Soil is biologically active — a home to countless microorganisms, invertebrates, and plant roots.
SOM also has important chemical properties that benefit soil. Cation exchange capacity is a measurement of the capacity of soil to store important plant nutrients such as N, K, Ca, and Mg and supply them to growing plants. Soil organic matter has a very high cation exchange capacity so just a small increase in the amount of SOM can have a big influence on the overall soil cation exchange capacity. Some fertilizers, acid rain, and other soil processes generate acidity in soil and tend to lower soil pH. Organic matter is able to buffer this acidity and help to keep soil pH from decreasing. When soil pH does decrease, the plant availability of Aluminum, Iron, and Manganese increases and these metals can become toxic to plants. Soil organic matter binds strongly to these metals and reduces their toxicity even in acidic soils.
The most important and abundant element in soil organic matter is Carbon, and carbon is energy. In soils, organic matter is food (energy) for the microbes that live there and give the soil life. The bacteria, fungi, actinomycetes and protozoa that live in soil are the organisms that decompose organic matter and also drive nutrient cycling. Humic and fulvic acids (part of SOM) have also been shown to stimulate root elongation and to increase the ability of plant roots to access both water and nutrients. These are just some of the benefits of soil organic matter. Now lets look a little more closely at what soil organic matter is and how it is formed.
Soil organic matter is all the material in soil that is made up of carbon (not including the carbon is in limestone or that is present as carbonates or bicarbonates). So, soil organic matter is all dead plant material, both above ground litter and roots, and all animal remains and excreta. Most scientists also include living soil microorganisms as part of the soil organic matter. These fresh organic materials have none of the properties of soil organic matter that we just mentioned. But over time they are decomposed by soil microbes and gradually turned into soil organic matter.
Photo credit: Collohmannia sp. Roy A. Norton, State University of New York at Syracuse File name: M8 Mites and Leaf
The process of degrading fresh organic materials added to the soil is a complex process that involves intricate interplay from many kinds of soil dwelling organisms. Decomposition of complex organic material like plant litter begins with mixing and shredding. Earthworms and other soil arthropods are very adept at this. Earthworms pull litter into their burrows and mix it with soil. Insects and other macro-arthropods feed on the litter pulling it apart into small pieces. Mixing the material into the soil brings it into contact with other soil degraders and greatly increases the surface area exposed to the degraders. In conventional tillage systems this mixing and shredding is done by the farmer. In no-till systems we rely on these critters to do the mixing and shredding.
When fresh organic material is mixed into the soil, bacteria respond almost immediately. They begin to feed on the simple organic compounds such as sugars, proteins, and amino acids. Bacterial numbers increase very rapidly in response to the food source. But the bacteria have a harder time with some of the more complex organic compounds in the litter, and these complex compounds sometime prevent the bacteria from getting at remaining material they could degrade. Its as if the food is locked in a cupboard.
So, lets bring on the fungi. Their populations increases more slowly than the bacteria, but they are able to degrade the complex compounds the bacteria could not get at. Things like hemicellulose, starch, and cellulose. The degrading work of the fungi helps to open up the locked cupboard and give other microbes access to the remaining simple compounds.
Another important function soil fungi is the strong influence they have on soil structure. Their hyphal strands help to hold soil aggregates together, and they also excrete organic substances that help cement the aggregates. This is demonstrated in these photos. On the left are soil aggregates in the presence of fungi. These soil aggregates are strong enough to hold up to being shaken in water. On the right is a soil that was similarly aggregated but without fungi. The structure could not stand up to being shaken in water.
Back to decomposition. The final degraders are the actinomycetes. They are the clean-up crew and come in at the final stages of decomposition. Like fungi they are able to degrade complex compounds like cellulose, lignin, and chitin.
No to be forgotten are the protists and nematodes. These are the predators, hunting around in the soil for the creatures that got fat from eating the plant litter. They feed on the bacteria and fungi and release nutrients into the soil.
The fungi use carbon from the plant to grow and make glomalin. In return, the fungi's hairlike filaments, called hyphae, extend the reach of plant roots. Hyphae function as pipes to funnel more water and nutrients—particularly phosphorus—to the plants.
Degradation of organic material involves in important balance between carbon and nitrogen in the material being degraded, in the degraders, and in the soil. When fresh litter is degraded, about 2/3 of the carbon is released as carbon dioxide, and about 1/3 goes into building new biomass. This cycle repeats over and over until the material is degraded to stable soil humus.
