The soil is important to Planet Earth for several reasons. It nurtures green plants, the basis of the terrestrial food chain, and plays a critical role in several biogeochemical cycles – notably the carbon cycle, which has such an important influence on the planet’s climate system and its stores of carbon. A healthy soil is the foundation of good health throughout the biosphere, which is more important than ever, as global warming intensifies. One reason why ‘sponge cities‘ – new, green-infrastructure projects involving the use of soils and plants – are gaining in popularity.
What Is Soil?
Soil is a varying mixture of dust, silt, sand, clay, small stones, or particles of rock that covers Earth’s land surface. The body of soil as a whole is known as the pedosphere, which is the top layer of the lithosphere, the hard outer-shell of the planet. Along with air and water, soil is a primary natural resource without which there could be little or no life.
What Does Soil Consist Of?
Soil consists of three basic components: (1) minerals from nearby or faraway rocks; (2) organic material, typically the remains of plants and animals that became buried; and (3) decomposers and other living organisms, such as algae, fungi, bacteria and lichens, as well as earthworms, ants, millipedes, beetles, moles and other burrowing creatures, and their waste. The abundance of life forms that exists in the soil exceeds that of any other ecosystem. In fact, there are more individual organisms alive in a tablespoon of soil than there are people on the Earth. 1 In addition, scientists believe that that soils hold a potential reserve of organisms and genetic material that can provide antibiotics and other important medications. 2
The soil is an important interface between the five sub-systems that shape the Earth’s climate and environment. These are the atmosphere (air), the hydrosphere (rivers, lakes, oceans), the cryosphere (frozen water and permafrost), the lithosphere and the biosphere (living things). In particular, it acts as a key pathway in the biogeochemical cycles of carbon, nitrogen, phosphorus, sulfur and water.
- What Is Soil?
- What Does Soil Consist Of?
- How Does Soil Form?
- How Long Does It Take Soil To Form?
- What Is Soil Profile?
- What Are Soil Horizons?
- How Deep Are Soil Horizons?
- What Does Soil Contain?
- Why Is Soil So Important?
- How Are Soils Affected By Climate Change?
- What Causes Soils To Form?
- Who Or What Are Pioneers In Soil Formation?
- How Do Organisms Improve The Soil?
- How Do Plants Affect Soil Formation?
How Does Soil Form?
The soils we see around us are made up of rock particles or granules, plus organic matter from plant and other organisms. So how does solid rock turn into tiny particles? Answer: Through a very slow process called weathering. This involves the gradual breakdown of rock by physical means (e.g. repetitive freezing and thawing, or erosion from the action of falling or flowing water), or chemical means (dissolving the rock through inorganically produced acids, like carbonic acid from CO2 and water). Chemical weathering can also be caused organically through acids produced by bacteria and fungi. 3 4
Other factors, notably climate and topography, assist in soil formation, due to their effect on temperature, rainfall and water run-off. 5
Does Soil Always Form From Local Rocks?
No. Not usually. Scientists estimate that up to 95 percent of the rocky parent material in the soil was transported to its present location – usually by wind, water or glaciers. Only 5 percent formed in situ from local parent material. 6
How Long Does It Take Soil To Form?
It depends what the parent material is, and how deep the soil is. A thin covering, for example, in the tropics or in a river delta region may take as little as 20 years to form. Conversely, a deep sandy soil could take up to 80 million years to form. 7 Generally speaking, in mild climates, it takes between 200 and 400 years to make 1 cm of topsoil. In tropical areas, soil formation is more rapid – perhaps 200 years for 1 cm. But, in either case, it takes longer to attract enough organic and inorganic material to make soils fertile. 8 According to other research, soils form continuously, but very slowly – on average, about one inch of topsoil forms every 500 to 1,000 years. 9
What Is Soil Profile?
A soil profile is the vertical cross section of the soil that is exposed after digging a vertical shaft from the surface of the soil to the foundation bedrock. It usually consists of a thin top layer of plant foliage, leaf litter and other organic residues (the organic layer), which sits above several distinct and thicker layers of soil and rocks, of differing widths, colours and textures.
Colour can be indicative of specific soil properties, regarding water and oxygen content. Dark soils, for example, tend to have a high content of organic matter; red and yellow soils are typically rich in iron; while grey, blue or green hues indicated a saturated, poorly aerated soil.
Texture often varies according to the mineral component of the soil, and helps to measure soil performance in respect of drainage and fertility. Larger particle and very fine particle soils tend to be less productive, with better productivity coming from mid-level soils.
