Pick up a handful of soil and you are holding four things at once: crushed rock, dead and living matter, water, and air. ESS does not treat soil as dirt. It treats it as a system — with things stored in it, things flowing in and out, and things moving and changing inside. Get that idea straight now and the rest of the topic falls into place.
📘 What you need to know
Soil is a mixture of inorganic matter, organic matter, water and air.
The inorganic part is mineral matter — rock fragments, sand, silt and clay — produced by the weathering of parent rock.
The organic part is living organisms (bacteria, fungi, earthworms) plus dead organic matter (decaying plants, animal remains, faeces).
Soil is a system, so it can be broken down into storages, flows (inputs and outputs), transfers and transformations.
A transfer changes the location of something. A transformation changes its chemical nature, state or energy.
Soils develop profiles made of distinct layers called horizons, which change in composition from the surface downwards.
Profiles usually run from organic-rich at the top to mineral-rich deeper down.
Profile development is influenced by climate, vegetation, parent material and time.
The four ingredients
Soil is a complex mixture of parts that interact with each other. Every one of those parts falls into one of four groups.
The exact numbers change from soil to soil. What matters is the idea: solids make up about half, and the other half is pore space shared between water and air.
The inorganic part
This is the mineral matter: rock fragments, and the three particle sizes you will meet again and again — sand, silt and clay. All of it comes from the weathering of parent rock, the rock sitting underneath the soil. Rain, frost, heat, roots and acids break that rock into smaller and smaller pieces.
The organic part
Two halves to this one, and students often only remember the first.
Living organisms — bacteria, fungi, earthworms and plenty more. Soil is a habitat, not just a material.
Dead organic matter — decaying plants, animal remains and animal waste (faeces). This is the raw material that decomposers turn into humus.
Water and air
Water is essential for chemical reactions and for life, and it carries dissolved minerals around the soil. Air supplies the oxygen and other gases that soil organisms and plant roots need to survive.
Here is the bit worth understanding rather than memorising: water and air share the same pore spaces. They are competing for the same gaps. So if a soil fills up with water, the air is pushed out, and roots and soil organisms are left without oxygen. That single fact explains waterlogging, why compacted soil is bad, and half of what comes later in this topic.
Soil as a system
Soils are dynamic systems sitting inside larger ecosystems. Like any system in ESS, you can simplify a soil by breaking it into four things:
Storages — what is held in the soil.
Flows — the inputs going in and the outputs coming out.
Transfers — something changes location.
Transformations — something changes its chemical nature, state or energy.
Draw this shape in your head for any system question: a box of storages, arrows in, arrows out, and processes working away inside.
Storages
Storage
What it is and why it matters
Organic matter
Plant and animal matter at various stages of decomposition. It supplies nutrients, improves soil structure and helps the soil hold water
Organisms
Microorganisms, fungi, bacteria, insects and other living things. They drive nutrient cycling, decomposition and the building of soil structure
Nutrients
Elements plants need to grow, such as nitrogen, phosphorus and potassium. They are held in the soil and released to plants by biological and chemical processes
Minerals
The inorganic part, derived from the weathering of rocks. Minerals set the physical properties of the soil and much of its fertility
Air
Held in pore spaces. Supplies oxygen for root respiration and for microbial activity
Water
The soil acts as a reservoir, holding water for plant uptake and keeping the habitat moist enough for soil organisms
Inputs and outputs
Flow
In or out?
What is moving
Dead organic matter
Input
Leaf litter, dead animal biomass and faeces adding to the organic content
Inorganic matter from rock
Input
Mineral material from the parent material (bedrock) and from weathering of exposed rock
Precipitation
Input
Rain or snow, carrying dissolved minerals into the soil
Energy
Input
Solar radiation and heat, which set soil temperature and the rate of biological activity
Anthropogenic inputs
Input
Human additions such as compost, fertilisers, agrochemicals and irrigation water
Leaching
Output
Dissolved minerals and nutrients washed out of the soil into streams, rivers, lakes and oceans
Uptake by plants
Output
Minerals and water absorbed by plant roots for growth
Soil erosion
Output
Soil particles removed by water or wind, taking topsoil and soil quality with them
Diffusion and evaporation
Output
Gases diffusing out and water evaporating from the soil surface
Notice that leaching appears twice in this topic. As a flow it is an output, because the nutrients leave the soil system altogether. As a process it is a transfer, because water is carrying dissolved minerals from one place to another. Same word, two jobs. Read the question and answer the one being asked.
Transfers and transformations
This is the distinction examiners come back to over and over, so learn the test rather than the list.
The test
Did it just move? → transfer
Did it become something different? → transformation
🧩
Easy way to remember it
Transfer = ferry. A ferry carries you across the water and you get off exactly the same person. A transformation forms something new.