But not all soil organic matter is equal. It is a long, slow process to convert fresh residues or manure into stable soil humus, and scientists have discovered there are different categories of organic matter. The active pool of organic matter is the freshest organic material, plant and animal residues that have just begun to decompose. This is the pool that gives the largest release of nutrients, where decomposition is most rapid and the largest amount of carbon dioxide is released back into the atmosphere. Decomposition of this pool also has the greatest effects on soil structure formation and stabilization. Thus many of the benefits of SOM are from this active or fresh pool – nutrient supply, improved structure, improved water infiltration, decreased erosion, stimulated microbial activity. This is the most dynamic part of the soil organic matter, the pool that undergoes the greatest change and turnover. At the other end of the line is the largest pool of soil organic matter, often called the passive pool. This organic matter is very stable. It has gone through many cycles of decomposition and the molecules that are left here are so complex that microbes have a hard time biting into them or using them as food energy. Scientists have been able to determine that some of this organic matter has been in the soil for as long as 5000 years. The passive organic matter is largely responsible for the increased cation exchange capacity and water holding capacity in soil. In between the active and passive pools of organic matter is the so called slow organic matter. In reality there are not three distinct pools, but rather a continuum of organic materials that ranges from active to passive. The slow organic matter pool has properties that are intermediate between active and passive. It also contributes to cation exchange capacity and water holding capacity.
1. Soil is an Ecosystem Slides from: Richard Stehouwer Marcus Buchanan Elaine Ingham Department of Crop & Soil Sciences Extension Viticulturist Soil Food Web Inc. Penn State University Oregon State University
2. <ul><li>SOM Improves Soil Chemical Properties </li></ul><ul><ul><li>Increases Cation Exchange Capacity so the soil is better able to store and supply plant nutrients </li></ul></ul><ul><ul><li>Increases pH buffering so the soil resists changes in pH </li></ul></ul><ul><ul><li>Reduces Aluminum, Iron, and Manganese toxicity in acidic soil </li></ul></ul>Why should you care about soil organic matter (SOM)?
3. <ul><li>SOM Improves Soil Biological Properties </li></ul><ul><ul><li>Greater abundance, diversity and activity of soil microbes </li></ul></ul><ul><ul><li>Increased nutrient cycling </li></ul></ul><ul><ul><li>Increased root elongation and abundance </li></ul></ul><ul><ul><li>Increased access to water and nutrients </li></ul></ul>Why should you care about soil organic matter (SOM)?
4. What is Soil Organic Matter? <ul><li>All material in soil that contains (reduced/organic) carbon </li></ul><ul><li>SOM is derived from </li></ul><ul><ul><li>Plant residue (both litter and roots) </li></ul></ul><ul><ul><li>Animal remains and excreta </li></ul></ul><ul><ul><li>Living soil microbes (microbial biomass) </li></ul></ul><ul><li>With time microbes transform fresh organic material into stable soil organic matter </li></ul>Crop residue Bacteria Fungi Actinomycetes SOM
5. Components of Soil Organic Matter Decomposing OM 33% - 50% Stabilized OM (humus) 33% - 50% Particle residues <10% Living organisms <5%
6. Organic Matter Decomposition <ul><li>Earthworms </li></ul><ul><ul><li>Mix fresh organic materials into the soil </li></ul></ul><ul><ul><li>Brings organic matter into contact with soil microorganisms </li></ul></ul>Corn leaf pulled into nightcrawler burrow <ul><li>Soil insects and other arthropods </li></ul><ul><ul><li>Shred fresh organic material into much smaller particles </li></ul></ul><ul><ul><li>Allows soil microbes to access all parts of the organic residue </li></ul></ul>Millepede Ants
