## Soil Biology: Key Educational Messages

Use this outline along with the Soil Biology Quick Facts below to
organize a soil biology presentation or to identify key points to
feature in printed materials.

I.     Soil is alive!

> A.   In most ecosystems, more life and diversity lives underground
> than above.
>
> B.   Energy flows from the sun, through plants, and through many
> trophic levels of soil organisms.
>
> C.   Soil organisms can be divided into six groups: bacteria, fungi,
> protozoa, nematodes, arthropods, and earthworms. Each group of
> organisms plays important roles. Even within each group, there is
> great diversity in form and function.
>
> D.   The rhizosphere is the interface between plant roots and the soil
> environment. It is the location of much soil biological activity and
> plant-microbe interactions including symbioses, pathogenic infection,
> and competition.
>
> E.   Soil organisms are part of a living system.  Ecosystem
> characteristics largely determine the structure of soil communities.
> Weather determines daily and seasonal variations in biological
> activity.
>
> 1.   The types of species present and their level of activity depends
> on micro-environmental conditions including temperature, moisture,
> aeration, pH, pore size, and types of food sources.
>
> 2.   Arid systems have few earthworms, but have termites, ants and
> other invertebrates that serve similar functions.
>
> 3.   Grasslands have near equal amounts of fungal and bacterial
> biomass, or may be dominated by bacteria. Coniferous forests may have
> 100 to 1000 times more fungal biomass than bacterial biomass.

#### II.      We need soil organisms for the services they provide. They play critical roles in plant health and water dynamics.

> A.   Soil organisms are an integral part of the cycling of nutrients
> through the environment. They drive:
>
>  1. Decomposition,
>
> 2\. Mineralization  (E.g., protozoa and nematodes excrete excess
> nitrogen into the soil when they eat nitrogen-rich bacteria and
> fungi.)
>
> 3\. Storage and release of nutrients,
>
> 4\. Degradation of pollutants before they reached groundwater or
> surface water,
>
> 5\. Carbon cycling, carbon sequestration, and soil organic matter
> transformations,
>
> 6.   Nitrogen cycling (N fixation, denitrification, nitrification).
>
> C.   Soil biological activity substantially affects soil structure
> including the size of soil pores, the stability of soil aggregates,
> and the existence of macropores. Soil structure impacts how water
> flows over, into, and through soil and how much water is held within
> reach of plant roots.
>
> 1\. Large, burrowing invertebrates (e.g., earthworms, ants, termites,
> beetles) create macropores that allow rapid flow of water into or
> through soil.
>
> 2\. Even tiny arthropods produce fecal pellets that are mixtures of
> soil and organic matter. These became stable soil aggregates.
>
> 3\. Fungi and bacteria produce substances that help bind soil
> particles together and stabilize soil aggregates.
>
> 4\. Soil organic matter can be physically protected from degradation
> within stable soil aggregates.
>
> D.   Plant pest dynamics depend on the whole mix of organisms in the
> soil. Some organisms prey on or compete with disease-causing
> organisms. Some bacteria release plant growth factors that directly
> increase plant growth.
>
> E.   In arid lands, biological soil crusts seem to be important for
> all the purposes listed above. They help fix nitrogen, stabilize the
> soil surface, affect water flow, and prevent the establishment of some
> exotic plant species.
>
> F.   Mycorrhizal fungi help plants acquire nutrients from the soil and
> they help stabilize soil aggregates.
>
> G.  Resilience is the ability of a soil to recover its functions after
> a disturbance such as fire, compaction, tillage, etc. The mix of
> organisms in the soil partially determine a soil's resilience.

#### III.    Management affects soil organisms

> A.   We know that land management practices change the soil community.
> The link between specific changes and soil function is less clear.
>
> B.   Soil biological crusts are very sensitive to trampling.
>
> C.   Reducing tillage tends to result in increased growth of fungi,
> including mycorrhizal fungi.
>
> D.   Soil compaction, lack of vegetation, or lack of plant litter
> covering the soil surface tends to reduce the number of soil
> arthropods.

 

## Soil Biology Quick Facts

-   The tips of small plant roots move through the soil with a twisting
    screw-like motion.  Mature trees can have as many as 5 million
    active root tips.

-   A single spade full of rich garden soil contains more species of
    organisms than can be found above ground in the entire Amazon rain
    forest.

-   Although the soil surface appears solid, air moves freely in and out
    of it. The air in the upper 8 inches of a well-drained soil is
    completely renewed about every hour.

-   The plants growing in a 2-acre wheat field can have more than 30,000
    miles of roots, greater than the circumference of the Earth.

-   The wonderful \"earthy\" smell of newly plowed ground is believed to
    result from chemicals produced by micro-organisms.  One of these
    chemicals, called geosmin, is produced by actinomycetes, organisms
    that have some properties of both bacteria and fungi.

-   Soil can act as either a sink or a source of greenhouse gases.  An
    estimated 30 percent of the carbon dioxide, 70 percent of the
    methane, and 90 percent of the nitrous oxide released to the
    atmosphere each year pass through the soil.

-   It takes about 4,000 to 6,000 pounds of crop residue per year to
    maintain the content of organic matter in a soil.

-   Modern farming practices that minimize soil disturbance (plowing)
    and return plant residues to the soil, such as no-till farming and
    crop rotations, are slowly rebuilding the Nation\'s stock of soil
    organic matter.

-   Of the carbon returned to the soil as plant residue, about 5 to 15
    percent become tied up in the bodies of organisms and 60 to 75
    percent is respired as carbon dioxide back to the atmosphere. Only
    10 to 25 percent is converted to humus in the soil.

-   "We know more about the movement of celestial bodies than about the
    soil underfoot."\
    \-\--Leonardo Da Vinci, circa 1500\'s

-   "Every time you take a step in a mature Oregon forest, your foot is
    being supported on the backs of 16,000 invertebrates held up by an
    average total of 120,000 legs." \
    --Dr. Andrew Moldenke, Oregon State University.

-   Even in agricultural soils, more than a thousand arthropod legs
    support your every step.

-   One cup of soil may hold as many bacteria as there are people on
    Earth.

-   Bacteria and actinomycetes are exceedingly tiny. Yet, because of
    their tremendous numbers, they make up half the living biomass in
    some soils. 

-   The weight of all the bacteria in one acre of soil can equal the
    weight of a cow or two. 

-   Actinomycetes have cells like bacteria, but grow as long filaments
    like fungi. Like fungi, they help degrade tough materials, but
    unlike fungi, they prefer high pH (over 7.0). 

-   A teaspoon of farm soil may contain tens of yards of fungi. The same
    amount of soil from a coniferous forest may hold tens of miles of
    fungi.

-   Nematodes are amazingly diverse. Twenty thousand species have been
    described, but it is thought that 500,000 species may exist. Five
    thousand soil species have been described.

-   Earthworms move from lower strata up to the surface and move organic
    matter from the soil surface to lower layers. Where earthworms are
    active, they can turn over the top 6 inches of soil in 10 to 20
    years.