What Are Soil Horizons?
A soil horizon is one of the layers visible in the profile, just below the thin organic layer (“O”). There are three basic soil horizons: (1) Topsoil, often called the “A” horizon. (2) Subsoil, the “B” horizon. (3) Parent material, the “C” horizon. The last one, mostly made up of large rocks, gets its name from the fact that horizons “A” and “B” developed from layer “C”. Beneath horizon “C” is bedrock, comprising a mass of solid rock.
How Deep Are Soil Horizons?
Topsoil (horizon “A”) is rated from “very shallow” (less than 10 inches deep), to “very deep” (more than 60 inches deep). The average is 5-10 inches deep. Plants grow best in topsoil rated “deep” (about 36 inches). This depth gives them room to develop stronger root systems for better stability and support. Deeper topsoil holds more nutrients and water than similar shallower soils.
Subsoil (horizon “B”) depths also vary considerably – anywhere between 10 and 60 inches, or more. The “C” layer is typically larger, anywhere between 30 and 80 inches or more. Soil may go down 10-15 feet before solid bedrock is reached.
What Does Soil Contain?
Although soils vary enormously according to biome or ecosystem, as well as the bedrock upon which they sit, a typical soil is roughly half solid, half space (voids or pores). Of the solid material, roughly 90 percent will be mineral, 10 percent organic matter. The non-solid pore space, which facilitates the infiltration and movement of air and water, both of which are critical for soil life, are occupied half by gas, half by water. 10 11 Where soil has become compacted, the pore space will be flattened, preventing air and water from supplying plant roots and soil organisms. 12
Why Is Soil So Important?
The main reason why the pedosphere is important is because it’s where plants establish their roots and photosynthesize.
Photosynthesis is critical for virtually all living organisms, including Man. Without photosynthesis, plants and phytoplankton could not survive and, without plants and phytoplankton, the Earth’s food web would collapse.
Soil nourishes and supports plants, creating a series of ground-level ecosystems that help to feed the planet. It is only because plants grow in soil, that large-scale food production and farming is possible. With world population set to increase by another 3.5 billion souls by 2100, the soil becomes an ever more precious asset, because soil health is vital for healthy food production.
The soil plays a hugely important role in nearly all biogeochemical cycles. Take the carbon cycle, for example. The majority of carbon recycling into and out of the atmosphere is bound up with biological reactions within the soil, involving the absorption, conversion, and release of carbon by plants and microorganisms, through photosynthesis, respiration, and decomposition. The geological carbon cycle takes place over hundreds of millions of years and involves the movement of carbon (including fossil fuels) through the various layers of the Earth. Scientists believe that more CO2 is stored in the world’s soils than is present in the atmosphere. The permafrost alone is estimated to hold about 1,500 billion tons of carbon. For more on this, see: Which is the Largest Carbon Reservoir?
Soil is also important to the nitrogen cycle. Although nitrogen is the world’s most abundant gas, it cannot be used as it is. It must be broken down into other forms in order to be used by living organisms. This breakdown process, known as “nitrogen fixing”, is mostly conducted by bacteria in the soil.
Phosphorus And Sulfur Cycles
In the phosphorus cycle, the soil is an important pathway for phosphorus take-up by plants and animals and also for its return to the environment through decomposition. The sulfur cycle benefits from the soil in the same way.
The part played by the soil in the water cycle is crucial for the maintenance of the biosphere, because it plays an important part in the storage and distribution of the water that reaches it. When water reaches the soil surface, it can do one of three things: (i) flow over the surface to reach the streams, lakes and rivers; (ii) infiltrate the soil, providing water for plants and the rest of the soil ecosystem; or (iii) pass straight down through the soil, into the aquifers or groundwater below. The first option, the least beneficial for the environment, is the option most closely associated with poor soil.
Soil is vital for the health of our agriculture and our drinking water, because it operates its own water-filtration system. For example, whenever rainwater is intercepted by natural groundcover or absorbed into the soil, it is cleansed as it infiltrates downwards to the groundwater table.
This cleansing process typically works in one of three ways: physically – soil particles can act as a filter and physically clean the water as it percolates; chemically – soil particles are negatively charged, so they can retain nutrients for future use, or prevent contaminants from reaching groundwater; or biologically – soil microbes can breakdown organic chemicals that need to be removed the soil. But if the soil is too thin or too poor, its cleaning capability is reduced, resulting in water flowing directly into streams and rivers without being filtered.
