Mycorrhizal Fungi Basics
Over 100,000 different kinds of fungi are known, and authorities suggest a million more
are out there waiting to be discovered. Most gardeners think of the mushrooms that appear
in their lawns or on the bark of their trees or they make think about the diseases fungi may
cause to their plants. But there is much more to the role of fungi as they play a key role in
the Soil Food Web.
Fungi, like higher plants and animals, are eukaryotes, organisms that have cells
with distinct, enclosed nuclei. Fungi usually grow from spores into thread-like structures called
hyphae. A single hyphal strand is divided into cells by walls, or septa.
The walls connecting hyphae cells are seldom completely sealed off from other cells in the strand,
thus allowing liquids to flow between cells.
Masses of invisible hyphae growing in close enough proximity form visible threads, or mycelia,
which you may have seen in decomposing leaf litter. A fungal hypha is considerably larger than a bacterium,
the average length being 2 to 15 micrometers with a diameter of 0.2 to 3.5 micrometers — still so thin that it
takes hundreds of thousands of individual hyphal strands to form a network thick enough for the human eye
A teaspoon of good garden soil may contain several yards of fungal hyphae, invisible to the naked eye. Millions
upon millions merge together to produce a mushroom in all its fruiting glory.
Fungi reproduce in many different ways, not just by spores, but never by seeds, as the most advanced plants
do. A fungal hypha is considerably larger than a bacterium, the average length being 2 to 15 micrometers with
a diameter of 0.2 to 3.5 micrometers—still so thin that it takes hundreds of thousands of individual hyphal
strands to form a network thick enough for the human eye to see.
Mycorrhizae are symbiotic relationships that form between fungi and plants. The
fungi colonize the root system of a host plant, providing increased water and nutrient absorption capabilities
while the plant provides the fungus with carbohydrates formed from photosynthesis. Mycorrhizae also offer
the host plant increased protection against certain pathogens.
Approximately 90% of all vascular land plants live in some association with mycorrhizal fungi. Mycorrhizal
associations are seen in the fossil record and are believed to be one of the contributing factors that allowed
early land plants, including Aglaophyton major, one of the first land plants to conquer the land.
Fungi are heterotropic organisms which means that they must absorb their food through their
cell walls. Fungi also have the ability to easily absorb elements such a phosphorus and nitrogen which are
essential for life.
Plants are autotropic, which means they are capable of producing their food in the form of
carbohydrates through the process of photosynthesis. However, plants often have difficulty obtaining and
absorbing many of the essential nutrients needed for life, specifically nitrogen and phosphorus.
In order to maximize both organisms abilities to thrive most plants allow, and indeed require, mycorrhizal
fungi to colonize their roots. In this symbiotic and intimate relationship the hyphae of the fungus greatly
increases the surface area that is open to nutrient and water absorption, maximizing the plants access to
these essential compounds and elements. In return, the plant supplies the fungus with carbohydrates for
use as energy.
This system of interdependence has evolved into many forms and now encompasses most land plants. Each
group of mycorrhizal fungi interacts and colonizes its botanical host in a slightly different way. These systems
of energy and nutrient exchange are often very complicated and very important ecologically, and have only
recently been heavily researched.
Fungal hyphae can travel over space measured in feet or meters, distances that are truly epic. Fungal hyphae are
able to bridge gaps and go short distances, which allows them to locate new food sources and transport nutrients
from one location to another, relatively far away from its origin.
The ability to transport nutrients is another key difference between fungi and bacteria. Fungal hyphae contain
cytoplasm, a liquid circulated throughout the septa in their cells. When a hyphal tip invades a
nematode, for example, it drains its hapless victim of its nutrients and distributes them in the
hyphal cytoplasm and from there through the main body of the fungus.
Nutrients are thus transferred from the tip of the fungal hypha to a wholly new location that can be several yards
away. Once inside the fungus, the nutrients are immobilized and will not be lost from the soil.
