Most people new to bonsai don't know about Mycorrhiza and the important role they play in a tree's ability to feed itself. Here's a quick introduction.
Some 70-90% of plants on earth will form a symbiotic relationship with Mycorrhiza fungi. The fungi colonize plant roots and act as an extension to the roots to search out and absorb water and nutrients from the soil - especially nitrogen and phosphorus. In exchange the tree provides sugars to the fungus that it uses to grow. The area covered by the fungus filaments is much larger than the tree's roots - perhaps 3-4 times as large. This means a much larger area of soil can be searched for nutrients than if only the tree's roots were involved.
The fungus filaments, or Mycelium, first penetrate the roots as shown below. Or they may grow sheaths that wrap around the outside of the tree's roots.
The other end of the filaments absorb water directly. When it encounters mineral salts, or rocks it can produce enzymes and even acids to turn those minerals into water soluble versions that can be transported back to the tree roots. Some mycorrhizal fungi attack dead plant material, such as leaf litter in a forest - or the fir bark as we use in our bonsai soil mix. The fungi are directly causing decay in this case.
The green parts of the tree use the water and minerals plus photosynthesis to produce more growth in the tree. As much as 30% of the sugars produced are delivered to the mycorrhiza for it's growth.
Here's what roots and mycorrhiza actually look like. You can see the roots as the tan lines. The much finer, while lines are the mycorrhiza filaments.
Here is a magnified view of the Mycelim tips, which search for and break down the nutrients. The very fine filaments connect to larger ones, which are often cross-connected to other filaments so that if one breaks the nutrients will continue to flow.
Interesting Facts: Mycorrhiza actually connect the root systems of many trees together - and can share nutrients between those trees. The area covered can be huge! There is an area in eastern Oregon that had a mycelium web that covered 2,400-acre [970-hectare] site - before logging roads cut through the site.
How This Works in Bonsai Soil
When we apply Osmocote or other granular fertilizers to the top of our bonsai soil small amounts are dissolved every time we water. When we use a liquid fertilizer we are applying mineral salts that are dissolved in the water that we water with. Much of these salts will be lost as the water drains out the bottom, but some will be retained by the pumice, akadama, fir bark, even within the space between the soil particles. The Mycelim can collect these nutrients.
Because our bonsai soil often contains fir bark, nutrients within the bark can be accessed via decay. And, as in the soil of your garden, the soil and rocks can be broken down although that will be slower than grabbing nutrients from the fertilizer salts. That's why it's important to fertilizer constantly, so that minerals are avaiilable for the Mycelim to collect and exchange with the roots and so the tree can grow.
Things to Watch For
Mycorrhiza can grow very rapidly. Over time there can be so many filaments that the flow of water thru the soil is impeded. This seems to happen a lot with pine trees. Remember the trick of sticking a chopstick into the soil to see if the tree needs to be repotted? Sometimes the soil has broken down into fine particles, but other times it's simply that there's too much mycorrhiza. In either case the tree will need to be repotted. Reuse some of the old soil when repotting. Also, collect some of the mycorrhiza filaments from the old soil and add it to the new pot.
If you have a tree that isn't growing as well as you think it should, maybe it needs more mycorrhiza. Transfer some soil from a healthy tree of the same species, e.g. pine to pine, juniper to juniper.
Adding Mycorrhiza to Your Soil
Some mycorrhiza spores will simply find your tree as they drift thru the air. You may also "seed" your soil by transplanting some mycorrhiza from another tree of the same species. This is especially easy to do with pines because the mycorrhiza is very visible. You can also add a fertilizer that containes a variety of different mycorrhiza species. The idea is that one of the species will be a match for your plant - and the rest wont' really do anything. Many fertilizers containing mycorrhiza contain manures. The following one doesn't.
Figures 1, 3 and 4 - The Magic of Mushrooms, BBC TV Four.
Professor Richard Fortey delves into the fascinating and normally hidden kingdom of fungi. From their spectacular birth, through their secretive underground life to their final explosive death, Richard reveals a remarkable world that few of us understand or even realise exists - yet all life on Earth depends on it. ...
Lower quality version available on YouTube
Figure 2 - Fungal Biology, 4th Edition (textbook) by J.W. Deacon, University of Edinburgh, School of Biological Sciences. See chapter MYCORRHIZAS