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Wikipedia gives a remarkably “lifeless” definition:

Soil is a natural body consisting of layers (soil horizons) of primarily mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics. In engineering, soil is referred to as regolith, or loose rock material. Strictly speaking, soil is the depth of regolith that influences and has been influenced by plant roots.

soil layers

Soil is composed of particles of broken rock that have been altered by chemical and mechanical processes that include weathering? and erosion. Soil differs from its parent rock due to interactions between the lithosphere, hydrosphere, atmosphere, and the biosphere. It is a mixture of mineral and organic constituents that are in solid, gaseous and aqueous states.

soil texture distribution

Soil is commonly referred to as earth or dirt; technically, the term dirt should be restricted to displaced soil. Soil forms a structure that is filled with pore spaces, and can be thought of as a mixture of solids, water and air (gas). Accordingly, soils are often treated as a three state system. Most soils have a density between 1 and 2 g/cm³. Little of the soil composition of planet Earth is older than the Tertiary and most no older than the Pleistocene.

From Bardgett’s book, adding life:

The soil also provides a medium in which an astounding variety of organisms live. These organisms not only use the soil as a habitat and a source of energy, but also contribute to its formation, strongly influencing the soil’s physical and chemical properties and the nature of the vegetation that grows on it. Indeed, along with vegetation, the soil biota is one of five interactive soil-forming factors: parent material, climate, biota, relief, and time (Jenny 1941).

That life is an important factor of soil formation, stabilization, and characteristics was already seen by Charles Darwin, whose last book (1881) is titled: The Formation of Vegetable Mould through the Action of Worms. This action is today called bioturbation.

From Meysman et al.:

In modern ecological theory, bioturbation is now recognised as an archetypal example of ‘ecosystem engineering’, modifying geochemical gradients, redistributing food resources, viruses, bacteria, resting stages and eggs. From an evolutionary perspective, recent investigations provide evidence that bioturbation had a key role in the evolution of metazoan life at the end of the Precambrian Era.



From Bardgett & Wardle’s book, p.13:

Soils absorb and release greenhouse gases (notably carbon dioxide and methane), and act as a major global carbon reservoir, storing some 80% of the Earth’s terrestrial carbon stock (IPCC 2007). Despite the importance of soils for carbon cycling, remarkably little is known about the factors that regulate the fluxes of carbon to and from soil, or about the role that interactions between plants and soil biota play in regulating soil-carbon cycling. (…) current models of the global carbon cycle seldom include these processes; rather, they simply treat net carbon emissions from ecosystems as the balance between net primary production (NPP) and heterotrophic respiration. An emerging challenge is therefore to use our advancing understanding of plant and soil microbial processes involved in carbon cycling to improve the representation of aboveground–belowground interactions in carbon-cycle models.

From Wikipedia:

Soil formation greatly depends on the climate, and soils show the distinctive characteristics of the climate zones in which they originate. Temperature and moisture affect weathering and leaching. Wind moves sand and smaller particles, especially in arid regions where there is little plant cover. The type and amount of precipitation influence soil formation by affecting the movement of ions and particles through the soil, and aid in the development of different soil profiles. The effectiveness of water in weathering parent rock material depends on seasonal and daily temperature fluctuations. The cycles of freezing and thawing constitute an effective mechanism that breaks up rocks and other consolidated materials. Temperature and precipitation rates affect vegetation cover, biological activity, and the rates of chemical reactions in the soil.

Here is an image from R. D. Bardgett’s article showing feedback effects:

soil climate factors

His own comments about these feedbacks:

Conversely, there is much current debate about the potential to increase the capacity of soils to sequester carbon from the atmosphere and hence mitigate climate change. Recent studies reveal that both of these processes, namely the loss and gain of carbon in soil, are strongly regulated by plant–microbial–soil interactions. Climate change can affect soil carbon through a variety of routes, both directly and indirectly (Figure 1). With regard to direct effects, recent studies show that even subtle warming (by approximately 1°C) can directly stimulate microbial activity causing an increase in ecosystem respiration rates in subarctic peatland. Likewise, it was recently shown that permafrost thaw over decades in an Alaskan tundra landscape has caused significant losses of soil carbon, despite increased plant growth and ecosystem carbon input.

Soil carbon loss due to warming

Here we use data from the National Soil Inventory of England and Wales obtained between 1978 and 2003 to show that carbon was lost from soils across England and Wales over the survey period at a mean rate of 0.6 per cent per year (relative to the existing soil carbon content). We find that the relative rate of carbon loss increased with soil carbon content and was more than two per cent per year in soils with carbon contents greater than 100 grams per kilogram. The relationship between rate of loss and carbon content held across the whole country and across all forms of land use suggesting a link to climate change. Our findings indicate that losses of soil carbon in England and Wales, and by inference other temperate regions, are likely to have been offsetting absorption of carbon by terrestrial sinks.

