Soil bacteria are the primary digestive system of the soil. Their activity is responsible for almost 90% of all biological and chemical actions. For instance, key macronutrients such as nitrogen, sulphur and phosphorus all require microbial transformation in the root zone to make them more available to the plant. Soil bacteria transform nutrients from "forms not usable by the plant" to "forms usable by the plant". Again, soil microbes improve aeration by loosening dense and compacted soils. Water is then able to better infiltrate and percolate. Most important, soil bacteria decompose organic waste materials such as leaves and manure into organic humus, which stores both moisture and nutrients. Further, microbes can balance soil acidity and alkalinity, create the carbon dioxide plants need, as well as produce vitamins, toxins, and hormones that both feed and protect the plant system.
Soil micro-organisms are living, breathing organisms and, therefore, need to eat. They compete with plants for nutrients including Nitrogen, Phosphorus, Potassium and micronutrients as well. They also consume amino acids, vitamins, and other soil compounds. Their nutrients are primarily derived from the organic matter they feed upon. The benefit is that they also give back or perform other functions that benefit higher plant life. Bacteria are able to perform an extremely wide range of chemical transformations, including degradation of organic matter, disease suppression, disease, and nutrient transformations inside roots (e.g. reducing bacteria in roots, bacteria cause nitrogen fixation). Azobacter, for example, is a genus of free-living bacteria that converts atmospheric nitrogen into ammonium, making it available for plant use. This process may only take place, however, if the following conditions are met: An easily degradable carbon source is available. Any nitrogen compounds such as ammonium or nitrate, are not already in present in substantial concentrations. Soil pH levels are between 6 and 9. High levels of phosphorus are present. Very low levels of oxygen are present. Azobacter is inhibited by a large range of toxic mineral and organic compounds, but may tolerate relatively high salinity and their activities are enhanced in the presence of clays.
In general, bacteria are the organisms in soil that are mainly responsible for transforming inorganic constituents from one chemical form to another. Their system of external digestion means that some of the metabolites released by the use of extracellular enzymes may be used by other organisms, such as plants. The bacteria gain nutrients and energy from these processes and provide other organisms with suitable forms of chemicals they require for their own processes, for example, in the conversions of nitrate to nitrite, sulphate to sulphide and ammonium to nitrite.
Bacteria are aquatic organisms that live in the water- filled pore spaces within and between soil aggregates. As such, their activities are directly dependent on relatively high soil water contents. Bacteria are normally found on the surfaces of mineral or organic particles or congregate around particles of decaying plant and animal debris. Most are unable to move and hence, their dispersion is dependent on water movement, root growth or the activity of soil and other organisms.
Rhizobia are one of the groups of micro-organisms living in soil. They are single celled bacteria, approximately one thousandth of a millimetre in length. Rhizobia belong to a family of bacteria called Rhizobiaceae. There are a number of groups (genera and species) of bacteria in this family. Rhizobia belong to a specific group of bacteria that form a mutually beneficial association, or symbiosis, with legume plants. These bacteria take nitrogen from the air (which plants cannot use) and convert it into a form of nitrogen called ammonium (NH4+), which plants can use. The nitrogenase enzyme controls the process, called nitrogen fixation, and these bacteria are often called "nitrogen fixers". Rhizobia are found in soils of many natural ecosystems. They may also be present in agricultural areas where they are associated with both crop legumes (like soybean) and pasture legumes (like clover). Usually, the rhizobia in agricultural areas have been introduced at sowing by applying an innoculum to the exterior of the seeds as liquid formations or pellets.
The nodulation process is a series of events in which rhizobia interact with the roots of legume plants to form a specialised structure called a root nodule. These are visible, ball-like structures that are formed by the plant in response to the presence of the bacteria. The process involves complicated signals between the bacteria and the host roots. In the first stages, the bacteria multiply near the root and then adhere to it. The small hairs on the root's surface curl around the bacteria and they enter the root. Alternatively, the bacteria may enter directly through points on the root surface. The method of entry of the bacteria into the root depends on the type of plant. Once inside the root, the bacteria multiply within thin threads. Signals stimulate cell multiplication of both the plant's cells and the bacteria and this repeated division results in a mass of root cells containing many bacterial cells. Some of these bacteria then change into a form that is able to convert gaseous nitrogen into ammonium nitrogen (that is, they can "fix" nitrogen). These bacteria are then called bacteroids. The shape of the nodules is controlled by the plant and nodules can vary considerably - both in size and shape. Most plants need specific kinds of rhizobia to form nodules. The rhizobia that form nodules on peas, for example, cannot form nodules on clover. Nodulation can be impeded by low pH, Al toxicity, nutrient deficiencies, salinity, water logging, and the presence of root parasites such as nematodes or genetic incompatibility with the plant host.
