A living, open system

Soil is a dynamic system within the larger ecosystem that has its own inputs, outputs, storages and flows. Matter and energy continually enter, leave, move and change within it. That’s what makes soil dynamic—not the simple movement of the soil itself.

A soil system is an open environmental system, exchanging matter and energy with its surroundings. It stores mineral particles, organic matter, organisms, water, air and nutrients. Through inputs and outputs, the system connects with the atmosphere, vegetation, underlying rock and nearby aquatic systems.

Soils provide resources for life, yet they differ greatly according to parent material, climate, organisms, relief, time and human management. These differences affect fertility and drainage, as well as carbon storage and the communities each soil can support.

The central idea in this topic is that soils sustain natural systems. They support plants and habitats, enable decomposition and nutrient recycling, and regulate water. Human activities can alter soil stability by changing inputs, removing upper layers, compacting the soil, accelerating erosion or disrupting transfers and transformations.

Four interacting components

Soil contains inorganic components, organic components, water and air. Rather than forming separate piles, these materials interact within a soil ecosystem that contains distinct soil organisms.

• Mineral matter is the inorganic component of soil formed from weathered parent rock, such as rock fragments, sand, silt and clay.
• Organic matter is carbon-containing material in soil derived from living organisms or their remains, including roots, microorganisms, animals, litter and partly decomposed material.
• Water fills pore spaces and transports dissolved nutrients.
• Air occupies pore spaces that aren't filled with water, supplying gases for respiration and other biological processes.

Rainfall, drainage, compaction and decomposition can alter this balance. In waterlogged soil, for instance, fewer pores contain air, so less oxygen is available.

Published identification keys use observable or measured properties to classify soils. For a key to work well, observations must be consistent. Colour, texture and other features should therefore be recorded according to the stated criteria, not guessed.

Development of a profile

A soil profile is the vertical development in soils characterized by different layers which can be distinguished by texture, structure, colour and pH. It is a vertical section through the soil, rather than a surface view.

A soil horizon is a horizontal soil layer with physical or chemical properties that distinguish it from adjacent layers. Over long periods, weathering and decomposition help create a relatively stable layered profile. Water movement, biological mixing and nutrient transfers also contribute. Organic material tends to be more abundant near the surface; further down, inorganic material increasingly dominates.

Comparing two B-horizon samples

To make the investigation fair, collect B-horizon samples from two contrasting sites: one in a garden or field and the other in a natural ecosystem. Keep the sampling depth, sample volume, equipment and procedures the same. Soil can vary over very short distances, so take replicates.

Investigate texture, organic matter, nitrogen, phosphorus and potassium concentrations, aeration, drainage and water retention. Use a feel test or sedimentation to examine texture, while nutrient kits can estimate NPK. Time infiltration or drainage using equal volumes of water. Changes in mass can then show how much water was retained.

Organic matter can be estimated to provide an operational estimate related to soil carbon. Dry a known sample to constant mass, then heat it safely in a crucible to burn off the organic matter. Cool it in a desiccator and reweigh it. Calculate:

The mass lost provides an estimate of organic matter, but it isn’t necessarily pure carbon because other volatile material may also be lost.

Wear eye protection and use heatproof equipment and tongs. Avoid inhaling soil dust or combustion products.

Matter and energy entering soil

An input is a flow of matter or energy entering a system. Inputs to soil can come from the surrounding ecosystem or another ecosystem. People may add them too.

Dead organic matter enters as plant litter, dead animal biomass or manure. Mineral material comes from the weathering of underlying parent rock, the deposition of wind-blown or waterborne particles, decomposition, and precipitation carrying dissolved minerals. Guano provides an external input of organic matter and nutrients. The soil system also receives gases, air, water, humidity and solar energy.

