5.1 Soil

Soil texture defines the physical make-up of the mineral portion of soil. It depends on the relative proportions of sand, silt, clay and humus it contains. Sand particles are the largest and feel gritty; silt particles are medium-sized and feel smooth, almost like flour; clay particles are the smallest, sticky when wet, and pack tightly together. Texture can be determined with a simple feel test, or more precisely in a laboratory by mixing a sample with water and letting the particles settle into layers by size.

The full sand/silt/clay classification triangle, used to name a soil's exact texture class, is in the HL Extension tab.

A soil's texture strongly influences how productive it can be, through three competing effects: how well it retains nutrients versus how easily nutrients leach away, how well it retains water versus how freely it drains, and how well it aerates versus how easily it becomes compacted or waterlogged. Humus, the dark, crumbly product of partially decayed plant material, contributes disproportionately to a soil's texture wherever it is abundant. Click each soil type below to see how it behaves.

Illustration: Kaerii, CC BY-SA 4.0.

Soils can act as a carbon sink (absorbing more carbon than they release), a store (holding carbon at a roughly steady level) or a source (releasing more carbon than they absorb), depending on the relative rates of dead organic matter input and decomposition. When input outpaces decomposition, carbon accumulates. When decomposition outpaces input, stored carbon is released, mostly as carbon dioxide.

In the tropics, heat and moisture keep decomposers extremely active, so dead organic matter is broken down and its nutrients recycled back into living plants within weeks. Almost all of the ecosystem's carbon stays locked in standing biomass rather than accumulating in the soil. In cold or waterlogged environments, tundra, peat wetlands and temperate grasslands, decomposition is slow: low temperatures or a lack of oxygen in saturated ground both suppress decomposer activity, so dead organic matter piles up over centuries to millennia instead of breaking down. Northern permafrost soils alone are estimated to hold roughly twice as much carbon as the entire atmosphere.

Run four real soils, not one generic one

Pick a real biome, set how much organic matter is going in, then watch 30 years unfold. Underneath, decomposition speed is set by a real number called k, borrowed from actual published decomposition studies, not a made-up scale.

What k means: k is the share of the stored carbon that breaks down in a year. A higher k means faster decomposition. Real studies put tropical rainforest at k ≈ 4.0/yr, temperate forest at k ≈ 0.3–0.5/yr, and tundra at k ≈ 0.17/yr, which is exactly why warm wet places lose carbon fast and cold wet places don't.

k values: global litter decomposition synthesis (Gholz et al. 2000, Journal of Plant Ecology review) and temperate deciduous forest decomposition studies. Model: simplified first-order decay (input minus k × current stock per year), not a full carbon-cycle simulation.

Application of skills, statement 5.1.3: sample and compare two subsoils.

The standard investigation samples two soils from the subsoil, the B horizon, one from a managed environment such as a garden or field, and one from a natural, undisturbed ecosystem. Both samples are then tested for texture, organic matter content, nitrogen/phosphorus/potassium (NPK) concentration, aeration, drainage and water retention, so the two can be compared directly.

One common part of this investigation determines the amount of organic carbon in a dry soil sample by burning off its organic matter and measuring the resulting change in mass. The sample is weighed, heated until only the mineral fraction remains, then weighed again; the mass lost is the organic matter that burned away.

Application of skills, statement 5.1.8: create a systems flow diagram representing the soil system.

A system diagram sorts every component of a system into three categories: inputs (matter or energy entering the system), processes (what happens to that matter or energy while inside the system), and outputs (matter or energy leaving the system). Sort the seven components below into the category each belongs to for the soil system.

Every term introduced in this subtopic. Terms marked HL are only required at Higher Level.

Soil

A complex mixture of inorganic and organic components, water and air, forming a dynamic system within the larger ecosystem.

Dynamic system

A system with its own inputs, outputs, storages and flows, in continuous motion rather than fixed or static.

Inorganic components

Mineral matter in soil, rock fragments, sand, silt and clay, produced by weathering of the parent rock.

Organic components

Living soil organisms and material from the decay of dead organisms.

Soil profile

The stable, layered structure a soil develops over long periods of time, made up of distinct horizons.

Horizon

A distinct horizontal layer within a soil profile, distinguished by its composition and appearance.

Input (soil system)

Matter entering the soil system, such as dead organic matter, weathered minerals or fertilizer.

Output (soil system)

Matter leaving the soil system, such as nutrients absorbed by roots, eroded particles or evaporated water.

