[permaculture] The Overstory #208--A fresh look at life below the surface

Lawrence F. London, Jr. lflj at intrex.net
Mon Jul 14 16:55:26 EDT 2008

The Overstory #208--A fresh look at life below the surface
         By Danny Blank


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The Overstory #208--A fresh look at life below the surface
         By Danny Blank


: --> Nutrient Retention
: --> Nutrient Cycling
: --> Improved Soil Structure, Air and Water Dynamics
: --> Pest and Disease Suppression



Soil is often described in textbooks as rock and minerals, air, water,
living organisms and decaying organic matter. Though an accurate
depiction, soil biology often takes a backseat to soil chemistry and
physics‹soils are classified largely on the presence or absence of
certain types and sizes of minerals. However, soil organisms play a huge
and underestimated role in the productivity and health of soils. When a
rainforest is cleared, burned, and the land subjected to annual tillage
and burning, we often see this once highly productive landscape now
barely able to support a maize crop. What happened? There is a growing
understanding that the answers to this all too common question are found
in the abundance and diversity of life hidden below the surface.


The soil foodweb is essentially the community of organisms that live in
the soil. Every agricultural field, forest, prairie, or pasture has its
own soil food web with a unique set of soil organisms. Healthy soils
contain massive populations of bacteria, fungi, protozoa, nematodes,
soil arthropods, and earthworms (Figure 1). A teaspoon (approx. one
gram) of productive soil contains between 100 million and 1 billion
bacteria. It contains around 25,000 species of bacteria and 8,000
species of fungi!

Just as the plants we see above ground differ from place to place, the
ratios and diversity of soil organisms change with region, climate,
vegetative succession, and soil disturbance. Grasslands and agricultural
fields generally have bacterial-dominated food webs while forests
usually have fungal-dominated soils. Healthy, highly productive
agricultural soils tend to contain about equal weights of bacteria and
fungi (Soil Biology Primer).

Soil life is dynamic and complex. Understanding this complex soil
foodweb‹the life in the soil‹ is critical to understanding how the plant
world grows and flourishes. It is the foundation for knowing how to
restore damaged lands, improve agricultural production and ultimately
improve the health and livelihoods of people. Soil microorganisms play a
big part in supporting healthy plant life through nutrient retention and
cycling, disease suppression, and improved soil structure, water
infiltration, absorption, and holding capacity.


--> Nutrient Retention

The ability of soil to hold nutrients is often measured by what is
called cation exchange capacity (CEC)‹a measure of a soil's negative
charge (usually in clays and organic matter). Rarely are soil organisms
mentioned with regards to nutrient retention. However, in a healthy soil
foodweb, vast reserves of important plant nutrients are stored within
the bodies of bacteria, fungi and other soil organisms. For example, no
known organism on the planet is more concentrated in nitrogen than
bacteria. Fungi are typically the second most concentrated in nitrogen
(Ingham, An Introduction to the Soil Foodweb). Along with nitrogen they
contain other critical plant nutrients‹high levels of phosphorus,
potassium, sulfur, magnesium, calcium, etc. Decomposition happens almost
exclusively by these two sets of organisms, which in turn store
nutrients from the decomposed organic matter in their own bodies,
immobilizing nutrients, and thereby reducing leaching. Another example
is calcium. Calcium is held incredibly tightly by fungal hyphae in the
soil. Without healthy fungal biomass, calcium is easily leached through
soils. The presence of decaying organic matter in soil‹broken down
leaves, roots, dead organisms, etc.‹along with diverse populations of
bacteria and fungi are key to immobilizing and storing nutrients in the
soil. These nutrient-rich organisms then become the basis for the
critical cycling of nutrients to plants.