People add compost, fertilizers, agrochemicals and irrigation water to managed soils. These inputs can increase productivity, though their effects depend on how much is used and how it is managed. Excess fertilizer may be leached. Agrochemicals can affect non-target organisms, while irrigation may contribute to salinization. Strictly speaking, salinization is the resulting transformation or condition; the direct input is usually salt-bearing water or added salts.

Losses from the system

An output is a flow of matter or energy leaving a system. In soil, outputs include losses of minerals, water and heat. Dead organic matter may also be lost or modified through decomposition.

Wind and water erosion can carry away mineral particles. Plant roots draw water and dissolved mineral nutrients from the soil store, while leaching moves soluble nutrients and water downward, potentially beyond the soil system. Gases diffuse between the soil and atmosphere. Water leaves through evaporation, and thermal energy is lost as heat.

Keep two ideas separate here. Erosion may remove the soil body itself. Root uptake, decomposition, leaching, gas diffusion and evaporation, by contrast, remove or transform particular components without necessarily taking away the whole soil. Repeated outputs can still reduce fertility, alter structure and degrade productive soil.

Movement without chemical change

A transfer is the movement of matter or energy between locations without changing its chemical nature. It may occur within a horizon or across horizon boundaries. Matter and energy can also move into or out of the soil.

• Infiltration is the transfer of water from the ground surface into soil.
• Percolation is the downward transfer of water through soil and permeable material under gravity.
• Groundwater flow is the subsurface transfer of water through saturated pores and fractures.
• Biological mixing is the redistribution of soil material by organisms, including burrowing animals, roots and microorganisms.
• Aeration is the exchange and movement of gases between soil pores and the atmosphere.
• Erosion is the detachment and transport of soil material by agents such as water or wind.
• Leaching is the downward transfer of dissolved substances through soil by percolating water.

These processes shouldn’t be confused. Infiltration takes water across the soil surface, while percolation carries it through the profile. Erosion moves particles; leaching moves dissolved material.

Processes that change soil components

A transformation is a process that changes matter or energy into a different form within a system. In soil, transformations alter individual components and may gradually change the character of the whole soil.

Decomposition is the biological and chemical breakdown of dead organic matter into simpler substances. Mineral nutrients are released for reuse, while respiration transfers some carbon to the atmosphere.

Weathering is the physical, chemical or biological breakdown of rock in place. This produces mineral particles and can release mineral nutrients into the soil.

Nutrient cycling is the repeated transfer and transformation of chemical elements between organisms and the physical environment. Plants take up mineral nutrients and incorporate them into biomass. These nutrients return through litter, death and decomposition.

Salinization is the accumulation of soluble salts in soil to concentrations that impair soil functioning or plant growth. Dissolved salts enter through irrigation water. Capillary movement can then draw saline water upward; when the water evaporates, the salts remain. Poor drainage worsens this accumulation.

Representing the soil system

A systems flow diagram is a model that represents storages as boxes and flows as arrows connecting them. Arrows that cross the system boundary show inputs or outputs, while arrows between boxes show internal transfers. Label each transformation beside the flow or storage where it occurs.

A soil diagram might show storages such as surface litter, soil organic matter, mineral nutrients, soil water and soil organisms. Precipitation, litter and weathering can act as inputs. Outputs may include leaching, erosion, evaporation, root uptake and heat loss, while infiltration, decomposition and nutrient release are internal flows.

Start by drawing a clear system boundary. Add the storages next, then connect them using labelled arrows. Different arrow widths should only be used when they genuinely show relative flow magnitude.

Soil connects the water, carbon and nitrogen cycles. It stores and transfers water, receives and decomposes carbon-rich organic matter, and changes nitrogen between organic and mineral forms. Damage to soil can therefore affect several larger Earth systems at the same time.

Foundation of terrestrial ecosystems

Soil anchors plant roots and provides a seed bank, a water store and a nutrient store. Air-filled pores within it also supply the oxygen roots need for respiration.

