5.1 Soil

A soil system needs a stated boundary so stores, inputs, outputs and internal flows can be tracked.

Choose the depth and area first. Then label water, organic matter, minerals and organisms as stores; rainfall, litter and fertilizer as inputs; leaching, erosion and gases as outputs.

For a farm plot, fertilizer enters, nitrate leaches downward, carbon is stored in humus and CO₂ leaves through respiration.

Inputs and outputs change when depth, area or time window changes.

A system diagram without a boundary cannot show whether matter is entering, leaving or merely moving internally.

Soil combines mineral particles, organic matter, pore water and pore air; it is not solid rock or solids only.

Rock fragments provide structure; humus stores carbon and nutrients; pore water transports dissolved substances; pore air supplies gases to roots and microbes. Their proportions control function.

A compacted soil may contain the same minerals but less pore air and water movement, restricting roots.

A complete account of soil composition includes mineral particles, organic matter, pore water and pore air; texture alone describes only part of the system.

Pore space is part of the system even though it is not a solid ingredient.

A soil profile records long-term processes with depth: upper horizons are usually richer in organic matter, while deeper horizons show mineral movement, accumulation and parent material.

Interpret colour, texture, roots and translocation together. For a fair investigation, sample the B horizon from one managed garden or field and one natural ecosystem at comparable depth and sample size; compare texture, organic matter, NPK, aeration, drainage and water retention.

The mass lost after heating estimates burned organic matter. Use dry samples and the same procedure for both soils; the estimate does not by itself identify which carbon compounds were present.

Colour alone cannot identify a horizon process or carbon content; combine profile observations with measured texture and chemistry.

Soil inputs can be natural internal recycling, externally transported material or deliberate management additions.

Plant litter, dead animals, manure and decomposition add organic matter; weathering, wind- or water-borne deposition and precipitation add minerals or water; air, gases, humidity and solar energy also cross the boundary. Managed systems add compost, fertilizer, agrochemicals and irrigation, which can also increase salinity.

Compost added to a field is a managed carbon and nutrient input, while leaf fall is a natural seasonal input.

Rainfall and irrigation both add water, but irrigation is a human-controlled input whose timing, dissolved salts and allocation can alter the soil system.

‘Natural’ describes source, not whether the input is beneficial; excess natural sediment can still bury soil.

A soil output changes a store through plant uptake, leaching, gas exchange, evaporation, heat loss or erosion.

Name the material and destination: decomposition reduces dead organic matter and releases gases; roots absorb water and minerals; nitrate leaches to groundwater; CO2 diffuses to air; water evaporates; heat leaves the system; and wind or water can export particles. Component loss is different from removal of the whole soil by erosion.

Heavy rain carries nitrate below roots, so soil nitrogen falls while groundwater nitrate rises.

Erosion exports soil particles and their contents, whereas evaporation removes water without directly removing soil mass.

An output is not disappearance; trace where the material goes.

Transfers move water, particles or dissolved substances; name the material and destination before choosing infiltration, percolation, leaching, flow, mixing, aeration or erosion.

Infiltration moves water into soil; percolation moves it through horizons; leaching transports dissolved substances; groundwater flow moves water below; burrowing organisms mix particles; aeration exchanges gases; and erosion detaches and exports particles.

Rain infiltrates, nitrate leaches with percolating water, and a stream later carries the dissolved load away.

Soil particles detached and transported by wind or water undergo erosion, not dissolved-solute leaching.

Infiltration and percolation are stages of movement, not interchangeable labels.

A transformation changes form or chemical identity; decomposition, weathering, nutrient cycling and salinization are not simple movement.

Decomposition changes organic matter into smaller compounds; weathering changes parent material; microbes transform nutrient forms; and evaporation after irrigation can concentrate dissolved salts until solid salts accumulate, producing salinization.

Dead leaves become mineral nutrients through decomposition; the nitrogen is chemically transformed before plants absorb it.

Nitrate moving downward unchanged is a transfer; a transformation changes its chemical or physical form.

A process can include both movement and transformation; identify which step the question asks about.

A useful soil diagram links stores to named inputs, outputs, transfers and transformations for one material and boundary.

Start with water, organic carbon or mineral nutrients; draw arrows with direction and process. Check whether each arrow changes location, chemical form or both.

