Mycorrhizal fungi occurs naturally in soil. They form a close symbiotic relationship with plant roots. They are called mycorrhizae ,from the Greek “mukés”, meaning fungus, and “rhiza,” meaning roots. Mycorrhizae are considered to be in a mutualistic relationship because both organisms benefit. The fungus receives the products of photosynthesis from the plant and is therefore does not need to find its own sources of energy. At the same time the fungus grows out into the soil and retrieves nutrients, especially phosphorus and nitrogen, and passes these back to the plant. It is usually seen as a network a very thin, white threads spread throughout the soil in close contact with roots.
However, in most soils that have been disturbed by residential construction, or intensive cropping practices with applications of fertilizers containing pesticides and other chemical products, the mycorrhizae content has considerably diminished, and has become insufficient to significantly enhance plant growth. Numerous experiments have shown that plants without mycorrhizae cannot cope as well with low mineral levels as those that have mycorrhizae.
In a Bonsai pot, the soil is a very controlled environment. The Bonsai artist determines what goes into the soil, what it consists of and how much water is given.
When mycorrhizal fungi colonize the plant’s root system, they create a network that increases the plant’s capacity to absorb more water and nutrients such as phosphorus, copper and zinc. This process in turn enhances growth and favors rapid development of roots and plants. Where this fungus is present in soil, a much faster establishment rate of new roots is observed as well as a reduced water need of up to 30%. The reason for this is that the micorrhizae ensures a much larger surface area is available for the absorption of water and nutrients. The plants are healthier and grow faster.
There is also evidence that these fungi can also improve the structure of the soil. The threads grow through the growing medium and aggregates the soil. This allows for more air to move through the soil as well as enhancing drainage.
Back to Bonsai cultivation. As mentioned before, the soil in a Bonsai pot can be very sterile as it could consist of particles that do not necessarily contain living organisms or lots of organic matter. These could include valuable soil bacteria (nitrogen fixing bacteria) as well as michorrizal fungus. In the absence of these, it is important that the Bonsai artist allows for more (more frequent) fertilization to take place. Michorrizal fungus spores can be bought (I have not seen it in New Zealand yet), but a much less expensive way to inoculate your Bonsai soil with the spores is to use a bit of the original soil in the Bonsai pot at planting time. I find that this is especially important when I plant Pines and other conifers.
In summary: Most plants in nature live in a relationship with Micorrhizal fungi. Both organisms benefit from this relationship. Most Bonsai soils will not contain this fungus and by adding the fungus to Bonsai soil, the plant will establish faster, take up more nutrients and will need less water. Something to think about when it comes to transplanting time.