Fungi produce special structures called mushrooms above ground or truffles below ground to disperse their
spores. Since fungi grow in all sorts of environments, they have devised some elaborate methods to achieve
spore dispersal, including attractive scents, triggers, springs, and jet propulsion systems. To ensure
survival, fungal spores can develop tough membranes that allow them to go dormant for years if the conditions
are not right for immediate germination.
Fungi occur universally. Some species even exist in the frozen region of Antarctica. Airborne dispersal of spores
helps explain why visitors from, say, Alaska, will recognize species of fungi growing in far-off Australia.
While dormant spores can be found around the world, they need the right conditions to germinate and grow. Thus,
fungal spores may be found continents away from their source, but they may not be functional because the
conditions for growth are not right.
While some fungi prefer the easier-to-digest sugars characteristic of the foods that feed bacteria,
most go for tougher-to-digest foods. Fungi produce phenol oxidase, a strong enzyme
that dissolves even lignin, the woody compound that binds and protects
Another characteristic of fungi is their ability to penetrate hard surfaces. Fungi have perfected
apical growth, that is, growth at their hyphal tip. Apical or tip growth is an incredibly
complex process, an engineering job akin to building a tunnel under a river and requiring great
coordination of events.
During apical growth, new cells are constantly being pushed into the tip and along the sidewalls,
elongating the hyphal tube. Materials for the growth of fungal hyphae are supplied to the advancing
tip by the cytoplasm, which transports vesicles loaded with all necessary construction supplies.
Of course, it is important to keep extraneous material from flowing into the hypha as well as out while
this growth is happening. All the while, powerful enzymes capable of dissolving all but the most
recalcitrant carbon compounds are released as the new cells are put into place.
Fungi can grow up to 40 micrometers a minute. Discount for the moment the speed, which is incredibly
fast for such tiny organisms, and compare the distance covered to the movement of a typical soil
bacterium, which may travel only 6 micrometers in its entire life.
As with the death of any organism in the soil, the death of fungi means the nutrients contained within
them become available to other members of the soil food web. But when fungi die, their hyphae leave
a subway system of microscopic tunnels, up to 10 micrometers in diameter, through which air and
water can flow.
These “tubes” are important safety zones for bacteria trying to elude protozoa. The protozoa are
considerably bigger than the tunnels.
Fungi are the primary decay agents in the Soil Food Web. The enzymes they release allow fungi to
penetrate not only the lignin and cellulose in plants (dead or alive) but also the hard, chitin shells of
insects, the bones of animals.
Bacteria can hold their own, but they require simpler-to-digest foods, often the by-products of fungal
decay, and often only after such food has been broken or opened up by fungi and others. Compared
to fungi, bacteria are in the minor leagues of decaying ability.
Soil Compaction. May gardeners lack appreciation for fungi because all soil fungi are very
fragile. Too much compaction of soil causes fungal tubes to be crushed and the fungi are killed.
Rototilling. This gardening rite of spring breaks up fungal hyphae, decimates worms, and
rips and crushes arthropods. It destroys soil structure and eventually saps soil of necessary air.
Compost Tea. Compost tea is the Soil Food Web gardener’s tool. Applying compost tea feeds
the microbes that feed the plants. If you keep the microbes happy, healthy, and diverse, you will have
Fungicides, Pesticides, Herbicides and Insecticides. The suffix ‘cide’ means to kill. Fungicides
kill fungi, so using them can harm mycorrhizal fungi. But so can herbicides, insecticides and other pesticides.
Water. Like us, fungi need water, but they also need air, so if your soil is so wet that there’s not enough air,
the fungi will suffer. But even if your soil is just consistently moist, that’s also an issue because the fungi won’t
have any reason to go searching deeper in the soil for moisture. You also face the same issue with plant roots. We want roots and fungi to spread out and go down, so you need to let the soil dry out a little between waterings.
Mulch. Maintaining a consistent mulch of leaves, straw and perhaps some wood chips will provide protection
and habitat for the fungi.
Understanding Soil Aggregates
How To Mulch Your Garden
Getting Started With Compost Tea