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Soil biota

Soil biota are living organisms that live in the soil and differ in their ecological functions and taxonomic position.

Soil biota includes:

  • microorganisms – bacteria, algae, fungi, actinomycetes;
  • protozoa – infusoria, flagellates, tapeworms;
  • worms;
  • arthropod insects;
  • mollusks, etc.

In well-cultivated soils, the number of soil biota can reach several billions per 1 g of soil, or 10 tons/ha.


The importance of soil biota

Soil biota is involved in:

  • soil fertility formation;
  • mineralization and humification of organic matter;
  • conversion of bound forms of nutrients to mobile forms;
  • nitrogen fixation;
  • movement of organic and mineral matter along the soil profile;
  • optimum soil structure formation;
  • processes of formation and transformation of phytoactive substances;
  • maintenance of the optimal nutrient regime of soils.

In the tilled soil, soil biota, due to partial binding of mineral elements and fertilizers, allows to retain nutrients in the arable layer, favoring the creation of an optimal nutrient regime and soil structuring.

Soil microflora

The first living microorganisms that appeared on Earth in ancient times started the soil-forming process. The first microbes derived their energy from the decomposition of chemical compounds and released strong acids in the process, which broke down and pulverized the parent rock, creating a new kind of structure. Over time, the weathered rock was enriched with organic matter.

In the arable soil layer, the mass of bacteria ranges from 3 to 7-8 t/ha.

Rhizosphere microorganisms recycle toxic substances released by plants during their life activity. Useful microorganisms convert insoluble compounds into compounds available for plant nutrition. Nitrogen fixers play an important role in plant nutrition, both those living on the roots of legumes and free-living ones.

Microorganisms are divided into autotrophic and heterotrophic according to their mode of nutrition. Autotrophic bacteria use photosynthesis or chemical energy of oxidation of minerals – chemosynthesis – to absorb carbon. Green and purple serobacteria, nitrifying bacteria, and iron bacteria are capable of photosynthesis. Heterotrophic bacteria absorb the carbon of already ready-made organic compounds. Most soil bacteria, actinomycetes, almost all fungi and protozoa are heterotrophs.

The process of oxidation of hydrogen sulfide, elemental sulfur and sulfur-containing compounds to sulfuric acid is called sulfofication. It is carried out by sulfur bacteria and thione bacteria. Sulfuric acid promotes the transition of hard-soluble mineral salts into soluble ones, or after neutralization in the form of sulfates is used in sulfur nutrition of plants.

Iron bacteria are involved in the transformation of iron and manganese salts.

Organic nitrogen, as a rule, is not available to plants. Mineralization of organic nitrogen (ammonification) takes place in the soil. Heterotrophic bacteria, actinomycetes and fungi take part in this process.

Ammonia released as a result of biochemical reactions of ammonification is partially adsorbed on clay-humus particles or neutralized by soil acidity, partially – used by soil biota. Part of the ammonia can be oxidized by autotrophs to nitrites, nitrates, and molecular nitrogen.

Autotrophs use mineral nitrogenous compounds such as ammonium salts and nitrates. There are specific microorganisms that can utilize nutrients from humus.

Optimal soil moisture for microbial development is 50-60% of maximum moisture capacity. Anaerobic microorganisms develop at a moisture content of 80 to 100%.

Aerobic and anaerobic microorganisms coexist in the soil simultaneously. This coexistence is possible when aerobic bacteria exist on the surface of a soil particle intensively absorbing oxygen. At the same time in the center of the particle there is an oxygen deficit and the conditions become anaerobic.

Various types of microorganisms are able to break down fiber and pectin substances, due to which the decomposition of plant residues takes place. Urobacteria transform urea into ammonium carbonate. Urobacteria are aerobic microorganisms, developing at pH 7-8, urea is a source of nitrogen for them, and organic acids and carbohydrates – carbon. A variety of soil microorganisms also break down hemicellulose, starch, and lignin. 