The main mineral nutrients stored in soil are nitrogen, phosphorus and potassium. Plants need nitrogen to make amino acids and other nitrogen-containing compounds. Phosphorus forms part of compounds used in energy transfer and genetic material, while potassium helps with processes such as water regulation and enzyme activity.

Carbon is the key exception. Rather than absorbing it from soil as a mineral nutrient, plants obtain most of their carbon from atmospheric $CO_2$. They convert this carbon into organic matter, some of which eventually enters the soil through litter and dead roots.

Plants form the producer base of terrestrial food webs. As a result, soil depth, water availability, aeration and nutrient supply affect productivity throughout terrestrial ecosystems.

Habitat and niche

A habitat is the environment in which an organism lives. A niche is the role of a species in an ecosystem, including how it uses resources and interacts with biotic and abiotic conditions. For many species, soil provides both.

Soil communities contain bacteria and other microorganisms, fungi, plant roots and animals of many sizes. Pore spaces, litter, aggregates and varying soil depths form contrasting microhabitats, with differences in moisture, oxygen, temperature, pH and food availability. As a result, species fill different niches as decomposers, detritivores, predators, grazers, parasites and ecosystem engineers.

Much of soil biodiversity remains undescribed. Apparently plain ground can support a remarkably complex community, and soil degradation may remove species before they are identified.

Recycling elements

A biogeochemical cycle is a pathway through which a chemical element moves between living organisms and the non-living environment. Within these cycles, soil acts as both a store and a processing zone.

Most organic input comes from dead plant material. Detritivores are consumers that ingest dead organic material and fragment it, which increases the surface area available to decomposers. Saprotrophs are organisms that digest dead organic matter externally and absorb the soluble products. Important soil saprotrophs include fungi and bacteria.

The sequence begins with litter input. Detritivores fragment the material, then saprotrophs decompose it chemically. Mineral nutrients are released, taken up by plants and incorporated into new biomass. Carbon may enter longer-term soil organic stores or leave through respiration; mineral nutrients can be retained, absorbed or leached.

Determining physical make-up

Soil texture defines the physical make-up of the mineral soil. It depends on the relative proportions of sand, silt, clay and humus. Put simply, texture describes the balance between coarse and fine particles, including the contribution made by humus.

A classification key or feel test can give an estimate of soil texture. Sand feels gritty. Silt has a smooth or floury feel, while wet clay is sticky and can be moulded. Although quick, the feel test is subjective, so every sample should be tested using the same procedure and under the same moisture conditions.

For a sedimentation test, mix the soil thoroughly with water and leave the particles to settle before measuring the separate layers. The larger sand particles settle first, followed by silt; clay stays suspended for the longest time. Organic material may float. Use the proportion of each mineral fraction with an appropriate classification key.

Texture controls growing conditions

Primary productivity is the biomass gained by producers in a specific area in a specific amount of time. Soil texture affects primary productivity through nutrient retention and leaching. It also controls water retention and drainage, as well as aeration, compaction and waterlogging.

Sand has relatively large particles and pores, so it usually drains and aerates well. However, it holds less water and fewer dissolved nutrients. Drought and leaching can therefore limit plant growth. Clay has very small particles and pores. It holds water and mineral ions effectively, but it may drain poorly, become compact and develop oxygen-deficient conditions. Silt has intermediate-sized particles and can retain water. When silty soil has a weak structure, though, it’s vulnerable to erosion and compaction.

Humus is a dark brown or black, loose and crumbly substance beneath leaf litter formed by the partial decay of dead plant material. When abundant, humus contributes strongly to soil texture. It retains water and mineral nutrients while improving aggregation and aeration.

A balanced soil contains water-holding pores alongside larger, air-filled drainage pores. No single texture is universally perfect because productivity also depends on climate, plant species and management. Even so, a mixture that prevents both rapid drought and prolonged waterlogging is often favourable.

The balance of inputs and decomposition

A carbon sink is a reservoir that absorbs more carbon than it releases over a stated period. A carbon store is a reservoir that contains carbon for a period of time. A carbon source is a reservoir or process that releases more carbon than it absorbs over a stated period.