Carbon sequestration potential

  • Rattan Lal et al., Soil Carbon Sequestration To Mitigate Climate Change and Advance Food Security, Soil Science 172 (2007), 943-956


Creative Commons or free acess

Abstract: Evidence is mounting to suggest that the transfer of carbon through roots of plants to the soil plays a primary role in regulating ecosystem responses to climate change and its mitigation. Future research is needed to improve understanding of the mechanisms involved in this phenomenon, its consequences for ecosystem carbon cycling, and the potential to exploit plant root traits and soil microbial processes that favor soil carbon sequestration.

Abstract: Soil dust is a major driver of ice nucleation in clouds leading to precipitation. It consists largely of mineral particles with a small fraction of organic matter constituted mainly of remains of micro-organisms that participated in degrading plant debris before their own decay. Some micro-organisms have been shown to be much better ice nuclei than the most efficient soil mineral. Yet, current aerosol schemes in global climate models do not consider a difference between soil dust and mineral dust in terms of ice nucleation activity. Here, we show that particles from the clay and silt size fraction of four different soils naturally associated with 0.7 to 11.8 % organic carbon (w/w) can have up to four orders of magnitude more ice nuclei per unit mass active in the immersion freezing mode at −12 ◦C than montmorillonite, the most efficient pure clay mineral.

Most of this activity was lost after heat treatment. Removal of biological residues reduced ice nucleation activity to, or below that of montmorillonite. Desert soils, inherently low in organic content, are a large natural source of dust in the atmosphere. In contrast, agricultural land use is concentrated on fertile soils with much larger organic matter contents than found in deserts. It is currently estimated that the contribution of agricultural soils to the global dust burden is less than 20 %. Yet, these disturbed soils can contribute ice nuclei to the atmosphere of a very different and much more potent kind than mineral dusts.

Abstract: First generation climate–carbon cycle models suggest that climate change will suppress carbon accumulation in soils, and could even lead to a net loss of global soil carbon over the next century. These model results are qualitatively consistent with soil carbon projections published by Jenkinson almost two decades ago. More recently there has been a suggestion that the release of heat associated with soil decomposition, which is neglected in the vast majority of large-scale models, may be critically important under certain circumstances. Models with and without the extra self-heating from microbial respiration have been shown to yield significantly different results. The present paper presents a mathematical analysis of a tipping point or runaway feedback that can arise when the heat from microbial respiration is generated more rapidly than it can escape from the soil to the atmosphere. This ‘compost-bomb instability’ is most likely to occur in drying organic soils with high porosity covered by an insulating lichen or moss layer. However, the instability is also found to be strongly dependent on the rate of global warming. This paper derives the conditions required to trigger the compost-bomb instability, and discusses the relevance of these to the concept of dangerous rates of climate change. On the basis of simple numerical experiments, rates of long-term warming equivalent to 10°C per century could be sufficient to trigger compost-bomb instability in drying organic soils.


These are possibly the most important books on soil:

  • Richard D. Bardgett: The Biology of Soil: A Community and Ecosystem Approach, Oxford University Press 2005

From back cover (emph. added):

(…) It describes the vast diversity of biota that live in the soil environment - the most complex habitat on Earth - and discusses the factors that act as determinants of this diversity across different spatial and temporal scales. The Biology of Soil also considers how biotic interactions in soil influence the important soil processes of decomposition and nutrient cycling . It demonstrates how interactions and feedbacks between diverse plant and soil communities act as important drivers of ecosystem form and function. The importance of these relationships for understanding how ecosystems respond to global change phenomena, including climate change, is discussed in depth. Much is still to be learned about the soil biota and their roles in ecosystems, (…)

  • David A. Wardle: Communities and Ecosystems: Linking the Aboveground and Belowground Components, Princeton University Press 2002

From back cover (emph. added):

Most of the earth’s terrestrial species live in the soil. These organisms, which include many thousands of species of fungi and nematodes, shape aboveground plant and animal life as well as our climate and atmosphere. Indeed, all terrestrial ecosystems consist of interdependent aboveground and belowground compartments. (…) Through ambitious theoretical synthesis and a tremendous range of examples, Wardle shows that the key biotic drivers of community and ecosystem properties involve linkages between aboveground and belowground food webs, biotic interaction, the spatial and temporal dynamics of component organisms, and, ultimately, the ecophysiological traits of those organisms that emerge as ecological drivers. His conclusions will propel theoretical and empirical work throughout ecology.

  • Richard D. Bardgett, David A. Wardle: Aboveground-Belowground Linkages: Biotic Interactions, Ecosystem Processes, and Global Change, Oxford University Press 2010

From book review in Eos, Vol. 92, No. 26, 28 June 2011

it is not accessible to undergraduates and may challenge some early-career graduate students who have not yet read extensively in the primary literature.