7. Organic Matter Decomposition <ul><li>Bacteria </li></ul><ul><ul><li>Population increases rapidly when organic matter is added to soil </li></ul></ul><ul><ul><li>Quickly degrade simple compounds - sugars, proteins, amino acids </li></ul></ul><ul><ul><li>Have a harder time degrading cellulose, lignin, starch </li></ul></ul><ul><ul><li>Cannot get at easily degradable molecules that are protected </li></ul></ul>Bacteria on fungal strands Spiral bacteria Rod bacteria
8. <ul><li>Fungi </li></ul><ul><ul><li>Grow more slowly and efficiently than bacteria when organic matter is added to soil </li></ul></ul><ul><ul><li>Able to degrade more complex organic molecules such as hemicellulose, starch, and cellulose. </li></ul></ul><ul><ul><li>Give other soil microorganisms access to simpler molecules that were protected by cellulose or other complex compounds. </li></ul></ul>Organic Matter Decomposition Soil fungus Fungus on poplar leaf
9. Fungi and Soil Structure <ul><li>Fungal hyphae (threads) help hold soil granules together </li></ul><ul><li>Fungal exudates (goo) help cement soil particles together </li></ul>Fungi absent - Soil structure is not maintained when immersed in water Active Fungi Present – Soil structure is maintained when immersed in water
10. <ul><li>Actinomycetes </li></ul><ul><ul><li>The cleanup crew </li></ul></ul><ul><ul><li>Become dominant in the final stages of decomposition </li></ul></ul><ul><ul><li>Attack the highly complex and decay resistant compounds </li></ul></ul><ul><ul><ul><li>Cellulose </li></ul></ul></ul><ul><ul><ul><li>Chitin (insect shells) </li></ul></ul></ul><ul><ul><ul><li>Lignin </li></ul></ul></ul><ul><ul><ul><li>Waxes </li></ul></ul></ul>Organic Matter Decomposition
11. <ul><li>Protists and nematodes, the predators </li></ul><ul><ul><li>Feed on the primary decomposers (bacteria, fungi, actinomycetes) </li></ul></ul><ul><ul><li>Release nutrients (nitrogen) contained in the bodies of the primary decomposers </li></ul></ul>Organic Matter Decomposition Amoeba Bacteria-feeding nematode Predatory nematode Rotifer
12. Mycorrhizae The fungi use carbon (energy) from the plant to grow. In return, the fungi's hairlike filaments, called hyphae, extend the reach of plant roots. Hyphae function as pipes to funnel more water and nutrients, particularly phosphorus, to the plant. Ecto - cover the root surface Endo - penetrate root
13. Organic Matter Decomposition Carbon and Nitrogen Cycling <ul><li>During each cycle of degradation about 2/3 of the organic carbon is used for energy and released as carbon dioxide (CO 2 ) </li></ul>Bacteria, Fungi Soil organic matter Nematodes, protists, humus CO 2 CO 2 Plant litter During each cycle of degradation about 1/3 of the organic carbon is used to build microbial cells or becomes part of the soil organic matter
14. <ul><li>Soil texture, pH, temperature, and moisture </li></ul><ul><li>The ‘quality’ of organic matter additions </li></ul><ul><li>C:N ratio and lignin content (L:N ratio) </li></ul><ul><li>Quantity of organic matter </li></ul><ul><li>Placement of materials (surface vs. incorporated) </li></ul><ul><li>Tillage </li></ul>Organic Matter Concepts Critical factors that control the rate of decay, nutrient release, and conservation of OM in soil
15. Organic Matter Quality Effects soil structure, N,P,S release, microbial populations, nutrient and water holding capacity
16. Soil Organic Matter Quality
17. All organic matter in soil is not equal <ul><li>Scientists describe 3 pools of soil organic matter </li></ul>Passive SOM 500 – 5000 yrs C/N ratio 7 – 10 Active SOM 1 – 2 yrs C/N ratio 15 – 30 Slow SOM 15 – 100 yrs C/N ratio 10 – 25 <ul><li>Recently deposited organic material </li></ul><ul><li>Rapid decomposition </li></ul><ul><li>10 – 20% of SOM </li></ul><ul><li>Intermediate age organic material </li></ul><ul><li>Slow decomposition </li></ul><ul><li>10 – 20% of SOM </li></ul><ul><li>Very stable organic material </li></ul><ul><li>Extremely slow decomposition </li></ul><ul><li>60 – 80% of SOM </li></ul>CO 2 Organic Material
18. Organic additions ( manure, compost, crop residues ) Below ground ( roots and poop ) Rapid Decay (Sugars, proteins) Resistant SOM Organic Residues ( leaf, shoots ) Decomposition (microbes) Soil Slow Decay (Cellulose, lignin) Slow SOM Active SOM Microbes Plants Decomposition (microbes) Humus Microbial Nutrients (N,P,S) Particulate Click in repetition to view the decay sequence