Most are not seen by the human eye. Others that are large enough to see are usually associated with the later breakdown stages. A succession of microbes and insects combine efforts to turn feedstocks, such as leaves, grass clippings, yard prunings and food waste, into the fully decomposed finished product known as compost. If you are composting for the first time, you may be surprised by the size and complexity of the community of small organisms that take up residence in your compost pile.
These organisms, which include many insects, bugs, slugs, bacteria, and fungi, form what is called a " food web. The macro-organisms you can see in or around your compost pile, such as mites, centipedes, sow bugs, snails, beetles, ants and earthworms, are physical decomposers; they grind, tear, and chew materials into smaller pieces.
However, micro-organisms such as bacteria, fungi, and actinomycetes--even though they go unnoticed in your compost pile--are responsible for most of the organic material breakdown. They are chemical decomposers because they use chemicals in their bodies to break down organic matter. The most abundant type of chemical decomposer in a compost pile is aerobic bacteria. When they break down organic material, they give off heat. Billions of aerobic bacteria working to decompose the organic matter in a compost pile causes the pile to warm up.
As the temperature rises, different organisms thrive. Psychrophilic bacteria are most active at around 55 o F. Mesophilic bacteria take over around 70 o F up to o F. When the compost pile temperature goes over o F, the heat loving thermophilic bacteria take over.
Thermophilic bacteria prefer a temperature between o F and o F. If the pile heats to more than o F, bacteria begin to die off and decomposition slows down.
This is why experts recommend turning your pile before the temperature exceeds o F. As thermophilic bacteria run out of food, the pile will cool and the makeup of the microbial community will shift back towards cooler-temperature bacteria. This is when fungi become more active in the compost pile. The soil zone located immediately around active roots is called the rhizosphere. This is an area of high microbial activity. Materials released from roots, called exudates, create a food-rich environment for the growth of microorganisms.
Rhizosphere microorganisms in turn help plants by fixing nitrogen from the soil air, dissolving soil minerals and decomposing organic matter, all of which allow roots to obtain essential nutrients. Some microbes have a specialized role in the rhizosphere. Rhizobia bacteria associate with the roots of legumes to form nodules. This symbiotic relationship provides the bacteria with a source of carbon in exchange for making nitrogen available to the plant.
Farmers are familiar with this process, and often encourage it by inoculating legume seeds with a commercial preparation of the Rhizobium species that is suited to the crop species they are planting. A special kind of fungus called mycorrhizae also associates with plants.
SOM includes plants, blue green algae, microorganisms bacteria, fungi, protozoa, nematodes, beetles, springtails, etc. Soil organic matter can be broken down into its component parts. One hundred grams g or pounds lbs of dead plant material yields about 60—80 g lbs of carbon dioxide, which is released into the atmosphere.
The remaining 20—40 g lbs of energy and nutrients is decomposed and turned into about 3—8 g lbs of microorganisms the living , 3—8 g lbs of non-humic compounds the dead , and 10—30 g lbs of humus the very dead matter, resistant to decomposition.
The molecular structure of SOM is mainly carbon and oxygen with some hydrogen and nitrogen and small amounts of phosphorus and sulfur. Soil organic matter is a by-product of the carbon and nitrogen cycles. SOM is composed of mostly carbon but associated with the carbon is high amounts of nitrogen and sulfur from proteins, phosphorus, and potassium.
SOM should be considered like an investment in a certificate of deposit CD. Soils that are biologically active and have higher amounts of active carbon recycle and release more nutrients for plant growth than soils that are biologically inactive and contain less active organic matter. Under no-till conditions, small amounts of nutrients are released annually like interest on a CD to provide nutrients slowly and efficiently to plant roots. However, with tillage, large amounts of nutrients can be released since the SOM is consumed and destroyed by the microbes.
Since SOM levels are slow to build, the storage capacity for nutrients is decreased and excess nutrients released are often leached to surface waters. SOM is a storehouse for many plant nutrients. Consider the following three scenarios. Soils typically turnover 1 to 3 percent of their nitrogen stored in SOM.
Tilled or unhealthy soils release a lower percent of nitrogen due to lower microbial activity. A soil that is more biologically active and has 4 percent SOM 4, lbs N may release 1. In tilled soils, excess nutrients released are often lost and the carbon stores are depleted so that future storage of nutrients is reduced. Farmers often see this occur when they till a virgin soil, an old pasture, or a fence row. For several years, crops on the newly tilled soil will grow better than the surrounding soils, but over time the soil will be depleted of carbon and the newly tilled soil will become less fertile because the carbon is oxidized as carbon dioxide and lost to the atmosphere.
Tillage results in the oxidation and destruction of carbon in the soil by increasing the soil oxygen levels, thereby promoting bacteria populations to expand and consume active carbon in the soil. SOM is affected by climate and temperature. Microbial populations double with every 10 degree Fahrenheit change in temperature. If we compare the tropics to colder arctic regions, we find most of the carbon is tied up in trees and vegetation above ground.
Moving north or south from the equator, SOM increases in the soil. Freezing temperatures change the soil so that more SOM is decomposed then in soils not subject to freezing. Moisture, pH, soil depth, and particle size affect SOM decomposition. Hot, humid regions store less organic carbon in the soil than dry, cold regions due to increased microbial decomposition.
The rate of SOM decomposition increases when the soil is exposed to cycles of drying and wetting compared to soils that are continuously wet or dry. Other factors being equal, soils that are neutral to slightly alkaline in pH decompose SOM quicker than acid soils; therefore, liming the soil enhances SOM decomposition and carbon dioxide evolution. Decomposition is also greatest near the soil surface where the highest concentration of plant residues occur.
At greater depths there is less SOM decomposition, which parallels a drop in organic carbon levels due to less plant residues. Small particle sizes are more readily degraded by soil microbes than large particles because the overall surface area is larger with small particles so that the microbes can attack the residue.
Bacteria are especially concentrated in the rhizosphere, the narrow region next to and in the root. There is evidence that plants produce certain types of root exudates to encourage the growth of protective bacteria.
Bacteria alter the soil environment to the extent that the soil environment will favor certain plant communities over others. Before plants can become established on fresh sediments, the bacterial community must establish first, starting with photosynthetic bacteria.
These fix atmospheric nitrogen and carbon, produce organic matter, and immobilize enough nitrogen and other nutrients to initiate nitrogen cycling processes in the young soil. Then, early successional plant species can grow. As the plant community is established, different types of organic matter enter the soil and change the type of food available to bacteria.
In turn, the altered bacterial community changes soil structure and the environment for plants. Some researchers think it may be possible to control the plant species in a place by managing the soil bacteria community. Certain strains of the soil bacteria Pseudomonas fluorescens have anti-fungal activity that inhibits some plant pathogens. They may produce a compound that inhibits the growth of pathogens or reduces invasion of the plant by a pathogen.
They may also produce compounds growth factors that directly increase plant growth. These plant growth-enhancing bacteria occur naturally in soils, but not always in high enough numbers to have a dramatic effect. In the future, farmers may be able to inoculate seeds with anti-fungal bacteria, such as P.
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