Overstory #208 - A fresh look at life below the surface
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.
Soil foodweb concept
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.
Soil foodweb functions
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.
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.
Implications for reforesting denuded hillsides
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 soil.
Selected references and recommended publications
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.
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). http://warnell.forestry.uga.edu/service/library/for00-003/index.html
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.
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.
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). http://www.agroforestry.net/overstory/overstory81.html
Ingham, E. 2002. An Introduction to the Soil Foodweb, CDs 1 and 2. Excellent audio recording of Elaine teaching, available through web-site.
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). http://soils.usda.gov/sqi/concepts/soil_biology/biology.html
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.
About the author
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:
Related editions to The Overstory
- 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