In the immediate vicinity of the roots of higher plants, a zone favorable for the development of soil microorganisms, the rhizosphere, is formed. Root excretions containing various organic substances and dead plant tissues become a breeding ground for rhizosphere microorganisms.

According to the data of V.T. Emtsev, the number of Clostridium bacteria in 1 g of fallow soil is 69,700, while in the rhizosphere it is 10,7 million. According to calculations, the mass of bacteria in the rhizosphere of alfalfa is twice as high as outside the rhizosphere, and is 5 and 2,25 t/ha respectively. The rhizosphere microflora of legumes is richer than that of cereals.

The predominant group of microflora living in the rhizosphere are non-porous bacteria: nitrobacteria, nodule bacteria, photosynthetic bacteria, butyric acid bacteria, mycobacteria, algae. More intensive development of algae is also noted in the rhizosphere. Ammonificators, denitrificators, and nitrificators also develop in the rhizosphere.

Under certain conditions, rhizospheric microflora can play a positive and negative role. Microorganisms, like plants, use minerals for nutrition. However, the size of this competition is usually not significant. Rhizosphere microorganisms act as biological “fixers” of nutrients from leaching and removal from the root layer of soil.

Table. Content of mineral compounds of phosphorus and potassium available to plants in the root zone and soil, mg per 100 g of dry soil

outside the roots
in the rhizosphere
outside the roots
in the rhizosphere
Winter wheat

The situation changes significantly when applying substances with a wide C:N ratio, such as straw manure or straw. In such cases, the number of microorganisms increases dramatically and the consumption of nitrogen, phosphorus and other macro- and microelements becomes significant. As a result, a deficit of plant nutrients can be created. This explains the observed decrease in crop yield in the first year after straw application.

The biological fixation of nutrients by microorganisms is short-lived. After dying off, the microbial cells are mineralized and the nutrients are released for later use by plants, mostly in the following year.

The microorganisms secrete enzymes, growth stimulants and vitamins that are absorbed by the plant roots and promote plant growth. In addition, they release antibiotics that inhibit the development of phytopathogenic flora.

Each type of soil, which emerged in the process of evolution, is characterized by a specific microflora.

Table. Number of microorganisms in different types of soils

Number of microorganisms, mln per 1 g of soil
Sod-podzolic, virgin soil
Sod-podzolic, cultivated
Black earth, virgin soil
Black earth, cultivated

Biological activity of soil

Biological activity of soil is an indicator characterizing the number of organisms living in the soil and quantifying the results of their life activity.

Under favorable conditions, soil biota are in constant and close interaction with each other, establishing a certain equilibrium. Requirements for living conditions of individual representatives can be both simple and complex, is in a symbiotic or antibiotic relationship with other representatives. The latter is often achieved through the release of phytoactive substances and is used in agriculture to regulate the soil biota by suppression of phytopathogenic microflora in order to create a favorable phytosanitary soil condition.

Quantitative assessment of soil biota is usually performed by counting the total number of organisms. Due to imperfect techniques and insufficient number of determinations over time, the results give only an approximate characteristic of soil biological activity. If necessary, individual species and groups of organisms are quantified, such as the number of nitrifying or cellulose-decomposing bacteria.

Other methods of quantitative assessment of soil biological activity are carried out on the basis of results of vital activity of soil organisms by determining the amount of absorbed oxygen and formed carbon dioxide, quantitative decomposition of cellulose, soil enzymes, ammonium and nitrate nitrogen and other compounds. In this case, the assessment characterizes the activity of individual species and groups of bacteria, and only indirectly allows us to judge the activity of soil biota as a whole.

In view of the diversity of soil organisms and their properties, the total assessment of soil biological activity is difficult. There are attempts to apply the so-called biological scores, which are the average of individual indicators of the state of biological processes of the soil.

High biological activity is a factor in improving soil fertility and its phytosanitary state. That is achieved by creating optimal conditions of vital activity of soil biota: the provision of nutrients, especially organic matter, moisture, heat and aeration of the soil.


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