--> Nutrient Cycling

As mentioned above, fungi and bacteria have considerably more nitrogen
in their bodies than other organisms. The carbon to nitrogen ratio for
bacteria is around 5:1 and for fungi is 20:1 (Ingham, Overstory #81).
Nutrient cycling happens when other sets of soil organisms (primarily
protozoa, bacterial and fungal feeding nematodes, micro arthropods, and
earthworms) are present to consume the nutrient-rich bacteria and fungi
and release nutrients in plantavailable forms. A healthy soil contains
diverse species and huge populations of protozoa, beneficial nematodes,
micro arthropods, and earthworms (Figure 1). For example, one gram of
healthy soil can contain 1 million protozoa (Soil Biology Primer). A
single protozoa, with a C:N ratio of 30:1, can consume 10,000 bacteria a
day. Because the protozoa need less nitrogen, the excess is excreted in
the form of ammonium ions. The ammonium ions are held more tightly to
the soil particles than are nitrate ions, the most common (and
leachable) form of nitrogen in commercial fertilizers. This
predator-prey relationship between protozoas and bacteria can account
for 40 to 80% of nitrogen in plants. (FAO Soil Bulletin #78). A similar
relationship has been documented with bacterial- and fungal-feeding
nematodes. With a consumption rate up to 5,000 cells/minute, these
beneficial nematodes (unlike plant-feeding types such as root-knot
nematodes) are thought to turn over nitrogen in the range of 20-130
kg/ha/yr, contributing immensely to plant available nitrogen. (FAO Soil
Bulletin #80). These rapid interactions and countless exchanges of
nutrients between soil organisms occur in root zones of plants where the
highest concentrations of organisms exist (because root exudates provide
food for the bacteria and fungi which in turn attract their predators‹
protozoa, nematodes, micro arthropods and earthworms).

Nutrient cycling by these predators also occurs with other valuable
plant nutrients such as potassium, phosphorus, calcium, sulfur and
magnesium, resulting in a less leachable form than what is usually
applied in synthetic fertilizers.

Other soil organisms are also involved in more direct forms of nutrient
cycling. Nitrogen-fixing bacteria convert air nitrogen into a useable
plant form as they colonize roots of legumes. Mycorrhizal fungi colonize
root systems of perennials such as coffee, staple grain crops as maize
and sorghum, and vegetables like onions. In so doing, these specialized
fungi cycle nutrients by secreting enzymes that solubilize calcium
phosphate and pump the phosphorus directly to the plants, thus making an
otherwise unavailable nutrient now available to plants (Ingham, An
Introduction to the Soil Foodweb). Mycorrhizae also benefit crops by
aiding in disease suppression and water absorption. In field trials at
Zamorano University in Honduras, mycorrhizal fungi were applied at the
time of planting and then one time a year thereafter. As a result,
coffee production increased by 30%, plantain production by 23%, and
jicama production by 35%. In addition, fertilizer use for avocado
nursery tree production was reduced by 50% (Personal communication with
A. Rueda).

--> Improved Soil Structure, Air and Water Dynamics

As bacteria populations increase, they secrete glue-like, sticky
materials that bind sand, silt, clay, and small SOM particles into
micro-aggregrates (micro-clusters). Fungi, the largest known organism on
the face of the earth (one organism can cover thousands of acres in a
forest), bind the microaggregates to form larger soil aggregate
structures, creating air and water passageways. Larger passageways
(pores) are created by bigger organisms like nematodes, soil arthropods
(e.g. sow bugs, termites, millipedes, roaches and soil mites), and
earthworms that burrow through the soil looking for food. Earthworms
"glaze" the passageways they create with a nutrient-rich and microbially
active slime layer that greatly enhances water-holding capacity and soil
structure (Ingham, An Introduction to the Soil Foodweb). Earthworms and
many soil arthropods also shred organic matter, grazing on the
microorganisms present, and then excreting the nutrients in a
plant-available form.

All these small channels and pores become a series of reservoirs and a
transportation network for air, water, nutrients, roots, and organisms.
Water use efficiency has been improved by as much as 50% in Australia by
reintroducing missing soil biology‹meaning the same amount of crop is
grown with half of the water due to the improved soil structure and
water dynamics that come with a healthy soil food web (Ingham, An
Introduction to the Soil Foodweb).

