Most people think of soil as dirt. In ESS it is a system, and that word is doing a lot of work. Soil has storages, things flowing in, things flowing out, and processes moving material around inside it. Get that framing right and half of Topic 5 becomes easy.
📚 What you need to know
Soil is a mixture of inorganic material, organic material, water and air.
The inorganic part comes from weathered parent rock: sand, silt, clay and rock fragments.
The organic part is living organisms plus dead organic matter from plants and animals.
Soil can be modelled as a system with storages, inputs, outputs, transfers and transformations.
A transfer moves material without changing what it is. A transformation changes its chemical nature, state or energy.
Examiners love asking you to sort named processes into transfers or transformations. Learn the list.
What soil actually is
Pick up a handful of healthy topsoil and you are holding four things at once. There is the mineral skeleton, ground down from the rock underneath. There is the organic fraction, which is small by volume but does most of the interesting work. And there is water and air sitting in the gaps between everything else.
Those gaps matter more than students expect. Roots need oxygen. Microbes need oxygen. If the pore spaces fill with water and stay full, the soil goes anaerobic and the whole system starts behaving differently.
The proportions shift constantly. A waterlogged soil has almost no air; a compacted one has almost no pore space at all.
The inorganic part
This is the bulk of the soil by weight. It comes from parent rock that has been broken down by weathering, and it arrives in three particle sizes: sand (biggest), silt (medium) and clay (smallest). There will also be larger rock fragments in younger or shallower soils.
The organic part
Two things live under this heading, and you should keep them separate in your answers.
Living organisms — bacteria, fungi, earthworms, insects, plant roots. These are the workforce.
Dead organic matter — fallen leaves, dead roots, animal remains and animal waste. This is the raw material.
Humus is what dead organic matter becomes once it has been partly broken down. It is dark, crumbly and it is the reason fertile soils look almost black. Humus holds water and holds nutrients, so a soil with plenty of it is a soil that grows things well.
If a question asks for the components of soil, do not just write “sand, silt and clay”. That is only the inorganic quarter of the answer. Give all four: inorganic, organic, water, air.
Soil is a system, not a substance
Here is the shift in thinking the syllabus wants. Instead of describing soil as stuff, describe it the way you would describe any other system in ESS: what is stored in it, what goes in, what comes out, and what moves around inside.
The same four boxes you use for any ESS system. Only the contents change.
Storages
These are the things held inside the soil at any moment: organic matter, organisms, nutrients, minerals, air and water. Notice that nutrients and minerals are listed separately. Minerals are the inorganic particles themselves; nutrients are the elements plants need, such as nitrogen, phosphorus and potassium, held in a form roots can take up.
Inputs and outputs
Inputs are what arrives: dead organic matter, weathered rock, rainfall, solar energy, and anything humans add such as fertiliser or compost. Outputs are what leaves: nutrients leaching away in drainage water, minerals and water taken up by plants, soil particles lost to erosion, and gases escaping to the atmosphere.
Human inputs are easy marks and easy to forget. Fertilisers, pesticides, irrigation water and compost are all inputs to the soil system, and mentioning them shows you can see the human dimension.
Transfers versus transformations
This is the distinction the examiners keep coming back to, and the test is simple. Ask yourself: has the material moved, or has it changed?
The testMoved but still the same thing → transfer Changed into something else → transformation
Process
Which is it?
What is happening
Infiltration
Transfer
Water moves from the surface into the soil
Percolation
Transfer
Water moves down through the soil layers
Groundwater flow
Transfer
Water moves sideways through deeper layers into aquifers
Leaching
Transfer
Dissolved nutrients are carried down and out of the root zone
Biological mixing
Transfer
Worms, burrowing animals and roots shift particles around
Aeration
Transfer
Air is circulated through the pore spaces
Erosion
Transfer
Wind or water detaches particles and carries them away
Decomposition
Transformation
Microbes break complex organic compounds into simpler ones
Weathering
Transformation
Rock is physically or chemically broken into smaller particles
Humification
Transformation
Partly decayed matter becomes stable, dark humus
Nutrient cycling
Transformation
Elements are taken up, released and reused in new forms
Salinisation
Transformation
Soluble salts build up, often after heavy irrigation and evaporation
⚠ Common mix-up
Leaching is a transfer, not a transformation. The nutrients are still nutrients — they have just gone somewhere you did not want them to go.
Erosion and weathering are not the same. Weathering breaks rock down in place (a transformation). Erosion carries the pieces away (a transfer).
Percolation and infiltration are not interchangeable. Infiltration is water getting in; percolation is water moving down once it is in.
Minerals are not nutrients. Minerals are the inorganic particles. Nutrients are the elements plants absorb.
EXAM QUESTION
Outline the main components of a soil system. [4]
Give all four categories
Inorganic material from weathered parent rock, in the form of sand, silt and clay.
Organic material, which is both living organisms and dead organic matter.Water, which carries dissolved nutrients and is needed for chemical reactions.Air in the pore spaces, supplying oxygen to roots and soil organisms.Four components, one mark each
EXAM QUESTION
Distinguish between a transfer and a transformation in a soil system, using named examples. [4]
Define both
A transfer moves material from one place to another without changing what it is.
A transformation changes the chemical nature, state or energy of the material.Then name an example of eachPercolation is a transfer: water simply moves downwards through the profile.Decomposition is a transformation: complex organic compounds become simpler ones, releasing nutrients.Definition + example for both sides
💡 Exam tip
Use the systems vocabulary. Writing “storage”, “input”, “output”, “transfer” and “transformation” signals straight away that you know the framework.
Sort before you write. If a question lists four processes, decide transfer or transformation for each before you start your sentence.
When asked to draw a soil system diagram, use boxes for storages and arrows for flows, and label every arrow.
Do not forget anthropogenic inputs. They are on the syllabus and they are the easiest mark on the page.
If you are unsure whether something is a transfer or a transformation, ask whether the substance would still have the same name afterwards.
Up next: How Soil Behaves and What It Does — now that you can describe what is in a soil, the next question is what that soil does for the ecosystem, and why texture changes the answer.
Want this explained one-to-one?
Book a free session with an experienced IB ESS tutor and get your trickiest topics made simple.
