Soil Health – The Value and Types of Soil Bacteria
ROLE OF SOIL BACTERIA
Microbes in the soil is the key to the processing of carbon and nitrogen. A teaspoon of fertile soil can contain from 100 million and up to 1 billion bacteria. Bacteria are tiny single-celled organisms with a width of about 0.2-2.0 microns (on average – 1 micron) and a length of about 1-10 microns. The size of the bacteria can be compared with clay particles (<2 microns) and sludge particles (2-50 microns). They grow and live in thin water films around soil particles and around the roots of plants, in an area called the rhizosphere. The small size of the bacteria allows them to grow and adapt to changing environmental conditions faster than larger and more complex microorganisms. Most soils are a kind of cemetery for dead bacteria. Since most bacteria live under constant starvation or water stress, they have learned to quickly adapt to environmental conditions and instantly reproduce when water and food are abundant. A population of bacteria can easily be doubled in 30 minutes. Bacteria are so simple in structure, sometimes they are called `bag of enzymes`. CLASSIFICATION OF BACTERIA BACTERIA ARE MAINLY DIVIDED INTO TYPES
To simplify, bacteria can be grouped into the following groups:
a) Bacteria depending on their form
Before the advent of DNA sequencing, bacteria were classified on the basis of their forms and biochemical properties. Most bacteria belong to three main forms: the rod (the core bacteria are called bacilli), the sphere (the spherical bacteria are called cocci) and the spiral (the spiral bacteria are called spirilla). There are also thin branching threads called actinomycetes. Some bacteria belong to different forms that are more complex than the above forms.
b) Aerobic and anaerobic bacteria
Bacteria that need oxygen to survive are called aerobic bacteria. Bacteria that do not require oxygen to survive are called anaerobic bacteria. Anaerobic bacteria can die if they are in an oxidized environment.
c) Gram-positive and gram-negative bacteria
The distribution of bacteria on gram-positive and gram-negative is based on the results of the Gram method. Gram-negative bacteria are the smallest and tend to be more sensitive to water stress, while gram-positive bacteria are larger in size, have a thicker cell wall, a negative charge on the outer surface, and tend to withstand water stress.
d) autotrophic and heterotrophic bacteria
This is one of the most important types of classification, it takes into account the most important aspect of the growth of bacteria and their reproduction. Autotrophic bacteria (also known as autotrophs) get the carbon they need from carbon dioxide. Some autotrophs directly use sunlight to produce sugars from carbon dioxide, while for others it depends on various chemical reactions. Heterotrophic bacteria obtain carbohydrates and / or sugars from the environment in which they are located.
e) Classification is based on fila
On the basis of morphology, DNA sequencing, necessary conditions and biochemistry, scientists have classified bacteria into 12 phyla. Each fila corresponds to the number of species and genera of bacteria. This classification includes bacteria that can be found in different types of media, for example
1. Bacteria that can survive in extreme temperatures (extreme cold and heat)
2. Bacteria that can survive in various environments (highly acidic and strongly alkaline environments)
3. Aerobic bacteria compared with anaerobic bacteria
4. Autotrophic bacteria compared to heterotrophic bacteria, etc.
FUNCTIONS OF SOIL BACTERIA
Bacteria perform important functions in the soil, decomposing organic residues from enzymes, is secreted in the soil. There are four main functional groups of soil bacteria:
1. Decomposers (destructors) – bacteria that consume simple sugars and carbon compounds, such as root excretions and fresh plant residues.
2. Mutualists – bacteria that form partnerships with plants; Example: rhizobia – nitrogen-fixing bacteria.
3. Lithotrophs (chemoautotrophs) – bacteria that receive energy from nitrogen, sulfur, iron or hydrogen compounds, and not from carbonaceous compounds.
4. Bacteria can also be plant pathogens.
Bacteria in the soil convert the energy of organic matter into forms that are beneficial to other organisms. A number of bacteria destructors (Decomposers) can destroy pesticide residues and some other pollutants in the soil. These bacteria are especially important for immobilizing or preserving nutrients, thereby preventing the loss of nutrients, such as nitrogen, from the root zone.
Bacteria of all four groups perform important functions related to the dynamics of water, the cycle of nutrients and disease resistance. Some bacteria produce substances that help to bind soil particles into microaggregates (2–200 µm). Stable aggregates improve water infiltration and increase the water holding capacity of the soil. Also, bacterial populations compete with pathogens in the roots and on the surface of plants.