Transfers — a change of location
Transfer
What happens
Infiltration
Water enters the soil from the surface
Percolation
Water moves through the soil and its layers, usually downwards through the profile
Groundwater flow
Water moves through the subsurface layers, often feeding aquifers and other groundwater reserves
Biological mixing
Soil organisms move particles and material around — burrowing animals, earthworms and growing roots. It mixes organic matter with minerals and improves structure
Aeration
Air is circulated through the soil and mixed with it
Erosion
Soil particles are detached and carried away by wind or water
Leaching
Minerals dissolved in water are moved downwards or sideways through the profile, stripping nutrients from the root zone. Worst where rainfall is high or irrigation is excessive
Transformations — a change of nature, state or energy
Transformation
What happens
Decomposition
Microorganisms break organic matter down, releasing carbon dioxide, water and nutrients. Complex organic compounds become simpler ones
Weathering
Physical and chemical processes break rocks and minerals into smaller particles. Physical weathering is mechanical breakdown; chemical weathering alters the minerals through chemical reactions
Nutrient cycling
Nutrients such as nitrogen, phosphorus and potassium are taken up, assimilated, released and recycled within the soil and plant system
Salinisation
Soluble salts build up in the soil, damaging plant growth and structure. Often caused by poor irrigation, high evaporation rates or natural mineralisation
Humification
Organic matter is converted into stable humus, which darkens the soil and improves how much water it can hold
Weathering catches people out. It sounds like something moving, but the rock is chemically and physically changed into smaller, different material — so it is a transformation. Erosion is the one that moves the pieces afterwards, and that is a transfer. Weathering breaks, erosion carries.
Soil profiles and horizons
Dig a hole down through a soil and you do not see one uniform brown material. You see bands. Those bands are horizons, and together they make a soil profile.
Horizons form because of long-term interactions inside the soil system. All those transfers and transformations, running for hundreds or thousands of years, sort material out by depth. Layers differ in composition and characteristics from the surface downwards, and that pattern is a record of how the soil formed.
The general trend is simple: organic-rich near the surface, because that is where dead plant material lands, and more mineral-rich as you go down, closer to the rock it all came from.
You are not asked to memorise horizon letter codes for ESS. You are asked to know that soils have distinct profiles built from individual horizons, and why.
What controls the profile
Climate — temperature and rainfall set how fast material rots and how much water washes through.
Vegetation — decides how much dead organic matter lands on the surface each year.
Parent material — the rock underneath supplies the minerals, so it shapes what the soil is made of.
Time — deep, well developed profiles take a very long time to build.
Place
What the soil looks like
Why
Tropical rainforest
Thick, organic-rich topsoil
Rapid decomposition and high biological activity in warm, wet conditions
Desert
Shallow, mineral-dominated, with clear horizons
Very little organic input and hardly any leaching
Boreal peat, e.g. Scandinavia
Thick layers of part-rotted organic matter (peat), acidic and nutrient-poor
Cold, wet conditions slow decomposition right down
Prairie, Great Plains USA
Deep, dark topsoil built over thousands of years
Organic matter from grassland vegetation piling up in a semi-arid climate
You do not have to learn these four examples. They are here to show how the same four factors produce completely different soils. If you can quote one confidently in an exam, that is a bonus, not a requirement.
Worked examples
WE 1
Outline the components of soil
Outline the main components of a soil. (4 marks)
Point 1: inorganic
Mineral matter such as rock fragments, sand, silt and clay, produced by the weathering of parent rock.
Point 2: organic, living
Living organisms including bacteria, fungi and earthworms.
Point 3: organic, dead
Dead organic matter from decaying plants, animal remains and faeces.
Point 4: water and air
Water for chemical reactions and life, and air in the pore spaces supplying oxygen to roots and organisms.
Mineral matter + organic matter + water + airsplit the organic part into living and dead — that is often two separate marks
WE 2
Transfer or transformation?
State whether each of the following is a transfer or a transformation, and justify your answer: percolation, humification, weathering. (3 marks)
PercolationTransfer. Water moves down through the soil layers but the water itself is unchanged — only its location changes.
HumificationTransformation. Organic matter is converted into stable humus, so its chemical nature changes.
WeatheringTransformation. Rock is physically and chemically broken down into different, smaller material.
Moved = transfer. Became something else = transformation.always justify with the words location or chemical nature — naming it alone rarely gets the mark
WE 3
Explain the shape of a peat soil profile
Explain why soils in cold, wet boreal regions develop thick layers of partly decomposed organic matter. (3 marks)
Step 1: the input continues
Vegetation keeps adding dead organic matter to the soil surface as an input.
Step 2: the transformation slows
Cold, waterlogged conditions slow decomposition, because decomposer organisms work slowly and waterlogging removes the oxygen they need.
Step 3: the imbalance
Organic matter arrives faster than it is broken down, so it accumulates as peat, giving acidic and nutrient-poor soil.
Input stays high, transformation rate drops, so the storage growsthis is a systems answer — use the words input, storage and rate and you are speaking the examiner’s language
💡 Exam tips
Learn the four components as a set: inorganic, organic, water, air. Questions often want all four.
Say where mineral matter comes from — weathering of parent rock. It is an easy extra mark.
Know the definitions of infiltration, percolation, decomposition and salinisation, and be clear which are transfers and which are transformations.
Use system language: storage, input, output, transfer, transformation. Marks are given for the vocabulary.
If asked about a soil profile, mention horizons and the organic-rich to mineral-rich trend down the profile.
Name the four controls on profile development: climate, vegetation, parent material, time.
⚠ Common mistakes
Calling weathering a transfer. Rock is broken down and changed, so it is a transformation. Erosion is the transfer.
Forgetting the living part of organic matter. Bacteria, fungi and earthworms are components of soil, not just visitors.
Saying soil is mostly solid. Roughly half of a healthy soil is pore space holding water and air.
Mixing up parent material and parent rock with the soil itself. Parent material is the broken rock the soil developed from, sitting below.
Describing a profile as random layers. Horizons form in a predictable order because of long-term processes.
Listing processes without saying what they do. A definition question wants the mechanism, not just the name.
Up next: How Soil Behaves and What It Does — the jobs soil performs in an ecosystem, how sand, silt, clay and humus decide the way a soil behaves, and the three ways of working out soil texture.
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