(College Level)

by Mrs. Eilisha Joy Bryson

    The soil is a complex system of organic and inorganic matter. It consists of various layers of this material, each varying in the amount of solid, gases, liquids, and organic matter. A general analysis of soil shows that about 40% are rocks and minerals, 25% gases, 25% liquid, and 10% organic matter. This combination creates the soil environment, which has several characteristics that make it worthwhile to study. It provides a place for plants to secure themselves. Soil acts as a filter that helps purify the water, and it is the place where many nutrients are recycled. Finally the topic of this paper, soil is the habitat for thousands of different organisms around the world.
    Soil Organisms are generally grouped into two categories: micro fauna/microorganisms and macrofauna. The main soil microorganisms include bacteria, fungi, and protozoa. The macro fauna include oligochaeta, arthropods, mollusks, and nematods. There are many other organisms that spend some time in the soil, but usually just for reproduction or feeding, and are not included in this paper. The organisms mentioned above play an essential role in soil formation and the soil environment. The larger organisms tend to depend more on the microorganisms that are part of the lower  trophic levels.  Therefore microorganisms will be discussed first to provide background information.

    Perhaps the most important microorganisms in the soil ecosystem are bacteria. They are single-celled organisms that are responsible for many important processes. One thing they do is decompose plant material as well as other organisms' waste. These waste materials contain nutrients, and the bacteria's cells are able to convert them from an unusable form into a form that can be used. One essential nutrient that is converted by bacteria is nitrogen. Nitrogen-fixing bacteria are able to use the nitrogen that exists in the atmosphere (N2) and change it through cellular processes into ammonia (NH3). Nitrifying bacteria mix ammonia (NH3) with water (H2O) to create ammonium ion (NH4+) and nitrate (NO3-), which can be used by other organisms. Protozoa are also present in the soil environment and contribute to distributing ammonium into the soil for use by plants and other organisms. They feed on bacteria but do not need as much nitrogen as the bacteria provide, and therefore release the excess ammonium. Plants need this to grow, so much so, that humans began putting more nitrogen into fertilizers through an industrial process created by Fritz Haber and Carl Bosch. Denitrifying bacteria are responsible for putting the nitrogen back into the atmosphere as N2.

    Bacteria are also responsible for decomposing other compounds as well.  One phylum of bacteria called
actinobacteria or actinomycetes are responsible for decomposing chitin, cellulose, and other difficult materials in dead plants, animals and fungi.
Other organisms cannot disintegrate these materials because of their tough structure. The bacteria are able to produce an enzyme allowing them to reduce the chitin, for example, from a polymer into simple sugars and ammonia
that other organisms can then use. Actinomycetes are a type of bacteria that used to be considered as fungi. They were thought to be so because they grow hyphae like fungi, and they can live in harsh environments in which other bacteria usually do not survive, like higher pH levels and dryer conditions. They since were reclassified and renamed actinobacteria.  These bacteria give the soil its earthy scent, which actually indicates a healthy environment. Bacteria such as this help to maintain healthy soil environments for plants, allowing them to get the nitrogen they need, and decomposing organic waste.
    Fungi serve in a similar capacity in the soil environment. They tend to be more abundant in places where the bacteria cannot thrive, such as environments that are acidic, have low amounts of nitrogen, have low moisture, and high complex carbohydrates (bacteria cannot break them down). Fungi fill the niche of decomposing tough organic matter such as chitin, cellulose, keratin and lignins. Again, most organisms cannot digest these materials, but fungi, like actinobacteria, can produce enzymes that allow them to degrade those strong compounds into sugars that can be digested.  Not only are they decomposers, but fungi also are responsible for supplying nutrients to other organisms, in this case plants. Fungi form a mutualistic relationship with plants called mycorriza, which allows both parties to benefit. The fungi grow on or within (endopyhtic) the roots of the plant. Because of the hyphae's high surface area to volume ratio, fungi are able to better access essential nutrients and deliver them to the plants. The plants, in turn, give the fungi food in the form of glucose.

    Already, the importance of these microorganisms is clear, but in addition to them converting nutrients into usable forms, they also are the food source for many of the macro fauna in the soil. The macro fauna depend on the nutrients that the microbes produce.
    The most beneficial, and probably most recognizable macro fauna in soil is the earthworm. Oligochaeta, a subclass of annelids, or segmented worms, contribute to the texture of soil as we know it. The earthworms burrow throughout the soil creating tunnels and shafts. These openings loosen the soil structure providing spaces where other organisms can travel and gain access to other areas. Water also fills these spaces, adding to the water absorption of the soil, increasing moisture, which many organisms need, including earthworms.  When water gets absorbed into the ground there is less runoff on the surface. While the worms burrow, they intake soil and dead plant material. They digest these nutrient-rich materials and produce waste called castings. The castings contain all of the nutrients that were present in the organic material: nitrogen, potassium, calcium, phosphorus, and magnesium, and contain microorganisms, such as bacteria and fungi.  The protozoa and nematodes feed on the microorganisms within the castings and release the nutrients within them. The worm castings are considered to be fertilizers because of their positive effect on the soil: they help to balance the pH levels, retain moisture, improve drainage, and control pathogens. Plant nurseries harvest and sell worm castings for this very reason.

    Other macro fauna include nematodes, mollusks, and arthropods. These include a wide variety of animals that help to maintain a balance in the food web. Some of these animals consume live or dead plant material, and some are predators of the microorganisms mentioned above. The larger bugs gain and release some of the nutrients from bacteria when they feed on them. In addition to this, macro fauna help maintain natural population sizes of their prey. One very interesting relationship is between macro fauna and bacteria. As the bacteria are eaten, the more their population grows. This is a negative feedback. This is clearly supported in the relationship between the arthropods and fungi in the following situation. Leaf cutter ants harvest basidiomycete fungi. They grow the fungi, feeding it leaves, providing it with a desirable living condition and protection from other organisms. In tern, the ants eat the fungi. The obvious question begs to be asked, wouldn't the fungi die off if it is constantly being eaten? In actuality, when it is eaten, it is more stimulated to grow, and it does just that. As long as it is fed (the leaves provided from the ants) it will continue to grow, and be satisfied living in this situation. This is the same relationship with bacteria and their predators.

    Soil organisms are not always beneficial. organisms that pose a threat to plants and microorganisms that aid the plants are viewed as pests. Fungi and bacteria have been known for posing many problems to soil organisms. They can form parasitic relationships with plants, attacking young or damaged roots and inhibiting their growth. The enzymes that enable these microorganisms to digest tough plant materials are the same enzymes, such as cellulase, that allow the bacteria and fungi to penetrate a plant's roots in a pathogenic relationship. Macro fauna are not faultless. Nematodes can also be parasites, feeding on plants and animals, including humans. They spread diseases and viruses amongst soil organisms, posing many problems for farmers and gardeners. Larger nematodes are often introduced as predators to smaller nematodes to control their population. An arthropod called symphyla, or wire worms, are pests in greenhouses and gardens. They also feed on plants and microorganisms. Mites pose similar concerns.

    As is described above, the organisms in the soil contribute greatly to the wellness of the plants that grow from the soil. They are dependent on each other and each has a niche to fill to ensure that the environment stays well-balanced. Organisms do not always behave in a helpful manner, but pesty organisms occur naturally in an ecosystem.  Each type of organism in the soil plays a role in keeping the soil an ecosystem that is diverse and worthy of our attention.


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Last Updadted: August 21, 2006