JARS v63n4 - Mycorrhiza, and the Gardening Benefits of Charcoal

Mycorrhiza, and the Gardening Benefits of Charcoal
Glen Jamieson
JARS Editor

In the article on soil carbonization in this JARS issue, it is stated that "...soil scientists discovered that a [microbial] community exists in symbiosis with the root hairs of plants in terra preta soils." This is more about that subject from http://en.wikipedia.org/wiki/Mycorrhiza.

A mycorrhiza (Greek for fungus roots coined by Frank, 1885; typically seen in the plural forms mycorrhizae or mycorrhizas) is a symbiotic (occasionally weakly pathogenic) association between a fungus and the roots of a plant. In a mycorrhizal association the fungus may colonize the roots of a host plant either intracellularly or extracellularly.

This mutualistic association provides the fungus with relatively constant and direct access to mono- or dimeric carbohydrates, such as glucose and sucrose produced by the plant in photosynthesis. The carbohydrates are translocated from their source location (usually leaves) to the root tissues and then to the fungal partners. In return, the plant gains the use of the mycelium's very large surface area to absorb water and mineral nutrients from the soil, thus improving the mineral absorption capabilities of the plant roots. Plant roots alone may be incapable of taking up phosphate ions that are immobilized, for example, in soils with a basic pH. The mycelium of the mycorrhizal fungus can however access these phosphorus sources, and make them available to the plants they colonize.

The mechanisms of increased absorption are both physical and chemical. Mycorrhizal mycelia are much smaller in diameter than the smallest root, and can explore a greater volume of soil, providing a larger surface area for absorption. Also, the cell membrane chemistry of fungi is different from that of plants. Mycorrhizae are especially beneficial for the plant partner in nutrient-poor soils.

Mycorrhizal plants are often more resistant to diseases, such as those caused by microbial soil-borne pathogens, and are also more resistant to the effects of drought. These effects are perhaps due to the improved water and mineral uptake in mycorrhizal plants.

Mycorrhizae form a mutualistic relationship with the roots of most plant species (although only a small proportion of all species have been examined, 95% of all plant families are predominantly mycorrhizal).

Plants grown in sterile soils and growth media often perform poorly without the addition of spores or hyphae of mycorrhizal fungi to colonise the plant roots and aid in the uptake of soil mineral nutrients. The absence of mycorrhizal fungi can also slow plant growth in early succession or on degraded landscapes.

Beneficial properties of adding charcoal to soil
(Modified from the Nov/Dec 2008 Bulletin of the BC Council of Garden Clubs)
Charcoal helps balance the acidity of a soil mix by absorbing large quantities of acid-forming products. If obtainable, growers should use a fine to medium grade of charcoal in their potting mixtures. Lacking a good grade of charcoal, activated carbon of the type used in aquarium filters is a good substitute. Beneficial properties of adding charcoal to soil:

• Absorbent - absorbs excess humidity, and some toxic elements that may be present in the soil.

• Antioxidant

• Reduces likelihood of root-rot

• Helps attain a proper porosity levels in soil mixes.

Uses of charcoal in gardening:

• Soil mix - Either mixed with potting soil, compost, peat or coir, charcoal is useful in lightening a planting medium and bringing out all its benefits. Due to its absorbing properties, charcoal is used as a water reservoir, an odour absorbent and in helping prevent rotting of roots. Charcoal is also often deposited on top of small rocks at the bottom of a pot in order to help with drainage.

• Orchids - Most orchids lovers around the world have discovered the virtues of charcoal in most potting mixes.

• African violets - Its is used by many African violet lovers and carbon seems to be particularly useful for this plant. A fine granular size is added to soil-less mixes in order to stabilize the humidity level and prevent it from fluctuating.

• Lawn - Fine charcoal powder used on lawns (e.g., golf greens) absorbs and functionally eliminates excess amounts of fertilizer and chemicals present in the soil.

• Transplanting shrubs or trees - Before transplanting a shrub or tree, some charcoal pieces at the bottom of the hole can help absorb and purify any stagnant water.