Soil micro-organisms, sometimes spelled as soil micro-organisms, are a very important element of healthy soil. Knowing what microbes in soil eat, the conditions they thrive in and the temperatures that they are most active in is important in organic gardening and organic lawn care. From a practical standpoint, it boils down to organic matter, but not just any organic matter. These facts below will help you plan your activities around the time they are most beneficial. Below is a partial list of important functions they perform.
Transforming raw elements from one chemical form to another. Important nutrients in the soil are released by microbial activity are Nitrogen, Phosphorus, Sulphur, Iron and others. Breaking down soil organic matter into a form useful to plants. This increases soil fertility by making nutrients available and raising CEC levels. Degradation of pesticides and other chemicals found in the soil. Suppression of pathogenic micro-organisms that cause diseases. The pathogens themselves are part of this group, but are highly outnumbered by beneficial microbes.
There are several types of micro-organisms in soil that benefit plants. Together they make up an immense population of living organisms. One teaspoon of soil may contain millions of various types. Below is a list of common soil micro-organisms found throughout the world. Bacteria These are small, single cell organisms that make up the single most abundant type of microbe. They have a very wide range of conditions that they live in from the Arctic wastelands to the steaming waters of volcanic hot springs. In soils, they multiply rapidly under the proper conditions. When conditions are wrong for one species, it is right for another. This is not always a good thing since a balance is what is required.
Fungi The largest microbe group in terms of mass. Some fungi are beneficial, called mycorrhiza, that form a symbiotic relationship with plant roots, either externally or internally. Within the fungi group are pathogen fungi. These are disease causing fungi, some of which can be quite devastating to plants. Protozoas Small single cell microbes that feed on bacteria. Actinomycetes Necessary for the breakdown of certain components in organic matter. Algae Beneficial groups such as blue-green algae, yellow-green algae and diatoms. Some of these can produce their own energy through photosynthesis.
It is a variety of natural substances including decomposed leaves, grass clipping, shed roots, wood chips, etc. Humus (well decomposed organic matter) is the richest source for plant growth. Organic matter comes in many different nutrient levels, especially Nitrogen. While soil microbes need carbon (C) to live, they also need the nitrogen contained in organic matter. Therefore, the Carbon to Nitrogen ratio (C:N) is very important.
Fungi are primarily organisms that cannot synthesise their own food and are dependent on complex organic substances for carbon. Specialized fungi can be pathogenic on the tissues of plants, while others form mutually beneficial relationships with plants and assist in direct nutrient supply to the plants (e.g. mycorrhizal associations). Many fungi play a very important role in the recycling of important chemical elements that would otherwise remain locked up in dead plants and animals. In the decomposition of plant debris, certain fungi are particularly important because of their ability to derive their carbon and energy requirements from the break down of dead and decaying plant cell walls, cellulose and lignin. They are much less dependent on water than other micro-organisms, but interactions with other microbes, temperature and nutrient availability will have an effect on their activity. Fungal activity is greatest in decomposing leaves and wood, and tends to diminish in the later stages of decomposition when bacteria become more dominant.
Even though a high proportion of both fungi and bacteria are decomposers in the soil, they degrade plant residues differently and have different roles in the recycling of nutrients. This is partly due to their different choice of habitats within the soil and the different types of organic matter they consume. Fungi are generally much more efficient at assimilating and storing nutrients than bacteria. One reason for this higher carbon (C) storage by fungi lies in the chemical composition of their cell walls. They are composed of polymers of chitin and melanin, making them very resistant to degradation. Bacterial membranes, in comparison, are phospholipids, which are energy-rich. They degrade easily and quickly and function as a food source for a wide range of micro-organisms. The different proportions of C and N (i.e. different C:N ratios) of bacteria and fungi might also play a role in the mineralisation and immobilisation processes of nutrients in the soil. Due to their structure and C:N ratio between 7:1 and 25:1, fungi need a greater amount of carbon to grow and reproduce and will therefore 'collect' the required amount of carbon available for this from the soil organic matter. Bacteria, however, have a lower C:N ratio (between 5:1 and 7:1) and a higher nitrogen requirement and take more nitrogen from the soil for their own requirements.