Leaf litter adds organic carbon and nitrogen to a soil store. Decomposition returns some carbon to the atmosphere as CO2 and mineralizes organic nitrogen into forms plants can absorb; infiltration, uptake and leaching link the same soil boundary to the water and nitrogen cycles.

An auditable arrow names the material, source, destination and process; unlabeled arrows cannot distinguish a transfer from a transformation.

A diagram is not complete because it has many arrows—every arrow needs a defined boundary and mechanism.

Soil provides anchorage, water storage, nutrients and a seed environment; atmospheric CO₂ supplies most plant carbon.

N, P and K support different functions, but fertilizer cannot replace light, water, roots or carbon fixation. Trace the resource to its source and process.

A seed bank and moist soil support germination, while leaves take CO₂ from air and photosynthesize biomass.

Fertilizer supplies selected mineral nutrients; most plant carbon enters as atmospheric CO2 fixed during photosynthesis, not from soil fertilizer.

Plant nutrition is not one soil input; distinguish water, minerals, anchorage and carbon.

Soil supports diverse organisms because pore size, depth, moisture, oxygen and food create different microhabitats.

Large pores drain and aerate; small pores retain water; surface litter supplies carbon; deeper layers differ in oxygen and chemistry. Microbes, fungi and animals partition these conditions.

A water-filled deep pore favours anaerobic microbes, while an oxygenated surface pore supports aerobic decomposers.

Compaction removes pore space and blocks oxygen and water pathways, reducing the range of soil microhabitats and niches.

Soil life is not evenly distributed; conditions vary over centimetres and depth.

Detritivores fragment litter; saprotrophs secrete enzymes and decompose it, releasing nutrients that plants can reuse.

Keep physical breakdown separate from chemical decomposition. The chain is litter → fragments → microbial breakdown → inorganic nutrients → plant uptake.

Earthworms shred leaves, fungi decompose the fragments, and nitrate released later enters plant roots.

Fungi and bacteria are saprotrophs that carry out extracellular chemical decomposition; detritivores such as earthworms mainly ingest and fragment litter.

Decomposition is not the same as eating litter; identify the chemical transformation step.

Sand feels gritty, silt smooth or floury, and clay sticky when wet; texture is the relative proportion of these particle sizes.

A settling test estimates proportions by how quickly particles fall. Humus modifies aggregation, water retention and nutrient behaviour but does not change the mineral-size label itself.

A jar with a thick slow-settling layer suggests more clay; gritty feel suggests sand-rich soil.

Colour may reflect humus or iron; texture requires particle feel or measured proportions.

Texture and structure are different: particle proportions do not fully describe aggregates and pore spaces.

Texture creates trade-offs: sand drains and aerates quickly, clay retains water and nutrients, and loam often balances both.

Clay’s surface area can retain ions and water but may drain slowly; sand can lose nutrients and dry rapidly. Humus and structure modify the prediction.

A sandy field may need frequent irrigation and nutrient management; a clay field may waterlog after heavy rain but retain more nutrients.

With comparable structure and compaction, sand-rich soil usually infiltrates water faster; clay-rich soil usually retains more water and nutrients but may drain and aerate poorly.

‘Clay is fertile’ is not universal; retention can coexist with poor aeration or waterlogging.

Compare carbon inputs and outputs: inputs greater than outputs make soil a sink, outputs greater make it a source, and equality is balance.

Compare inputs from litter and roots with losses through decomposition, respiration and erosion over a stated depth and period. Tropical forest soils often store little carbon because warm, moist conditions speed decomposition; tundra and waterlogged wetlands accumulate carbon because cold or low-oxygen conditions slow decay, while temperate grasslands add substantial carbon through dense roots.

A soil is a sink while inputs exceed outputs, a source while outputs exceed inputs, and a store describes the carbon stock present; the same soil can hold a large store yet currently be a source.

Classify sink or source from the net rate of stock change, not from dark colour or the absolute size of the carbon store.

A sink is defined by net rate, not by having a large carbon store.

Classify a soil profile by horizon depth, colour, organic matter and translocation evidence, not one visual cue.

Brown-earth-type profiles commonly show organic-rich surface and active cycling; oxisol-type profiles are deeply weathered, iron-rich and nutrient-poor. Treat these as evidence patterns, not labels without data.

