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.

Organic matter plus pore water and air; texture alone is incomplete.

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

A soil profile records processes with depth: surface layers are usually richer in organic matter, while deeper layers show mineral accumulation and parent material.

Interpret colour, texture, roots and translocation as evidence of inputs, decomposition, leaching or accumulation. A horizon difference is a process clue, not a name to memorize.

A dark surface horizon suggests organic inputs and humus; a clay-enriched layer below suggests downward movement and deposition.

Removal of soluble minerals or clay by percolating water; confirm with texture and chemistry evidence.

Colour alone cannot identify a horizon process; combine several profile clues.

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

Litter and root residues are biological inputs; dust, rainfall and flood sediment are transported inputs; fertilizer, manure and irrigation are managed inputs. State source and pathway.

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

Both add water, but irrigation is a human-controlled input with different timing, quality and allocation implications.

‘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: nitrate can leach to groundwater, CO₂ diffuses to air, water evaporates, and topsoil erodes downslope. The same process can help one store while depleting another.

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

Erosion exports particles; evaporation would remove water, not 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 enters soil; percolation moves through it; leaching carries dissolved substances; groundwater flow moves below; aeration exchanges gases; erosion exports particles.

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

Erosion; the material is detached and transported, not dissolved 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 converts organic matter into smaller compounds; weathering breaks rock into minerals; microbes change nutrient forms; evaporation can concentrate salts in soil water.

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

No, it is a transfer; the chemical form remains nitrate during that movement.

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 enters organic carbon, decomposes to mineral nitrogen, plants take it up, and respiration returns carbon as CO₂.

It names source, destination, material and process; unlabeled arrows cannot be audited.

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.

No. Fertilizer supplies mineral nutrients; carbon enters mainly as atmospheric CO₂ through photosynthesis.

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.

It removes pore space and oxygen pathways, shrinking the range of available 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.

A saprotroph such as a fungus; detritivores mainly ingest and fragment material.

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.

Sand-rich soil, assuming comparable structure and compaction.

‘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.

Include plant residues, root inputs, decomposition, respiration, erosion and management over a defined time and depth. Stock change is a rate/balance question, not colour or biomass alone.

If soil receives 50 units of carbon and loses 40, the stock rises by 10 for that period.

Yes, if current outputs exceed inputs; a large stock can be declining.

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.

O and A, because litter and humus-rich surface material are exported first.

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 organic matter; relief changes drainage and erosion; parent material supplies minerals; time allows horizons and chemistry to develop.

Steep wet slopes may remain shallow because erosion removes developing soil faster than weathering builds it.

Relief, organisms, drainage and time can change 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.

Particle percentages describe texture; organic matter estimates carbon-rich material; water content and infiltration describe movement; bulk density indicates compaction; pH describes acidity. State what the result cannot show.

To test compaction, bulk density and infiltration are more useful than colour; to test nutrient availability, pH and chemical extraction are needed.

Infiltration (with texture, structure and moisture context), not organic matter alone.

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 exposes organic matter and can accelerate oxidation; wetland drainage exposes carbon to oxygen. Warming can increase decomposition, release greenhouse gases and intensify climate pressure, but rates depend on moisture and substrate.

Drainage lowers waterlogging, increases oxygen and can shift a wet soil from CH₄ production toward CO₂ release.

Warming or disturbance releases GHGs, which increases warming and can accelerate further soil carbon loss.

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