Transfer

Matter moving into, out of, or across the horizons of a soil without changing what it is, such as infiltration or leaching.

Transformation

A process that changes soil components or the soil system itself, such as decomposition or weathering.

Seed bank

The reserve of viable seeds present within soil, one of the ways soil supports plant growth.

Detritivore

An organism, such as an earthworm, that feeds on dead organic matter and breaks it into smaller fragments.

Saprotroph

An organism, such as a fungus or bacterium, that chemically decomposes dead organic matter.

Biogeochemical cycle

The cycling of chemical elements between living organisms and the physical environment; soils play a central role in these cycles.

Soil texture

The physical make-up of the mineral portion of soil, determined by the relative proportions of sand, silt, clay and humus.

Humus

A dark, crumbly substance formed by the partial decay of dead plant material, lying beneath the leaf litter.

Carbon sink

A soil (or other system) absorbing more carbon than it releases, so stored carbon increases over time.

Carbon source

A soil (or other system) releasing more carbon than it absorbs, so stored carbon decreases over time.

O horizon HL

The uppermost organic layer of a soil profile, made up of undecomposed or partially decomposed plant litter.

A horizon HL

The layer just beneath the O horizon, rich in organic matter and also known as the mixed layer or topsoil.

B horizon HL

The mineral subsoil layer beneath the A horizon, with little organic matter but often rich in leached minerals.

C horizon HL

The layer of weathered parent rock beneath the B horizon, not yet broken down into true soil.

Topsoil HL

Another name for the A horizon; the most nutrient-rich and biologically active layer, but also the most vulnerable to erosion.

Cation-exchange capacity (CEC) HL

A measure of how many positively charged nutrient ions a soil can hold and make available to plants; clay has a far higher CEC than sand.

Soil texture triangle HL

A diagram used to classify a soil's exact texture class from its percentages of sand, silt and clay.

Leaching HL

The downward movement of dissolved nutrients through soil horizons by percolating water, removing them from the upper layers.

Parent material HL

The underlying geological material, rock or sediment, from which a soil's mineral component is derived through weathering.

Methane clathrate HL

An ice-like structure trapping methane molecules within underlying geological formations; its breakdown under warming is a potential carbon tipping point.

Tipping point HL

A threshold beyond which a system shifts to a substantially different state, sometimes irreversibly.

Soil profile diagrams record the depth, color and character of each horizon, and that record can be used to classify a soil into a type. Because climate and vegetation are two of the strongest controls on how a soil profile develops, each soil type tends to recur under the same biome wherever it forms. Match each soil profile description to the biome it typically forms under.

A mature soil profile develops four distinct horizons, each shaped by a different balance of inputs, outputs, transfers and transformations. Click each horizon to see its detail.

Diagram: US Department of Agriculture, adapted by Hridith Sudev Nambiar and Wilsonbiggs, CC BY-SA 4.0.

The A horizon is also called topsoil; it is the most nutrient-rich and biologically active layer, but also the most vulnerable to erosion, since it sits at the surface once the O horizon is removed or absent.

Five factors, working together over time, determine what kind of soil forms in a given location: climate, organisms, geomorphology (the shape of the landscape), geology (the parent material) and time itself. Match each scenario below to the factor it illustrates.

Sand, silt and clay particles differ enormously in size, and that difference in size drives a difference in surface area. Clay particles are smallest and most numerous per gram, giving them by far the largest surface area, and therefore the most negatively charged sites available to bind positively charged nutrient ions such as calcium, magnesium and potassium. This capacity is called the cation-exchange capacity (CEC). Organic matter adds further exchange sites on top of whatever the mineral texture provides, which is one reason humus-rich soils are so fertile.

The soil texture triangle is a ternary diagram: each side represents the percentage of sand, silt or clay in a sample, and the point where all three percentages intersect gives the sample's texture class, for example loam, sandy loam or clay. Use the sand/silt/clay percentages given below to work out each sample's texture class.

Diagram: cmglee and Mikenorton, adapted from US Department of Agriculture data, CC BY-SA 4.0.

When decomposition outpaces new organic input, stored soil carbon is released back to the atmosphere, mostly as carbon dioxide. Under some conditions it can instead be released as methane, a gas that traps substantially more heat per molecule than carbon dioxide over short timescales. One proposed mechanism involves methane clathrates, ice-like structures that trap methane within certain geological formations; rising temperatures can, in principle, destabilize these structures and release the trapped methane, a possible climate tipping point: a threshold beyond which a system shifts to a substantially different, and potentially irreversible, state.