The role of the living phase in the formation of soil fertility

How the living phase of the soil creates its soil fertility

Soil fertility directly depends on its living phase — a complex of macro- and microorganisms. Their activity provides plants with access to mineral nutrition by breaking down soil parent material, and also incorporates solar energy into the biological cycle. It is these organisms that process plant residues and form humus — the main energy fund of any field.

The vital activity of the underground community also directly affects the phytosanitary state of the field due to the diverse enzymatic apparatus of microorganisms. However, for the stable operation of this biosystem, three factors are necessary: a constant presence of organic residues, optimal humidity, and temperature. In regions with low temperatures, soil life activity fades, while in temperate, subtropical, and tropical zones, it reaches its maximum.

• Mass of microorganisms in chernozems — 5.2 t/ha
• Mass of microorganisms in serozems — 5 t/ha
• Mass of microorganisms in sod-podzolic soils — 3.5 t/ha
• Proportion of bacteria among soil microorganisms — 70%

The living population of the soil is divided into macro- and microstructures. The macroflora includes the roots of cultivated and wild plants, while the macrofauna consists of large field inhabitants. The microfauna is represented by nematodes, protozoa, and rotifers, which process organic matter at the microlevel.

• Soil macrofauna: rodents, insects, insectivores, millipedes, woodlice, mites, gastropods, centipedes, spiders, and earthworms.
• Soil microflora: bacteria, actinomycetes, fungi, algae, viruses, and bacteriophages.

The number of microorganisms varies significantly depending on the soil type and its organic matter content. In humus-rich horizons, the density of the microbial population is several times higher than in poor soils. Below is the population dynamics of the arable layer by soil type.

Bacteria: the invisible engine of soil nutrition

Bacteria make up the overwhelming majority of soil microflora. Their influence on yield is determined by the colossal contact surface with the soil solution, which is measured in hundreds of thousands of square meters per hectare of arable land. Bacteria ensure the fixation of atmospheric nitrogen, and also initiate the processes of ammonification, nitrification, and the transformation of sulfur and iron compounds.

Although bacteria penetrate the soil profile to a depth of up to 5 meters, the bulk of them is concentrated in the top 20-centimeter layer. This is the warmest, best-aerated, and organic-rich zone. The survival capacity of these single-celled organisms is unique: they are adapted to develop both during frosts and at temperatures close to the boiling point of water.

Upon the return of favorable conditions, bacteria instantly resume their vital activity. Some species are capable of doubling their population in just 20 minutes. Due to the high reproduction rate and labile enzymatic apparatus, they adapt quickly to changing environmental conditions and maintain the biological activity of the arable land.

Actinomycetes, fungi, and algae: destroyers of organic matter and builders of structure

Once readily available nutrition is consumed by bacteria, more powerful organic matter decomposers enter the process. Actinomycetes occupy an intermediate position between bacteria and molds (they are also called ray fungi or mold-like bacteria). They are present in neutral or alkaline, well-aerated, moisture-depleted soils with a sufficient amount of organic matter. Thanks to a rich enzymatic apparatus, these microorganisms easily mineralize hard-to-dissolve compounds and actively participate in the formation and mineralization of humus. In addition, actinomycetes produce antibiotics, suppressing pathogens of crops and maintaining biological balance in the soil.

Fungi represent an extensive group of heterotrophic organisms devoid of chlorophyll. They actively decompose plant and animal residues, contributing to the accumulation of organic acids and humus substances. Fungi are adapted to survival in conditions of high acidity, where bacterial life practically ceases. Being aerobes, they develop mainly on the surface of the substrate, easily tolerate drying out, but prefer a moist environment for active growth. Fungi reproduce both vegetatively and sexually.

Microscopic chlorophyll-containing algae inhabit all types of soil — from the tundra to deserts. Given a favorable combination of heat, light, moisture, and nutrients (usually in spring and autumn), massive reproduction of algae occurs, visible to the naked eye as the greening ("blooming") of the soil. Folk signs associate this phenomenon with a good future harvest. After dying off, the accumulated algal biomass is quickly involved in the nutrient cycle, nourishing soil microflora and crops.

Algae perform the crucial role of pioneers in soil formation, being the first to colonize volcanic lava, ash, industrial waste, and construction debris. In the process of photosynthesis, they saturate the soil solution with oxygen, stimulating the vital activity of beneficial aerobic microorganisms. Of greatest value to agronomy are blue-green algae, capable of fixing atmospheric nitrogen and enriching the soil with it.

Viruses and bacteriophages: hidden parasites and microflora regulators

Viruses are a group of ultramicroscopic obligate intracellular parasites capable of reproducing only within the cells of living organisms. They cause diseases in humans, animals, plants, insects, protozoa, and microorganisms. In the soil, viruses do not reproduce and lack their own metabolism, but they can persist in it for a long time. Reproduction of a virus requires only its nucleic acid, which uses the host cell's ribosomes to create the parasite's proteins.

Viruses possess the following specific features:

• they are not retained by bacteriological filters;
• they lack cellular structure;
• they are incapable of growth and binary fission;
• they lack their own metabolic systems;
• they contain only one type of nucleic acid — DNA or RNA;
• their reproduction requires only nucleic acid;
• they use the host cell's ribosomes to form their own proteins;
• they do not reproduce on artificial nutrient media and can exist only within the body of a susceptible host.

Bacteriophages are viruses of bacteria and actinomycetes. Their external form resembles a comma or a ping-pong paddle. The mass of a bacteriophage is approximately 1000 times less than the mass of a bacterium. Phages are highly specific: they infect only certain types of cells or groups of closely related organisms, acting as natural regulators of the population of soil microorganisms.

