay, currently it's improved (a lot of sand and compost thinned down the clay), but when I get the perfect soil in my garden - I'll consider giving up digging.

    Thanks for sharing!

    1. Dewberry, I love that name, it creates interesting images of you
      I have just visited your own blog and liked your dactylorhiza article. Mycorrhiza are specially significant to orchid propagation.
      Looking at your soil It looks to me time you did convert to no dig- but them you know what camp I am in!

    2. Oh, yes - I dig only the vegetable beds, the flower beds are left to their own existence. I only weed them from time to time :)

      Thank you for your kind words!

  2. You never seem to see TV gardeners plant any type of shrub without adding mycorrhizal fungus now do you? We've never actually used it though.

    1. Nor have I Sue. I rarely watch gardening tv. I tend to get cross too often when they say something I disapprove of! The producers of these products have certainly got their sales propaganda over very well! I would like to see more evidence that they do any good in domestic garden situations.
      There is a certain logic in using them and I will be quite prepared to eat humble pie if they really do make a difference.

  3. Somewhat unrelated to the main topic of this post .... my father first noticed a patch of white bluebells (if that is not a contradiction in terms) in amongst his blue flowers some years ago. Since then the number of white flowers has been increasing whilst the blue flowers have been decreasing. Is this some thing you observe too or is his a 'one-off' white dominance colony?

    A totally unscientific observation on the use of mycorrhizal fungus. I used some when I planted a number of fruit trees in 2014 (the pack was found under the sink in a house my daughter rented). These trees are all doing well - but then they were good healthy specimens, bought from a reputable supplier.

  4. Thanks for your two interesting questions. I hope you will try some more!
    I have used the contradictory description 'white bluebells' in my cemetery lecture for years
    I can't give you a definite answer about the bluebells but you have got my mind racing. I feel a blog post coming on! Watch for it but it might be a few weeks!
    Glad your fruit trees did well. But as you say not very scientific )- (smilie intended)


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