The soil is a living, breathing ecosystem, populated by a heaving host of microbial and invertebrate organisms, as well as more complex animal biota. Soil microorganisms known as decomposers play an extensive role in the breakdown and decomposition of organic matter, recycling of nutrients and energy and elemental fixation. Many microorganisms form symbiotic relationships with plants and animals serving as nitrogen fixers and gut bacteria, functioning as a significant part of the food web.
Soil microbiologists separate the 7,000 or so soil species into 5 groups according to size. The smallest are protists, which include bacteria, actinomycetes, and algae. Next up are the microfauna, which include single-celled protozoans, flatworms, rotifers, nematodes and tardigrades (eight-legged invertebrates). Next comes mesofauna, such as springtails, nematodes, mites, proturans and pauropods. The fourth group, the macrofauna, include the potworm, a white, segmented worm, as well as slugs, snails, and millipedes, and centipedes, beetles and their larvae, and the larvae of flies. The largest soil organisms are the Megafauna, which include the largest earthworms, as well as mice, moles, rabbits, lizards, gophers, badgers and the like.
It has been estimated that one gram of soil contains more organisms than there are human beings on the planet. 13
How Are Soils Affected By Climate Change?
Impact on Soil Carbon Levels
In general, changes in the atmospheric CO2 concentrations, temperatures, and precipitation will determine decomposition rates, varying the soil-plant system. In turn, this will have an impact on the amount of organic carbon levels in soils. This is important because organic carbon affects important soil qualities, such as soil fertility and microbial population. See also: 7 Effects of Climate Change on Plants.
Researchers have found that soils heated 5-20 cm deep, tend to release 9-12 percent more CO2 than normal, whereas soils 100 cm deep release as much as 37 percent more CO2 than normal when they experience a 4°C temperature increase. This is because deeper soils contain more than half the global soil carbon. For instance the first 3 feet or so of soil holds about 1420 billion tons of carbon, while the vegetation and dead organic matter just above/below ground level store about 460 billion tons of carbon. 14
Healthy soil absorbs carbon. This is because healthy soils enable plants and vegetation to photosynthesize at an optimum rate and so grow at a healthy rate. This is important because during the photosynthesis process, plants convert atmospheric carbon dioxide (CO2) into carbohydrates, removing CO2 from the atmosphere. Also, healthy soils absorb CO2, preventing it from escaping into the atmosphere. On the other hand, unhealthy soils – caused by deforestation, environmentally damaging land use, excessive use of herbicides, pesticides and fertilizers, erosion and over-frequent tillage – discharge carbon into the atmosphere, thus boosting global warming. 15
But note that one study thinks that some climate models may overestimate the soil’s capacity to absorb carbon by as much as 40 percent. Using data obtained from 157 soil samples, the study found that the average age of soil carbon is significantly older than previous estimates. It may therefore take centuries for soil to absorb large amounts of carbon from the atmosphere. 16
Impact on Agriculture
Climate change has a significant effect on soils and the functions they perform. In agriculture, for instance, crop production will be adversely affected as changes in soil, air temperature and rainfall impede the ability of crops to reach maturity. Some of the loss of harvest involved will be offset by the longer growing season for some crops in some regions.
However, with rising temperatures, freshwater may become scarce which may reduce or even prevent irrigation. On top of this, deforestation and land degradation involving loss of trees and vegetation, can lead to soil erosion, desertification, salinisation, or loss of peat and permafrost soils, further reducing the ability of soils to produce proper yields.
Impact on Ecosystems
As soils suffer, so too will their ability to support their local ecosystems. This is bound to lead to a shrinkage or even outright loss of habitat for plants and animals alike. For example, plants and animals suited to wetter conditions may not be able to compete with rivals able to cope with drier conditions.
The effects of global warming are exacerbated by man-made changes to soils, such as the drainage of peatlands and other wetlands as part of infrastructure development (bridges, motorways, roads and commercial units) or housing expansion.
Erosion, Runoff and flooding
Higher rainfall can be equally problematic. A study published in the journal Nature last year showed that regional increases in rainfall due to climate change may lead to less water infiltration into the soil, more runoff and erosion, and a higher risk of flash flooding. 17
What Causes Soils To Form?
How soil formation evolves and how long it takes is largely conditioned by five interconnected factors.  They are: (1) Parent material (the original rocks the soil comes from) and its breakdown by physical and chemical means. (2) Climate, mainly temperature and precipitation. (3) Topography, chiefly altitude and slope. (4) The impact of plants, animals and decomposers on soil structure. (5) Time.
