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Soil is full of life. It is often said that a handful of soil has more living organisms than there are people on planet Earth. Soils are the stomach of the earth, consuming, digesting, and cycling nutrients and organisms.
On first observation, however, soil may appear as a rather inert material on which we walk, build roads, construct buildings, and grow plants. On closer observation, we observe that soil is teeming with living organisms. Living organisms present in soil include archaea, bacteria, actinomycetes, fungi, algae, protozoa, and a wide variety of larger soil fauna, including springtails, mites, nematodes, earthworms, ants, insects that spend all or part of their life underground, and larger organisms such as burrowing rodents. All of these are important in making up the environment we call soil and in bringing about numerous transformations that are vitally important to life.
The links between soil organisms and how they impact soil chemical and physical properties is complex. Soil biologists study a variety of things.
Only 5% of what is produced by green plants is consumed by animals, but the 95% is consumed by microorganisms. One gram of fertile soil can contain up to one billion bacteria. There are many different types of bacteria, and most of them have not even been discovered yet! Most of these bacteria are aerobic, meaing that they require oxygen from the soil atmosphere. However, other bacteria need to live without oxygen, and other types can live both with, and without oxygen. The growth of these bacteria is limited by the food that is available in the soil.
Soil fungi are also large component of the soil that come in various sizes, shapes, and colors. Mushrooms have underground roots (mycelium) that absorbs nutrients and water until they are ready to flower in the mushroom form. They tolerate acidity, which makes them very important to decompose materials in very acidic forests, that microbes cannot do, they can also decompose lignin, which is the woody tissues for decomposing plants.
Soil animals are consumers and decomposers because they feed on organic matter and decomposition occurs in the digestive tract. Some animals feed on roots, and others feed on each other. There are several types of worms. Earthworms are the easiest to identify. They eat plant material and organic matter, and excrete worm castings in the soil as food for other organisms. They also leave channels that they burrow in, which increases infiltration. Earthworms can weigh between 100-1,000 pounds per acre! There are also microscopic worms called nematodes, or roundworms. These worms live in the water around soil particles. There are several different types of nematodes, some of them eat dead materials, others eat living roots, and some eat other living organisms. Some nematodes are bad, and can cause severe root damage or deformation.
Aside from worms, another large body of insects are arthoropods that have exoskeletons and jointed legs. These include mites, millipedes, centipedes, springtails, and grubs.
Nurtient Cycling is the exchange of nutrients between the living and nonliving parts of the ecosystem. Soil biologists measure how plants and microbes absorb nutrients, and incorporate them into organic matter, which is the basis for the carbon cycle. There are two main processes. Immobilization is when soil organisms take up mineral nutrients from the soil and transform them into microbial and plant tissues. The opposite process is mineralization, which is what happens when organism die and release nutrients from their tissues. This process is rapidly changing, and very important in providing nutrients for plants to grow. The carbon cycle and nitrogen cycle are both very important to soil microbiologists.
Plant roots leak a lot of organic substances into the soil from dead materials. These provide food for the microorganisms, and create zones of activity around the root called the rhizosphere. In this zone, plant growth or toxic substances can be produced, but most of these organisms are benefical.
Other scientists study soil diseases of plants and animals found in the soil. Bacteria and fungi can cause plants to wilt or rot. The Great Potato Famine in Ireland in 1845 was caused by a fungus that caused the potato blight! These organisms don't just impact plants. Humans can get sick if certain types of bacteria, like E-Coli, are present in our waste, and that waste isn't treated properly.
Some fungi "infect" plant roots, but the relationship is symbiotic, meaning that it is beneficial to both the plant and the root. These are called mycorrhiza, and they help plants absorb more water and nutrients, increase drought resistance, and reduce infection by diseases.
Another symbiotic relationship involves nitrogen. There is a lot of nitrogen in the atmosphere, but it is not easy for plants to get. There are certain species of bacteria that absorb nitrogen gas from the atmosphere, and form a nodule. These are called nitrogen fixing bacteria. When the die, the nitrogen that they used are released for plants.