Soil Health – Soil Microbes, Organic Substances and Nutrient Recycling


Soil microorganisms are found in the soil in large quantities. Their number directly depends on the carbon source for energy. Due to their small size, they have little biomass. Actinomycetes similar in biomass to bacteria. Populations of fungi dominate in structurally undisturbed soils. Bacteria, actinomycetes and protozoa are more hardy than mushrooms, respectively, can tolerate soil damage; they dominate tilled soils. Fungi and nematodes tend to dominate no-till soils.

Relative abundance and biomass of microorganisms in the soil (0-15 cm)



The decomposition of organic matter provides energy for the growth of microorganisms and supplies carbon to form new cells. Soil organic matter consists of three fractions: microorganisms, fresh residues of plant or animal material, and humus. Humus is a long-term fraction that is resistant to decomposition for a very long time. Also, the organic matter of the soil can be divided into two groups: active agents (about 35%) and passive agents (about 65%). An active agent consists of microorganisms and fresh residues of plant or animal material that is food for bacteria and consists of sugars and proteins. A passive agent is resistant to degradation by microorganisms.

Microbes require regular supplies of active agents to survive in the soil. Long-term No-Till soils have a higher level of microbes, contain more active carbon and more OM, than ordinary tilled soils. Most of the microbes in the soil exist in conditions of starvation and, thus, they are usually at rest (especially in the tilled soils).

Plant residues and plant nutrients also become food for germs in the soil. Soil organic matter is all organic matter, as well as: plants, algae, microorganisms (bacteria, fungi, protozoa, nematodes, etc.) and fresh degraded plant, animal and microorganism degradation.

All the organic matter of the soil can be divided into its component parts. So 100 grams of dead plant material can give 60-80 carbon dioxide that is released into the atmosphere. The remaining 20-40 g are 3-8 g of microorganisms, 3-8 g of NOT-humic compounds and 10-30 g of humus. The molecular structure of OM is mainly carbon, oxygen, hydrogen, nitrogen, and small amounts of phosphorus and sulfur. Soil organic matter is also a by-product of carbon and nitrogen cycles.


OB is the storage of many plant nutrients. A chemical agent consists of carbon, oxygen, hydrogen, nitrogen, sulfur, phosphorus, and potassium. Biologically active soils contain large amounts of active carbon and produce more nutrients for plant growth than soils that are biologically inactive and contain less active organic matter. When tillage, a large amount of nutrients can be released, since the chemical is consumed by bacteria and destroyed by their action.


RH also depends on climate and temperature. Microbial populations can double as the temperature changes by 50 ° C. You can compare the tropics with the cold arctic regions: in the tropics, the topsoil contains very little RH, since high temperatures and high humidity quickly decompose RH. Moving north or south of the equator increases the RH value. Tundra near the Arctic Circle; in this region, soils have a high OV value, since cold temperatures freeze and store OV.

The decomposition of organic matter is affected by: moisture, pH, soil depth and particle size. Hot and humid regions retain less organic carbon in the soil than dry and cold regions (due to an increase in microbial decomposition). The rate of decomposition of organic matter increases when the soil is subjected to cycles of drying and wetting. In neutral or slightly alkaline soils, the decomposition of organic matter is faster than that of acidic soils; therefore, soil liming enhances the decomposition of organic matter and the release of carbon dioxide. More decomposition occurs near the soil surface, where the highest concentration of plant residues is located. At greater depths of decomposition of organic matter is worse, due to the smaller amount of plant residues.

In forest soils, most of the organic matter is distributed in the uppermost several centimeters. Steppe soils contain a high OM value throughout the soil profile. These soils are sufficiently productive because they have a higher percentage of agents (especially active carbon), may contain more nutrients, contain more microbes and have a better soil structure due to the growth of the fungal population.


The destruction of organic residues by microbes also depends on the ratio of carbon to nitrogen (C: N). Consider two examples, young alfalfa and oats / wheat. Young alfalfa contains a lot of raw protein, amino acids and sugars in the stem, which are easily digested by microbes. Also, young alfalfa contains a lot of protein nitrogen (amino acids and proteins with a high content of nitrogen and sulfur), so it has a low carbon to nitrogen ratio (less carbon, more nitrogen). Oats / wheat contain a lot of lignin (which is resistant to microbial decomposition), a little crude protein and even less sugar in the stem, respectively, a higher ratio of carbon to nitrogen (more carbon, less nitrogen). Straw is decomposed by microbes, but this requires additional time and nitrogen to destroy this carbon source.

A low nitrogen content and a high carbon-to-nitrogen ratio are associated with a slow decomposition of OM. Young plants contain a higher nitrogen content and a low carbon to nitrogen ratio, and rapid decomposition of the RH. For good compost, the carbon to nitrogen ratio should be less than 20, which allows organic materials to decompose quickly (4 to 8 weeks). A carbon to nitrogen ratio of more than 20 requires additional nitrogen and slows decomposition. So if we add high carbon content and low nitrogen content to the soil, the microbes will bind the soil nitrogen. That is why the ratio of carbon to nitrogen is less than 20.

For most soils, this ratio is 10: 1, which indicates available nitrogen for plants. The carbon to nitrogen ratio for most plant residues tends to decrease over time (<10). This is due to the loss of carbon in the form of carbon dioxide (CO2). Thus, the percentage of nitrogen in residual OM increases as decomposition progresses. Bacteria are the first microbes that can process new organic plant and animal residues in the soil. Bacteria have a high content of nitrogen in their cells (from 3 to 10 carbon atoms per 1 nitrogen atom). Under favorable conditions (temperature, moisture and food sources) they can multiply very quickly. Bacteria are less effective at converting organic carbon in new cells. Aerobic bacteria absorb about 5-10 percent of carbon, and anaerobic bacteria can absorb only 2-5 percent. Fungi tend to release less carbon dioxide into the atmosphere and therefore are more efficient at converting carbon to form new cells. Mushrooms usually get more energy from the RH (40-55 percent of total carbon). Most fungi can consume organic matter in the form of cellulose and lignin, which are slower and more difficult to decompose. The lignin content in most plant residues may be more important in predicting the rate of decomposition. Mushrooms have a large surface area and help in transporting mineral nutrients and water to plants. The life cycle of fungi is more complex and longer than that of bacteria. Mushrooms are not as hardy as bacteria and require a more permanent food source. Fungi populations tend to decline using traditional processing. Fungi have a higher carbon to nitrogen ratio than bacteria (10 carbon atoms per 1 nitrogen atom). They are more efficient at converting carbon to soil organic matter. The simplest can reproduce in 6-8 hours, and nematodes from 3 days to 3 years (30 days on average). After simple nematodes consume bacteria or other microbes (high in nitrogen), they release excess nitrogen in the form of ammonium cation. Plants absorb ammonium and ground nitrates for food through the fungal mycorrhizal network. Microbial populations can change in the soil as quickly as they can quickly add, consume, and process these microorganisms. The amount, type and availability of organic matter will determine the microbial population and its development. Each individual organism (bacteria, fungi, protozoa) has its own specific enzymes and chemical reactions that help these organisms absorb carbon.

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