Soils cover most of the Earth’s landmasses. Soil is a complex system, under continuous evolution and in perpetual relation to the atmosphere, hydrosphere, lithosphere, and biosphere. It is polyphasic, composed of a solid (mineral and organic) fraction, but also of liquid and gaseous fractions. A key aspect is its stratification into several horizons. This complex structure determines the living conditions being provided to the local biota, but also a major diversification of soil types on our planet.

Microorganisms form the bulk of soil biota. Having densities of hundreds of millions per gram, bacteria are the dominant organisms. They are the starting point of most trophic chains, they ensure a major part of soil functionality as an ecosystem and they effectively create soil, especially by breaking down organic matter. Their taxonomic diversity is enormous, such as the ecological one: heterotrophs, photoautotrophs, chemoautotrophs, etc. Besides them, there are also archaea, viruses, different types of microalgae, and the eclectic group we usually call “protozoans”. 

The most abundant multicellular organisms in the soil are fungi (although the group also contains unicellular members – yeasts). Fungi are an extremely diverse group of heterotrophic organisms. Most are saprophytic, playing key roles in decomposition and pedogenesis processes. We can add parasitic species, as well as lichens, and photoautotrophic symbiotic associations. Similar to an organization and way of life are mycetozoa and pseudofungi (oomycetes and their relatives). Plants are present in the soil only through their underground organs (roots, rhizomes, bulbs, etc.), but have an essential contribution to the genesis and functioning of soil, once their various underground and aboveground components decompose. Finally, animals are some of the main consumers of soil. Here we may find nematodes, annelids, insects, and other arthropods and some species of vertebrates that use soil as a temporary or permanent living environment.

The entire organic chemistry is based on carbon, an essential element in all substances that form living matter. The carbon cycle in soil comprises a wide variety of processes, involving diverse groups of micro- and macroorganisms. Photoautotrophy and chemoautotrophy are weakly represented, although they are not a negligible source of organic matter. The dominant process, however, is decomposition. Sugars, lipids, proteins, and other categories of compounds are being constantly degraded, through the enzymatic activity of various heterotrophic microorganisms, forming humus (a relatively stable organic complex) and, then, simpler and simpler products. In weakly oxygenated soils, specific phenomena such as methanogenesis and its reverse, methanotrophy, occur. 

Nitrogen is another bioelement of crucial importance, being a part of amino acids, proteins, nucleic acids, and many other organic compounds. It is also an object to a complex natural cycle occurring, among others, at the soil level. Ammonification is one of the main components of this cycle, consisting of the decomposition of nitrogencontaining organic compounds and releasing ammonium ions, essential nutrients for plants and microorganisms. Another nitrogen input source in soils is the fixation of atmospheric dinitrogen by some bacteria, free-living or in symbiosis with certain plant species. Nitrification converts ammonium into nitrate, another valuable nutrient, soluble and easily disseminated in soil solution and hydrosphere. Microbial denitrification closes the cycle by transforming nitrate into nitrogen oxides and, then, dinitrogen that returns to the atmosphere. 

Phosphorus and sulfur, together with alkaline, alkaline earth and transitional metals are important nutrients and constituents of living matter. The sulfur cycle involves an atmospheric phase, but its main stages are due to terrestrial biota, through processes like sulfate reduction, sulfide oxidation, anoxygenic photosynthesis, etc. The cycles of phosphorus and metals lack significant atmospheric stages. The main biogeochemical processes involving soil microorganisms are those of solubilization and precipitation, that determine their general bioavailability.

Between soil organisms, various interspecific relationships are formed, some of which are positive, and others are negative for at least one of the parts involved. A mutually positive relationship is symbiosis. Endo- and ectosymbioses between nitrogen-fixing bacteria and plant roots, actinorrhizae, and, most of all, mycorrhizae play extremely important roles in plant productivity and maintaining soil quality. Relationships between plants and various groups of microorganisms at the rhizosphere level also ensure a matter and energy flow from one plant to another, leading to a true ''mycorrhizal Internet”. Knowledge and control over these complex relationships also have practical applications, in producing biofertilizers, biopesticides, and in bioremediation, etc.