Soil Organisms Improve Garden Tilth
Rather than being an inert material, soil houses a dynamic living ecosystem. The
1-5% organic matter found in soils includes 0.2% living organisms. Although
most soil organisms are invisible to the naked eye, they help gardeners in multiple
ways. One major benefit to gardeners is their ability to help improve soil tilth.
Soil tilthis the suitability of a soil to support plant growth, especially as it relates
to ease of tillage, fitness for a seedbed, impedance to seedling emergence and root
penetration. Soil organisms also play a central role in making nutrients available
to plants. The community of soil organisms is varied, versatile, and adaptable to
changing conditions and food supplies.
Types of Soil Organisms
Soil contains an enormous number of living organisms. One cup of undisturbed
native soil may contain:
Bacteria 200 billion
Protozoa 20 million
Fungi 100,000 meters
Soil organisms are naturally active during certain times of the year. Most are
active when the soil is warm and moist, like during late spring and early summer.
If the soil dries out during the summer months, soil organism activity naturally
declines. During fall, if there is rain or snow that moistens the soil while it is still
warm, soil organisms may resume partial activity. As the soil cools in the fall,
many soil organisms go dormant. Gardeners should note that fertilizers that
require processing by soil organisms will be more available to plants when the soil
is warm and moist and less available when the soil is cool or dry.
Despite their small size, soil organism activities have great impact on plant growth.
Soil organisms can be grouped into three categories: 1) organisms that are
beneficial to plants—directly or indirectly, 2) neutral organisms—those whose
activities have no effect on plants, and 3) organisms that are harmful to plants.
Harmful organisms are often described as pathogens, such as the soil fungi that
cause wilt diseases, or plant pests, such as white grubs that feed on plant roots.
Directly Beneficial Soil Organisms
Some soil organisms have a close, mutually beneficial (symbiotic) relationship
with plants. Two examples include rhizobia and mycorrhizae. Rhizobia are
bacteria that form symbiotic associations with legumes such as beans and peas.
The bacteria form nodules on the roots of the host plant in which they fix nitrogen
gas from the air. Rhizobiasupply the plant with nitrogen and in turn the plant
supplies the bacteria with essential minerals and sugars. It may be helpful to add
Rhizobiain the first planting of beans and peas in a soil area. Afterwards they will
Mycorrhizae are specific fungi that form symbiotic associations with plant roots.
Found in most soils, they are very host-specific (i.e., each plant species has specific species of mycorrhizae associated with it).
The Latin word mycor means fungus and rhizais root. The terms “mycorrhiza”
(singular) or “mycorrhizae” (plural) refer to the tissue that forms when fungi and
roots develop a mutually beneficial relationship. Enlarging the surface-absorbing
area of the roots by 100 to 1,000 times,mycorrhizae create filaments or threads
that act like an extension of the root system. This makes the roots of the plant
much more effective in the uptake of water and nutrients such as phosphorus and
zinc. In exchange, the fungus receives essential sugars and compounds from the
roots to fuel its own growth. Some species of mycorrhizae can be seen on roots,
while most are invisible to the naked eye.
Mycorrhizae improve plant health. They enhance the plant’s ability to tolerate
environmental stress (like drought and dry winter weather) and reduce transplant
shock. Plants with mycorrhizae may need less fertilizer and may have fewer soilborne diseases.
A by-product of mycorrhizal activity is the production of glomalin, a primary
compound that improves soil tilth. In simple terms, glomalin glues the tiny clay
particles together into larger aggregates, thereby increasing the amount of large
pore space, which in turn creates an ideal environment for roots. For additional
details, refer to U.S. Departmentof Agriculture web site at
Mycorrhizal cocktails are sometimes incorporated in planting or post planting care
of trees and landscape plants. However, in research studies, results are variable.
Over time, additional research will help clarify what procedures result in improved
plant health and vigor.