--> Pest and Disease Suppression

Soil organisms break down toxic compounds in the soil, produce
plant-growth promoting hormones and chemicals, out-compete and suppress
disease causing organisms, and buffer soil pH. When there is a healthy
balance and abundance of soil organisms in the foodweb, pests and
diseases can be out competed or preyed upon. One of our worst pests in
Florida (and on the ECHO farm) is the root feeding nematode. This pest
has numerous predators in the food web‹bacteria like Pasteuria and
Burkholderia, predatory nematodes, and multiple nematode-eating fungi
species such as Trichoderma (Guerena, M. Nematodes: Alternative
Controls). Commercial formulations of these biocontrols are increasingly
available. When a balance is not maintained (for example, if fungal
diversity and biomass is reduced), micro arthropods and fungal-feeding
nematodes whose main food source is normally fungi foods may attack
plant roots instead. Most of us are aware of beneficial organisms like
ladybugs, spiders, and wasps that attack crop pests above ground. There
are far greater concentrations of organisms in the soil. Maintaining a
healthy soil foodweb is essential for long-term, sustainable crop health
and production.


At the beginning I mentioned a scenario where a tropical forest was
cleared for an agriculture field, but after one or two seasons, the land
could barely support a maize crop. So what happened? For centuries,
there existed a dynamic forest system that never once needed any
fertilizer, lime, or other chemical input. Under the forest's tall giant
canopies, in the deep shade and protection of leaf litter, the soil was
teeming with an abundant, diverse balance of soil organisms. Nutrients
at the surface were rapidly recycled; complex humic substances were
formed; an extensive mychorrizal fungi biomass was present; and
countless other species were present to perform all the necessary and
important life-supporting functions that exist in mature forest systems.

With the disappearance of the forest, removal of the litter layer, and
rapid oxidation of the remaining organic matter due to damaging
agricultural practices, the number and diversity of soil organisms
dramatically declined. Their habitat and food were gone. The soil
remained exposed to the sun and impact of rain, further limiting the
potential for restoration. With declining organic matter and soil
biology, and with continued bad practices of fire, tillage, and
exposure, the ground became increasingly compacted. Anaerobic conditions
developed, resulting in further soil acidification and toxic compounds
being produced. With the biology largely missing, the soil became
defined by the leftover mineral composition of the soil‹low CEC, low pH,
low water-holding capacity, low fertility, etc. Chemical inputs now
become the norm and a devastating cycle of dependence develops. Hope is
described as the next fertilizer or lime subsidy.

If we only knew that life below the surface is what supports life above
the surface, many would find that in a short time, damaged lands can be
restored to their productive potential without expensive inputs. Land
care practice would change to be truly that, care for the land,
patterned after the marvelous and elaborate design in the meadows and
prairies and forests, that causes them to flourish.


The measure of a healthy soil should include the presence of organic
matter and of a full supporting cast of bacteria, fungi, protozoa,
beneficial nematodes, worms and arthropods. Organic matter is the food.
Soil biology is the life that makes it happen. The remedy for so many
damaged agricultural lands, especially in the tropics where solar
radiation is intense throughout the year, is to keep the soil covered,
minimize tillage, practice rotation, maximize organic matter and
reintroduce needed soil biology to bring breath and life back into the


African Conservation Tillage Network‹ACT Information Series No. 1-9.
These short publications are extremely welldone and I highly recommend
those working with farmers to take the time to read this material.
Available on-line (English). <http://www.act.org.zw/infoseries.html>

Coder, K. D. 2000. Soil Compaction & Trees: Causes, Symptoms & Effects,
University of Georgia. I found this publication very helpful in
explaining the finer points of soil compaction and how serious a problem
it is. Available on-line (English).

Conservation agriculture: Case studies in Latin America and Africa, FAO
Soil Bulletin 78. Lots of helpful case studies with an incredible
appendix about the soil foodweb. Available online (English).
<http://www.fao.org/DOCREP/003/Y1730E/y1730e00.htm#P- 1_0>

Cooperband, L. R. 2000. Composting: Art and Science of Organic Waste
Conversion to a Valuable Soil Resource. Laboratory Medicine. Vol.
31:283-290. This is a good general guide about composting available
on-line (English). <http://www.region18.com/composting.htm>