Whether soil acts as a sink, store or source depends on the balance between inputs of dead organic matter and outputs from decomposition. When inputs exceed decomposition losses, the carbon store grows and the soil acts as a sink. When the two rates are balanced, the store stays broadly stable. If decomposition and other losses become greater than inputs, the store shrinks and the soil acts as a source.

Tropical forest soils usually contain relatively little carbon. Warm, moist conditions allow rapid decomposition, while released nutrients are quickly absorbed into biomass. As a result, the forest may store considerable carbon above ground despite having a modest soil carbon store.

Tundra soils contain more carbon because low temperatures reduce decomposer activity. Frozen or waterlogged conditions also restrict oxygen. Wetland soils are saturated and often oxygen-poor, which slows decomposition and allows partly decomposed organic matter to accumulate. In temperate grasslands, dense root systems provide substantial below-ground inputs, allowing carbon to build up in the soil.

Warming, drainage or disturbance may speed up decomposition, turning a former sink or stable store into a source. Soil stability is therefore directly connected to climate regulation.

Reading the whole profile

Soils are classified and mapped by looking at the entire soil profile, not just a handful taken from the surface. Evidence comes from the sequence and thickness of horizons, along with colour, texture, structure, organic content and signs of leaching or mineral accumulation.

A profile drawing works as a descriptive model. Show the layers in the correct vertical order and at their relative thicknesses. Then annotate the transfers and transformations that produced the visible features—for example, litter input, decomposition near the surface, downward leaching and mineral accumulation deeper in the profile.

Profile diagrams can help connect soils with biomes. Brown earth profiles are linked to temperate deciduous forest conditions. Here, litter decomposition and biological mixing create a relatively well-developed upper soil. Oxisol profiles occur under tropical rainforest conditions, where intense chemical weathering and leaching form deeply weathered, mineral-rich soil that is often nutrient-poor.

To classify an unfamiliar diagram, focus on the evidence in the profile rather than its name. Look at which horizons are present, where organic matter occurs, and whether leaching or accumulation dominates.

O, A, B and C horizons

Horizons are horizontal strata that are distinctive to the soil type. The key horizons are organic layer, mixed layer, mineral soil and parent rock (O, A, B and C horizons). Together, these layers show the effects of inputs, transfers and transformations.

• The O horizon forms the organic surface layer. It contains litter and material at different stages of decomposition.
• The A horizon is the organic-rich mixed layer, or topsoil. It contains mineral particles and humus, with abundant biological activity.
• The B horizon is the mineral subsoil. It commonly receives clay or dissolved minerals transferred from the layers above.
• The C horizon lies beneath the developed soil and consists of partly weathered parent material.

Natural soil systems commonly keep all four horizons: O, A, B and C. With intensive agriculture, repeated cultivation, vegetation removal and erosion may remove or severely degrade the O and A horizons, leaving mainly B and C material. The effects are substantial. The soil loses its seed bank, humus, soil organisms, nutrients, water-holding capacity and productive structure. Fertilizer inputs alone cannot recreate the full function of the upper soil ecosystem.

The productive mixed layer

The A horizon is the layer of soil just beneath the uppermost organic humus layer, where present. It is rich in organic matter and is also known as the mixed layer or topsoil. Here, mineral particles mix with humus. Roots and soil organisms closely connect the layer to plant growth and decomposition.

For plant growth, this is normally the most valuable horizon. It contains nutrients, biological activity and a seed bank, while storing water and providing a structure that allows root penetration and aeration. Along with the O horizon, it is the layer most exposed to erosion by wind and water.

Removing or degrading the A horizon destabilizes the soil system. Nutrient and carbon stores decline, infiltration may fall, and runoff and erosion can increase. Primary productivity also drops. Sustainable management keeps the layer covered, limits compaction and disturbance, maintains organic inputs and reduces erosive runoff.