• Share of actinomycetes in microflora — 25–30%
• Biomass of soil fungi — up to 1.5 t/ha
• Annual production of algae — 50–1500 kg/ha
• Size of soil viruses — from 15–20 to 300–400 μm

Microflora of rice paddy soils. Flooding rice paddies with a layer of water causes a change in the water, air, and temperature regimes of the soil. The processes occurring in the soil after flooding can be conditionally divided into two consecutive phases. In the first phase, starting immediately after the formation of the water layer, a decrease in the amount of O2 and the reduction of NO3, Mn4+, and Fe3+ occur sequentially. This phase lasts from two to three days and depends on the amount of iron oxide in the soil. In the second phase, the formation of sulfides occurs, and hydrogen and methane fermentation develop. As a result, quantitative and qualitative changes in the microflora take place, expressed in the succession of aerobic and anaerobic groups. In the first phase, aerobic and facultatively anaerobic microorganisms predominate in the soil. Immediately after flooding a rice paddy, an increase in the population of both anaerobic and aerobic microorganisms occurs in the top 5 cm layer of soil. Subsequently, the number of aerobes gradually decreases. However, they do not completely disappear from the soil throughout the entire growing season of the rice.

The qualitative composition of bacteria inhabiting the soil under rice is diverse, but facultative anaerobes predominate, mainly – Bac. megatherium, Bac. mycoides, Bac. subtilis, Bac. mesentericus, Bac. idosus, Bac. cereus. Spore-forming bacteria develop in the second growing season period, and in the first — mainly non-spore-forming ones. The number of ammonifying microorganisms in flooded soil is always relatively high; however, they are more numerous in the first period after flooding, when there is still a sufficient amount of organic substances in the soil. Subsequently, the population of these microorganisms decreases to a certain level, after which it remains practically unchanged. Among ammonifying microorganisms in the flooded soils of rice paddies, representatives of the genus Bacillus predominate – Bac. megatherium, Bac. mycoides, Bac. zotaus, Bac. glutinosus.

Nitrifying bacteria, being aerobic microorganisms, are found in large quantities in the soil during the first few days after flooding, when there is still a sufficient amount of oxygen in the environment. In the subsequent period, their numbers drop sharply; however, nitrifiers, like other aerobic microorganisms, are always present in flooded soil under rice, which is explained by the high aeration capacity of this crop's roots.

Denitrifying microorganisms are constant companions of rice fields. Immediately after flooding, the number of this group increases, especially if mineral fertilizers were previously applied to the soil.

Fungi are found in the soil during all phases of the rice growing season. In the second half of the growing season, when reduction processes reach their maximum, a noticeable decrease in their number is observed. From this group of microorganisms, representatives of the following genera are present in rice field soils: Penicillium, Aspergillus, Mucos, Alternaria, Fusarium, Phisopus. In the soils of the Kuban occupied by rice crops, in addition to those listed, fungi of the genera Сephalosporium and Cladosporium predominate. Among actinomycetes, Act. albus sterilis, Act. sterilis ruber, Act. viridis, Act. globisporus griseus, Act. parvus, and Act. intermedius are constantly present in rice field soils.

The group of cellulose-decomposing microorganisms in rice fields is represented by both aerobic and anaerobic forms.

The development of sulfate-reducing bacteria in flooded soils is in direct dependence on the intensity of the reduction processes occurring within it. Since the most reduced conditions in the soil usually occur in the second half of the growing season, the number of sulfate-reducing bacteria also increases by this time.

Oxidation of sulfur compounds also takes place in flooded soil. Two types of microorganisms participate in the process of sulfur oxidation: anaerobic – Thiobac. denitrificans and aerobic – Thiobac. thioparus. Manganese compounds are oxidized and reduced in flooded soil with the participation of actinomycetes.

Fertilizers and soil microflora. The application of organic and mineral fertilizers to the soil not only improves plant nutrition but also changes the conditions for the existence of soil microorganisms, which also need mineral elements for their vital activity. The complex nature of the ecosystem's response to the application of fertilizers is shown in Figure 54.

Fig. 54. Pathways of fertilizer impact on soil microflora

It is quite obvious that fertilizers can have not only a direct but also an indirect effect on microorganisms – through plants and livestock animals.

Under favorable climatic conditions, the number of microorganisms increases significantly after soil fertilization (Table 31; Fedorov M.V., 1963).

Table 31 – Influence of manure on the number of microorganisms in the soil, thousand cells/g of soil Total number of microorganisms Bacteria Actinomycetes Fungi

 No fertilizers           1180             470          700            10
 Manure, 30 t/ha          2550             830         1700            20
 Manure + lime            1983             760         1200            23
 Manure + NPK             1101             366          700            35

Fertilizing the soil with manure contributes to an approximately two-fold increase in its total content of microorganisms. At the same time, the number of not only bacteria increases, but also actinomycetes and fungi.

Mineral fertilizers also have a positive effect on soil microflora. However, their effect depends on the following factors:

• type of fertilizer;
• application rate;
• time of application;
• method of application.

The nature of the influence of various forms of nitrogen fertilizers on soil microorganisms:

How nitrogen and phosphorus fertilizers change the composition of the microbiota

By applying mineral fertilizers, an agronomist directly influences the activity and composition of soil microflora. Different forms of nutrients stimulate the development of specific groups of microorganisms. This is important to consider when planning plant nutrition, as the speed of converting elements into available forms depends on the composition of the soil biota. The nature of this influence for different forms of nitrogen is presented in the table:

Phosphorus fertilizers also have a significant impact on soil microflora, although to a lesser extent than nitrogen ones. Their application increases both the total number of soil microorganisms and the quantity of their individual groups. The greatest increase in numbers under the influence of phosphorus is shown by ammonifying bacteria and actinomycetes, which mineralize the organic matter of the soil.