How Are Rocks Moved To Their Final Soil Destinations?
The rocks from which a soil forms are called parent material. Rock is pretty important stuff. It is the source of all soil minerals as well as the source of all plant nutrients (except for carbon, hydrogen and nitrogen). Anyway, most parent material (perhaps as much as 95 percent) has been transported to its present site by a combination of wind, water, ice or gravity. Before, during and after this transportation process, the parent rock is physically and chemically weathered, eroded, fragmented, crushed, partially dissolved, or precipitated, gradually turning into a soil.
Aeolian (wind) processes commonly blow large amounts of silt many hundreds of miles, to form new loess soils. 18 The phenomenon is seen in the American Midwest, north-western Europe and China. Something like 10 percent of the Planet’s land surface is covered by loess or similar soils. 19
Interestingly, each year winds blow an average of 182 million tons of phosphorus-rich dust from the African Sahara to the Amazon rainforest 1,600 miles (2,600 km) away, across the Atlantic Ocean. 20 The windblown material is a handy replacement for an equivalent amount of phosphorus which is typically washed out of the ground every year by rain and floods. 21
Another major mover of parent rock is water. For example, run-off after high rainfall, underground streams, swollen rivers, ocean storm surges and delta floodwaters all play a part in washing along rocks, silt, sand and other rigolith to new destinations, where they eventually form soil.
Glaciers also transport stones and rocks, often grinding and crushing them in the process, thus accelerating their breakdown. Gravity is another method of moving parent rock.
How Are Rocks Broken Down Into Soil Material?
There are three ways that rocks are broken down. By physical means, chemical means and biological means. Physical breakdown or weathering is a constant process that never stops. Compaction, extreme heat, freezing/thawing, fire, lightning are naturally occurring events that – over thousands or millions of years – can reduce the hardest rock to powder. So too can ocean waves. Chemical breakdown results from chemical reactions (mostly acidic) between rock and other agents, nearly always in the presence of water. When atmospheric CO2 dissolves in rainwater to produce carbonic acid, it will gradually eat away at the rock surfaces it lands on.
Other chemical weathering processes include hydrolysis and carbonation (most effective), as well as hydration, oxidation.
Climate is the dominant factor in soil formation, and soil profiles reveal the distinctive characteristics of the biomes in which they form. In a nutshell, the hotter and wetter the climate, the more chemically active the entire environment – including the soil.
Temperature and moisture impact directly on the metabolism of plants and microorganisms around them, as well as the leeching of nutrients and other material from the soil. The downward infiltration of rainfall, in conjunction with chemical weathering leaches magnesium, iron, and aluminium from the soil, and flushes them into the groundwater, a process known as podzolization.
Photosynthesis, a key biogeochemical mechanism, increases in warmer climates (notably the Amazon Rainforest) where there is greater light intensity, higher carbon dioxide levels, and more green pigment (chlorophyll) in the plants and trees. 24 In these forests, trees and plants can generate as much as 800 grams of carbon per square meter per year. 25
The topography of a locality or region – for our purposes, this includes the altitude, slope, and general relief of the area – will amplify or diminish the effect of climate.
Steep slopes experience high rates of run-off and low rates of filtration, leading to low mineral deposits and poor soils, compared to soils on more level sites in the vicinity. 26 Furthermore, soil on slopes suffers greater leeching and erosion, with material being deposited on the flat areas at the foot of the slope.
On the other hand, super-flat terrain is sometimes associated with saturated, infertile soils and bad drainage, leading in extreme cases to the formation of peat bogs or saline marshes.
Who Or What Are Pioneers In Soil Formation?
In simple terms, they are nature’s first responders: the organisms that are equipped to begin the process of breaking down the rock by chemical means, chiefly the use of acid. The biotic impact on the breakdown of parent rock starts with lichen and other microorganisms such as mosses, green algae, numerous fungi and bacteria, who secrete a variety of acids – such as oxalic, acetic, and citric – over the rock. 27 28
Others pioneers include: lyme grass, sea couch grass and Marram grass, all of which colonize barren sand; as well as swordfern, lichen (Stereocaulon vesuvianum and Placopsis gelida) and moss (Racomitrium ericoides), who help to breakdown solidified lava rock. Other acids that contribute to early-stage chemical weathering include phenolic, humic and fulvic acids. All these acids accelerate chemical decomposition by reacting with the rocks in a process known as chelation.