Indirectly Beneficial Soil Organisms
In addition to directly beneficial organisms such as rhizobia and mycorrhizae, there
are a large number of soil organisms whose activities indirectly help plants. Soil
organisms collectively decompose organic matter, resulting in two principal
First, as soil organisms decompose organic matter, they transform nutrients into
mineral forms that plants can use; thus this process is called mineralization.
Without soil microorganisms, insects, and worms feeding on organic matter, the
nutrients in organic matter would remain bound in complex organic molecules that
plants can’t utilize.
Second, as soil organisms break down organic matter, their activities help improve
soil structure. Improved soil structure provides a better environment for roots,
with less soil compaction and better water and air movement. Many gardeners
know that organic matter improves soil, but it is important to note that its
beneficial properties are only released after being processed by soil organisms.
Soils naturally contain these decomposers.Adding decomposers to the soil or
compost pile is not necessary. Rather nurture them with food (organic matter) and
good aeration and drainage (air and water).
Soil Organic Matter
Soil organic matter is composed of a wide variety of organic substances. Derived
from plants, animals, and soil organisms, the soil organic matter “pool” can be
divided into four categories. First are the living organisms and roots, making up less
than 5% of the total pool. Second are the residues from dead plants, animals and soil organisms that have not yet begun to decompose (<10%). Third is the portion
undergoing rapid decomposition (20-45%). Fourth is the stabilized organic matter
(humus) remaining after further decomposition by soil microorganisms
The stabilized organic matter, or humus, is the pool of soil organic matter that has
the longest lasting benefits for gardeners. After rapid decomposition occurs, a mix
of stable, complex organic compounds remains, which decomposes slowly over
time (about 3% per year). Humus is a mix of tiny solid particles and soluble
compounds that are too chemically complex to be used by most organisms.
Humus contains a potpourri of sugars, gums, resins, proteins, fats, waxes, and
lignin. This mixture plays an important role in improving the physical and
chemical properties of soil.
Humus improves the physical and chemical attributes of soil in several ways,
• Humus improves soil structure by binding or “gluing” small mineral particles together into larger aggregates creating large soil pores for improved air and water infiltration and movement.
• Humus improves water retention and release to plants.
• Humus slowly releases nitrogen, phosphorus, and sulfur over time, which
plants then use for growth and development.
• Because of its positive surface charge, humus improves soil fertility by retaining nutrients.
• Humus buffers the soil pH so it remains stable for plant roots.
• Humus can chelate or bind metals in soil, preventing metal toxicities.
As a point of clarification, garden stores sometime carry soil amendments labeled
as humus. In reality these are generally “compost” and do not meet the soil
scientist definition of humus as given above.
Gardeners can purchase products at garden centers that are intended to introduce
soil organisms to an existing soil. Adding decomposing bacteria from a purchased
product is generally not necessary, since decomposing soil organisms are already
present in the soil. Even if their populations are low due to unfavorable conditions,
as soon as organic matter and water become available their populations rapidly
increase. Thus, soil biologists encourage gardeners to nurture existing
communities rather than introducing external organisms through purchased
In addition, inoculating with rhizobia is generally not needed, unless a vegetable
gardener is planting a leguminous crop for the first time. In this case, the gardener
should purchase the appropriate inoculant (bacteria) for the leguminous vegetable
being planted. Inoculation in future years is not needed, since rhizobia produce
survival structures to over-winter.
Myccorhizal products are considered highly experimental at this time, and are thus
not recommended by CSU Extension for general use.