Guerena, M. 2006. Nematodes: Alternative Controls.
http://attra.ncat.org/attra-pub/nematode.html. This particular article
(in English) was helpful for the portion on nematodes. ATTRA is an
incredible source for information related to sustainable agriculture.
This has been one of the most useful websites for help in managing
ECHO's farm. <http://attra.ncat.org/> (website includes information in
English and Spanish)

Haynes, R.J., Mokolobate M.S. 2001. "Amelioration of Al toxicity and P
deficiency in acid soils by additions of organic residues: a critical
review of the phenomenon and the mechanisms involved." Nutrient Cycling
in Agroecosystems. I included this because I used it as a reference for
the portion on soil acidification. Though I only had access to the
abstract, I still found this brief summary very helpful. Available
on-line (English). <http://www.springerlink.com/content/g52v9p31n6728582/>

The Importance of Soil Organic Matter: Key to Drought Resistant Soil and
Sustained Food Production, FAO Soil Bulletin 80. This is one of FAO's
conservation farming publications. It is not as detailed as I hoped, but
still contains useful information emphasizing SOM from a reputable
source. Available on-line (English).

Ingham, E. The Soil Foodweb Approach. This is the official Soil Foodweb,
Inc. website (English). It is a good place to start for information and
see when future courses are taught.

Ingham, E. The Overstory #81, The Soil Food web: Its Role in Ecosystem
Health. This is a concise summary of the soil food web approach.
Available on-line (English).

Ingham, E. 2002. An Introduction to the Soil Foodweb, CDs 1 and 2.
Excellent audio recording of Elaine teaching, available through

Ingham, E. 2001. The Compost Foodweb. CDs 1 and 2. Excellent audio
recording of Elaine teaching, available through web-site.

Soil and Water Conservation Society (SWCS). 2000. Soil Biology Primer.
Rev. ed. Ankeny, Iowa: Soil and Water Conservation Society. This short
book can be purchased or read on-line. It explains well the different
roles and functions of the major soil organism groups. Extremely well
done USDA publication (in English).


This article was excerpted with the kind permission of the author and
publisher from:

Blank, Danny. 2007. A Fresh Look at Life below the Surface. ECHO
Development Notes, Issue 96.


Danny Blank has worked with ECHO (Educational Concerns for Hunger
Organization) since 1994. ECHO is a non-profit, inter-denominational
Christian organization located on a demonstration farm in North Fort
Myers, Florida, USA. ECHO has been assisting a global network of
missionaries and development workers since 1981 and is currently serving
agricultural workers in 180 countries. Danny is in charge of the ECHO
farm, directing all planting and planning. He can be contacted at:

17391 Durrance Road
North Fort Myers, Florida 33917
Phone: 239-543-3246; Fax: 239-543-5317
Web site: <http://www.echonet.org>
E-mail: <dblank at echonet.org>


Soil Foodweb offers an extensive soils bibliography, publications, soils
testing, calendar of lectures, and subscriptions to a very informative
monthly newsletter with articles by Elaine Ingham:

USDA Natural Resources Conservation Service's "Soil Biology Primer":

Appropriate Technology Transfer for Rural Areas (ATTRA) Agronomy Resource
List for Alternative Soil Testing Laboratories:

Fred Moor's soils page has extensive links on soil life:

The US Department of Agriculture Natural Resources Conservation Service
(USDA NRCS) offers several publications on soil biology:

Cornell University Worldwide Portal to Information on Soil Health
has an excellent list of links:

Management of Organic Inputs in Soils of the Tropics (MOIST)
covers green manures and cover crop research and exchange. This
site also maintains the high quality Mulch-L discussion forum on
green manures/cover crops:

Centro Internacional de Información Sobre Cultivos de Cobertura
(CIDICCO) has green manure/cover crop information for small
farmers: <http://www.cidicco.hn/>


The Overstory #155--Mycoforestry
The Overstory #131--Microsymbionts
The Overstory #119--Five Fertility Principles
The Overstory #102--Mycorrhizas
The Overstory #86--Role of Mushrooms in Nature
The Overstory #81--Soil Food Web
The Overstory #78--Reforestation of Degraded Lands
The Overstory #70--Rhizosphere
The Overstory #61--Effects of Trees on Soils
The Overstory #42--Improved Fallow


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