A deep red, strongly weathered profile with low nutrient retention supports an oxisol interpretation more than a dark, moderately leached profile.

Iron, moisture and organic matter can all alter colour; combine depth, texture and chemistry.

A profile type is an inference from processes, not a name assigned from one horizon.

O is organic litter, A is topsoil mixed with minerals, B is a zone of accumulation, and C is weathered parent material.

Use the sequence to explain farming loss: erosion and tillage can remove O/A material while leaving deeper B/C horizons, reducing roots, humus and biological activity.

A field with only B and C near the surface has lost much of its organic topsoil, even if mineral soil remains.

Erosion usually exports the O and A horizons first, removing litter, humus-rich topsoil, roots and biological activity before the deeper B and C horizons.

Horizon letters describe material and process, not fixed universal thicknesses.

Topsoil combines humus, roots, microbes, oxygen, water and nutrients; losing its structure cannot be replaced by fertilizer alone.

Fertilizer adds selected nutrients but not depth, pore habitat, organic carbon, water storage or the biological networks that regulate release and decomposition.

Adding nitrogen to eroded subsoil may raise growth briefly, but poor roots and low water retention still limit the crop.

Cover, organic inputs and erosion control rebuild structure and biology over time.

Fertilizer can supplement soil; it cannot recreate lost topsoil volume and function quickly.

Soil formation reflects parent material, climate, organisms, relief and time acting together.

Climate controls weathering and leaching; organisms add and transform organic matter; relief changes drainage, erosion and deposition; parent material supplies minerals and influences pH; and time allows horizons and chemistry to develop. These controls interact rather than acting independently.

Calcareous parent rock rich in calcium carbonate tends to form alkaline soil that may have low organic matter, nitrogen and phosphorus. Volcanic material can weather to release minerals such as magnesium and potassium, often supporting fertile young soils.

Parent material does not determine soil alone: climate, organisms, slope, drainage and time modify weathering, inputs and losses.

No single factor ‘determines’ soil; identify the interacting controls and the process each changes.

Clay particles are much smaller than silt and sand and generally provide more charged surface for cation exchange than quartz-rich sand.

Higher CEC means greater ability to retain positively charged nutrients such as K⁺, Ca²⁺ and NH₄⁺. Humus can also raise CEC, so texture is not the only control.

A clay-humus soil can retain ammonium after rain, while coarse quartz sand is more likely to leach it.

Smaller particles provide more reactive charged surface per mass; confirm mineralogy and organic matter.

High CEC does not guarantee good drainage or fertility; retention and availability are different.

Match each test to the property it measures: texture, organic matter, water, infiltration, bulk density, colour or pH answers different questions.

Use sand, silt and clay percentages that total 100% to locate a point on the soil texture triangle. Other tests answer different questions: loss on heating estimates organic matter, mass change estimates water content, infiltration measures entry rate, bulk density indicates compaction, colour is descriptive and pH measures acidity.

Local practice data give Soil B as 20% clay, 40% silt and 40% sand. Plotting those three percentages on the texture triangle classifies it as loam; that label does not by itself determine organic matter, pH or compaction.

Choose the test from the question: infiltration helps assess water entry, bulk density compaction, pH acidity, and the texture triangle mineral particle proportions.

A measured proxy is not the whole process; name the limitation and context.

Aerobic decomposition mainly releases CO₂; anaerobic conditions can produce CH₄, and warming or drainage can alter the balance and feedback.

Tillage can expose organic matter to oxygen and accelerate CO2 release; wetland drainage similarly increases aerobic decomposition, while saturated anaerobic soils can produce CH4. Warming may accelerate decomposition, although the response depends on moisture, oxygen and available substrate.

A reinforcing feedback occurs when warming increases soil greenhouse-gas release, which adds further warming and carbon loss. The syllabus also identifies a potential tipping point where rising temperature destabilizes methane clathrates in underlying geological structures, causing an additional methane release.

Treat a tipping point as a threshold risk, not a guaranteed outcome: the feedback depends on temperature, carbon form, water and oxygen conditions.

‘Wet soil always emits more greenhouse gas’ is too simple; distinguish CH₄, CO₂, oxygen and net radiative effect.