Nitrogen-fixing bacteria (rhizobia) form symbiotic associations with the roots of legumes. Rhizobia is a Gram-negative bacteria. Nodules visible to the eye are created in places where bacteria infect the growing root of the plant. The plant supplies simple sugars to the bacteria, and the bacteria convert atmospheric nitrogen from air into the nitrate and ammonium forms that the plant can use. When the leaves or roots of a plant decompose, the amount of nitrogen in the soil increases. Anaerobic conditions are necessary for bacteria to fix atmospheric nitrogen.
Nitrifying bacteria first convert ammonium into nitrite, and then nitrate, which is the best form of nitrogen for most row crops. Nitrifying bacteria need soils with excessive aeration. Nitrate is easily leached from the soil, so some farmers use nitrification inhibitors to reduce the activity of nitrifying bacteria. Denitrifying bacteria convert nitrates to atmospheric nitrogen or nitrous oxide. Denitrifiers are anaerobic bacteria, that is, they are active in the absence of oxygen, for example, in compacted soils or inside soil microaggregates. In heavy clay soils up to 40-60% of nitrogen may be lost during denitrification.
Although there are many bacteria in the soil, only a small specialized group of nitrogen-fixing bacteria can fix atmospheric nitrogen. Nitrogen fixation cannot occur without the participation of specific nitrogenase enzymes of specific bacteria. Nitrogen-fixing bacteria are present in most soil types (as symbiotic species), however, they usually constitute a very small percentage of the total number of microorganism populations and have a low nitrogen fixation capacity.
Sulfur, like many other nutrients, is transformed in the soil in the same way as nitrogen. Special bacteria in anaerobic conditions make sulfur less accessible to plants by converting sulfur to hydrogen sulfide in water-saturated soils, depositing sulfur from the soil in the form of insoluble sulfides of various metals. In well-aerated conditions, bacteria convert sulfur from metal sulphides to the sulphate form through intermediate stages to form elemental sulfur and thiosulphate.
Actinomycetes is a large group of bacteria that grow like fungi and functionally similar to mushrooms. Actinomycetes are smaller in size (1-2 microns) than fungi (10-50 microns) and are quite sensitive to antibacterial agents. When farmers plow the soil, it is the actinomycetes responsible for the specific “earthy” smell, the source of which is streptomycin. A number of antibiotics are made from actinomycetes, including streptomycin. Actinomycetes decompose a lot of substances and are more active at high pH values. Actinomycetes are particularly important in the degradation of difficult to decompose compounds such as chitin, lignin, keratin, fungal cellulose, and animal polymers. At low pH, mushrooms are more active in the degradation of such compounds. Actinomycetes are important in the formation of stable humus, which increases the structure of the soil, improves the supply of nutrients in the soil and improves the quality of the soil to retain water.
ADVANTAGES OF SOIL BACTERIA
Different types of bacteria thrive on different food sources and in different microenvironment. In general, bacteria are more competitive when easily digestible (labile) substrates (simple sugars) are present in the rhizosphere. These are fresh plant residues and compounds that are located next to living roots. Bacteria (especially rod and gram-negative bacteria) and actinomycetes are concentrated in the rhizosphere around the roots. Actinomycetes can make up from 10 to 30% of the total volume of microorganisms in the rhizosphere of the soil, depending on the nutritional availability. Some plants produce certain types of root excretions to stimulate the growth of protective bacteria.
Many bacteria produce a layer of polysaccharides and glycoproteins that cover the cell surface. Some form a mucous layer, while others form a thick gel-like capsule, which reduces the loss of water from the bacterial cell. These substances play an important role in the cementing of sand, silt and clay soil particles into stable microaggregates that improve the structure of the soil.
In order for bacteria to survive in the soil, they must adapt to different microenvironment. Oxygen concentrations in soil can vary widely. Large pores filled with air, provide a high level of oxygen, promotes the formation of aerobic conditions. At the same time, small micropores can be an anaerobic environment where there is not enough oxygen. This diversity in soil microenvironment allows bacteria to thrive at different levels of soil moisture and oxygen content, since even after flooding (soil saturation, lack of oxygen) or soil treatment (oxygen diffusion) there are small microenvironment where different types of bacteria and microorganisms can exist.
As the natural continuity occurs in the plant community, the continuity also occurs in the soil. Bacteria have the ability to change the soil environment in favor of certain plant communities. On fresh sediments, photosynthetic bacteria capable of fixing atmospheric nitrogen and carbon produce organic substances and other nutrients in order to initiate nutrient cycling processes in young soil. Bacteria dominate tilled or destroyed soils, soils with a high pH value and soils with high availability of nitrate nitrogen, which is an ideal place for weeds.
As the soil breaks down less and the diversity of plants increases, the ground food becomes more balanced and diverse, which makes the nutrients of the soil more accessible to higher plants. Various microbial populations and fungi, protozoa and nematodes support the utilization of nutrients and disease-causing organisms under control.