Soil Food Web
Within the soil, organisms function within an ecological food web (the smaller
becoming the food for the larger) cycling nutrients through the soil biomass. This
soil food web is the basis of healthy, living soil. Significant soil organisms
involved in the soil food web include: 1) bacteria, 2) fungi, 3) protozoa, 4)
nematodes, 5) arthropods, and 6) earthworms
Bacteria are simple, single-celled microorganisms. Bacteria inhabit a wide variety of habitats, including soil. In fact, a teaspoon of productive soil can contain from
100 million to 1 billion bacteria. Soil-inhabiting bacteria can be grouped as
decomposers, mutualists, pathogens, or chemoautotrophs. Bacteria that improve
soil quality feed on soil organisms, decompose organic matter, help keep nutrients
in the root zone, enhance soil structure, compete with disease-causing organisms,
and filter and degrade pollutants in soil.
Fungi are a diverse group of multi-cellular organisms. The best known fungi are
mushrooms, molds, and yeast, but there are many others that go unnoticed,
particularly those living in soil. Fungi grow as long strands called hyphae (up to
several yards long), pushing their way between soil particles, rocks and roots.
Fungi can be grouped as decomposers, mutualists, or pathogens. Fungi that
improve soil quality decompose complex carbon compounds, improve
accumulation of organic matter, retain nutrients in soil, bind soil particles into
aggregates, compete with plant pathogens, and decompose certain types of
Protozoa are microscopic, single-celled microbes that primarily eat bacteria. The
bacteria contain more nitrogen than the protozoa can utilize and some ammonium
(NH4) is released to plants. Protozoaalso prevent some pathogens from
establishing on plants and function as a food source for nematodes in the soil food
Nematodes are small, unsegmented round worms. Nematodes live in water films
in the large pore spaces in soil. Most species are beneficial, feeding on bacteria,
fungi, other nematodes, but some cause harm by feeding on plant roots.
Nematodes distribute bacteria and fungi through the soil as they move about.
Predatory nematodes can consume root-feeding nematodes or prevent their access
Soil arthropods are small animals such as insects, spiders, and mites. They range
in size from microscopic to several inches in length. Most live near the soil
surface or in the upper three inches. Arthropods improve soil quality by creating
structure through burrowing, depositing fecal pellets, controlling disease-causing
organisms, stimulating microbial activity, enhancing decomposition via shredding
organic matter and mixing soil, and regulating healthy soil food web populations.
Soil arthropods can be shredders(millipedes, sowbugs, etc.), predators(spiders,
scorpions, pseudoscorpions, centipedes, and predatory mites, ants and beetles),
herbivores(symphylans, root-maggots, etc.), or fungal-feeders(springtails and
turtle mites). Most soil-dwelling arthropods eat fungi, worms, or other arthropods.
Refer to the CMG GardenNotes #218, Earthworms
Ways to Encourage Beneficial Soil Organisms
Creating a favorable environment for soil organisms improves plant growth and
reduces garden maintenance. Encouraging their efforts is central to building a
healthy fertile soil supportive to optimum plant growth.
• Add organic matter to the soil.Soil organisms require a food source from
soil amendments (compost, crop residues) and/or mulch.
• Use organic mulch. It stabilizes soil moisture and temperature, and adds
organic matter. Mulches may help prevent soil compaction and protect soil
oxygen levels needed by soil organism and roots.
NOTE: The term mulchrefers to material placed on the soil surface. A mulch
controls weeds, conserves water, moderates soil temperature and has a direct
impact on soil microorganism activity. Soil amendmentrefers to materials mixed
into the soil.
• Water effectively. Soil organisms require an environment that is damp (like a
wrung out sponge) but not soggy, between 50 – 90F. Soil organism activity
may be reduced due to dry soil conditions that are common in the fall and
winter. Avoid over-irrigation since water-logged soils will be harmful to
beneficial soil organisms.
• Avoid unnecessary roto-tilling, as it will destroy the mycorrhizae and soil
structure. Instead of tilling, mulch for weed control.
• Avoid unwarranted pesticide applications. Some fungicides, insecticides
and herbicides are harmful to various types of soil organisms.
• Avoid plastic sheets under rock mulch.This practice discourages
microorganism activity by reducing water and air movement and preventing
the incorporation of organic matter.