Although it may not be obvious, healthy soils are chock-full of living
organisms. Some are visible to the naked eye, like earthworms, beetles,
mites and springtails, but the majority of soil-dwellers are very, very
small. They’re also very, very important to soil fertility.
Just a few grams of soil, less than a teaspoonful, may contain hundreds
of millions to billions of microbes. Not only is the total number of microorganisms
in fertile soil quite high, but together, they weigh a lot, too. Soil microbial
biomass can range from several hundred to thousands of pounds per acre.
By far, the most numerous microbes in soil are bacteria, which have
just one cell. Also abundant are fungi, which produce long, slender strings
of cells called filaments, or hyphae. The actinomycetes are in-between
these two organisms. They are advanced bacteria that can form branches
like fungi. It’s the actinomycetes that give soil its characteristic earthy
smell. Fungi and actinomycetes are good at starting the decomposition of
organic residues, working on materials that are tough to break down. Bacteria
finish the job by eating the more digestible ingredients.
Many other microbes can be found in smaller numbers in soil, including
algae, cyanobacteria (often called blue-green algae), and protozoa (one-celled
organisms that decompose organic materials and also consume bacteria).
Nematodes are microscopic roundworms; some of these are beneficial and
some are plant parasites.
The soil zone located immediately around active roots is called the
rhizosphere. This is an area of high microbial activity. Materials released
from roots, called exudates, create a food-rich environment for the growth
of microorganisms. Rhizosphere microorganisms in turn help plants by fixing
nitrogen from the soil air, dissolving soil minerals and decomposing organic
matter, all of which allow roots to obtain essential nutrients.
Some microbes have a specialized role in the rhizosphere. Rhizobia
bacteria associate with the roots of legumes to form nodules. This symbiotic
relationship provides the bacteria with a source of carbon in exchange
for making nitrogen available to the plant. Farmers are familiar with this
process, and often encourage it by inoculating legume seeds with a commercial
preparation of the Rhizobium species that is suited to the crop species
they are planting.
A special kind of fungus called mycorrhizae also associates with
plants. By colonizing large areas of roots and reaching out into the soil,
mycorrhizae aid in transfer of soil nutrients and water into the plant.
This is especially important in situations where nutrient availability
or moisture is limited.
Microbes have a lot to do with maintaining good soil structure, which
promotes infiltration and drainage of water, soil aeration, and vigorous
root growth and exploration. Gummy substances produced by soil microbes
(complex sugars and mucilages) help cement soil particles together into
aggregates, which contribute to soil structure. This cement also makes
aggregates less likely to crumble when exposed to water. Fungal hyphae
further stabilize soil structure as their threadlike structures spread
through the soil, surrounding particles and aggregates like a hairnet.
The proportion of the different kinds of organisms present in your
soil depends on conditions such as available moisture, aeration, organic
matter levels and the type of plants present. Chemical conditions such
as acidity and alkalinity will greatly affect soil organism populations.
For example, fungi often prefer acidic soils, while actinomycetes thrive
in more alkaline conditions.
In order to encourage microbial activity on the farm, soil has to
be managed to create a favorable environment for both crops and microbes.
This can be done by timely and appropriate tillage that avoids compaction;
irrigation and drainage practices that keep the soil moist but not waterlogged,
liming to maintain a near-neutral pH, and frequent addition of organic
(carbon-containing) residues to provide energy for the microbes.
In general, the abundance of microbes in soil is proportional to
the organic matter content. Soils that have large amounts of organic residues
regularly added to them tend to support a larger microbial population.
However, there is usually an explosion in microbial numbers after the addition
of available carbon ‘fuel’, followed by a population crash as that fuel
is consumed. Some of the fuel is incorporated into microbial cells and
some is given off as carbon dioxide. Later, the microbial cells become
food for other microbes and then they, too, are decomposed through microbial
activities. So eventually, microbial activity returns to a low level unless
more residues are added. The good news is that the microbes are always
there, ready to leap into service when environmental conditions are suitable
and there’s a source of energy.
For more information on soil microbes, soil management and soil fertility, refer to "Building Soils for Better Crops,"
by Fred Magdoff and Harold van Es, available from the Sustainable Agriculture Research and Education (SARE) Program at: www.sare.org/Learning-Center/Books/Building-Soils-for-Better-Crops-3rd-Edition.