Pioneering fauna generally will only move into a locality once flora and fungi have inhabited the area for a while. Soil fauna, from microscopic protists to larger invertebrates, play an active part in soil formation and the recycling of nutrients. Bacteria and fungi are the most important actors in the decomposition of organic debris left by primary producers like moss and algae. As the profile develops, earthworms and ants aerate the soil, converting large amounts of organic debris into rich humus, improving fertility and facilitating the transit of gases and water.
How Do Organisms Improve The Soil?
Soil teems with living organisms. Estimates of the number of species per gram, range from 50,000 to over a million, with the actual number varying according to location, soil type and depth. 29
The major contribution of soil animals is to rotovate the soil (a process known as pedoturbation), create holes for water and gas, support plant life and enrich the ecosystem with their waste. In addition, they feed on dead plants and leaf litter strewn about on the surface, mixing the organic material with the upper soil layers.
Smaller animals include: mites, nematodes, springtails, proturans, pauropods, rotifers, tardigrades, small araneidae, enchytraeidae such as insect larvae, potworms, small isopods and myriapods. Larger animals include snails, worms, slugs, woodlice, tenebrionid beetles, termites, millipedes and moles.
Plant roots perform a similar function by penetrating soil horizons. Plants with deep roots can penetrate several metres through the different layers to excrete or suck up nutrients from deep down. Roots rapidly decompose, adding organic matter to soil, a process known as rhizodeposition.
How Do Plants Affect Soil Formation?
Vegetation contributes to soil health in several ways. Plants reduce the risk of surface runoff. 30 They provide shade and exert a cooling effect. 31 They also act as a buffer against extremes of precipitation. 32 For example, during the driest months, plants protect soils from drying out; while during the wetter months they help to prevent saturation. (See also: Is more CO2 good for plants?)
The “O” Horizon
“O” stands for organic matter. It is the top layer, open to the atmosphere, and usually consists of large amounts of organic debris in varying stages of decomposition. O horizons typically contain about 20 percent organic carbon.
The “A” Horizon (Topsoil/Surface Soil)
Average depth: 5-10 inches. (Maximum 60 inches)
Topsoil is the upper layer of soil, usually the top 5-10 inches (13-25 cm). It has the highest concentration of dark decomposed organic matter, called “humus”. The hummus helps to make the topsoil soft and porous enough to hold sufficient air and water. In this layer, the seeds germinate and plant roots are everywhere. As a result, organisms such as earthworms, potworms, arthropods, nematodes, millipedes, centipedes, fungi, and many species of bacteria are concentrated here, many clustered around plant roots.
The biggest environmental danger to the topsoil is wind erosion, since without topsoil, plant life becomes almost impossible. Unfortunately, conventional agriculture seems to encourage the depletion of topsoil by having it ploughed and replanted each year. The United States alone loses almost 3 tons of topsoil per acre per year. 33 Sustainable farming techniques attempt to reduce this depletion by planting cover crops in order to accumulate organic matter in the soil. Note that: one inch of topsoil takes between 500 and 1,000 years 34 to form naturally. Based on 2014 trends, the world has about 60 years of topsoil left. 35
The “B” Horizon (Subsoil)
Average depth: 10-30 inches. (Maximum 60 inches, or more)
The B horizon consists of mineral layers which are significantly altered by the formation of iron oxides and clay minerals. Typically, brownish or reddish due to the iron oxides, the “B” horizon is harder and more compact than the overhead topsoil. Even if not marked by browns or reds or browns it is generally paler in colour than the topsoil owing to the relative absence of humus. But if less organic, this horizon is richer in minerals which tend to sink down from the topsoil. The subsoil will contain deeper roots of trees and the like, but most plant roots remain within the topsoil. The lack of humus and roots means that this layer attracts comparatively few soil organisms.
The “C” Horizon (Parent Material)
Average depth: 30-48 inches. (Maximum 80 inches)
Mostly made up of large rocks, the defining feature of the “C” horizon is that it is so little affected by pedogenesis, compared to horizons “A” and “B”. Clay illuviation (the spreading of clay across the profile under the influence of water), if present, is not a significant issue. The formation of this horizon is either the result of a specific deposit (from flood or landslide) or it formed from the fragmentation of the bedrock, below. It is called the “parent material” because the upper layers developed from this layer.
The “R” Horizon (Bedrock)
This horizon, which sits at the foot of the soil profile, is made up of a large solid mass of rock, usually only partially weathered. It contains no organic matter.
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