TutorForAll

Soil layers and ground cover

Years 3–8Stage 2 to Stage 4Move it

Soil lies in layers over rock. What covers it changes where rain goes and how much topsoil washes away.

Demonstration: a simplified modelNot to scale1 · The soil profile

A slice through a gentle slope covered with grass. From the top: a thin organic layer, topsoil with grass roots and earthworms, subsoil, weathered rock and bedrock. A dashed line in the weathered rock marks the water table; below it, groundwater fills the spaces between the rock pieces. A pit dug from the surface near the top of the slope shows how far the digging has gone. Dug so far: Bedrock (R horizon). At the foot of the slope an empty tank, its rim level with the ground, waits to catch any water that runs off. No rain is falling.

A slice through a grassy slope, dug to bedrock. Soil lies in layers over rock, each one different.

  1. O: organic layer
  2. A: topsoil
  3. B: subsoil
  4. C: weathered rock
  5. Water table
  6. R: bedrock
  7. Tank for runoff

Key Solid blue arrows pointing down: rain soaking into the soil. Dashed dark blue arrows along the ground: rain running off. Short blue lines above the ground: falling rain. Brown dots just above bare ground: soil splashed up by raindrops. Blue dashed line: the water table, the top of the groundwater. Blue in the weathered rock, below the water table: groundwater filling the spaces between the rock pieces. The white gap near the top of the slope: a pit dug down from the surface. Blue in the pit: groundwater seeping in below the water table. In the tank: the water that ran off, with the soil it carried settled at the bottom. Brown water is muddy. White lines in the topsoil: grass roots. Pink curves in the topsoil: earthworms. Yellow pieces on top of the soil: straw mulch.

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The idea, step by step

Not to scale

  1. A slice through a gentle slope covered with grass. From the top: a thin organic layer, topsoil with grass roots and earthworms, subsoil, weathered rock and bedrock. A dashed line in the weathered rock marks the water table; below it, groundwater fills the spaces between the rock pieces. A pit dug from the surface near the top of the slope shows how far the digging has gone. Dug so far: Topsoil (A horizon). At the foot of the slope an empty tank, its rim level with the ground, waits to catch any water that runs off. No rain is falling.
    2 · Organic layer and topsoilOrganic layer: fallen plant material rotting down. Topsoil below, dark with humus, holds most roots and soil animals.1 O: organic layer · 2 A: topsoil · 3 Tank for runoff
  2. A slice through a gentle slope covered with grass. From the top: a thin organic layer, topsoil with grass roots and earthworms, subsoil, weathered rock and bedrock. A dashed line in the weathered rock marks the water table; below it, groundwater fills the spaces between the rock pieces. A pit dug from the surface near the top of the slope shows how far the digging has gone. Dug so far: Subsoil (B horizon). At the foot of the slope an empty tank, its rim level with the ground, waits to catch any water that runs off. No rain is falling.
    3 · SubsoilDeeper is the subsoil, with less humus and fewer roots. Clay and iron compounds carried down by water collect here.1 O: organic layer · 2 A: topsoil · 3 B: subsoil · 4 Tank for runoff
  3. A slice through a gentle slope covered with grass. From the top: a thin organic layer, topsoil with grass roots and earthworms, subsoil, weathered rock and bedrock. A dashed line in the weathered rock marks the water table; below it, groundwater fills the spaces between the rock pieces. A pit dug from the surface near the top of the slope shows how far the digging has gone. Dug so far: Bedrock (R horizon). At the foot of the slope an empty tank, its rim level with the ground, waits to catch any water that runs off. No rain is falling.
    4 · Weathered rock and bedrockWeathered rock: groundwater fills its spaces below the water table. Bedrock lets little through, so groundwater collects above it.1 O: organic layer · 2 A: topsoil · 3 B: subsoil · 4 C: weathered rock · 5 Water table · 6 R: bedrock · 7 Tank for runoff
  4. A slice through a gentle slope covered with grass, with topsoil, subsoil, weathered rock and bedrock under a thin organic layer, and groundwater below the dashed water table in the weathered rock. A pit shows how far the digging has gone. Dug so far: Bedrock (R horizon). Heavy rain falls on the grass. Arrows show rain soaking down into the topsoil and a little running along the ground. Rain soaking in: Most. Rain running off into the tank at the foot of the slope: A little. Soil carried into the tank: A little.
    5 · Heavy rain on grassGrass breaks the raindrops' fall and slows the water: most soaks in. Roots hold the topsoil, so little washes off.1 O: organic layer · 2 A: topsoil · 3 B: subsoil · 4 C: weathered rock · 5 Water table · 6 R: bedrock · 7 Tank for runoff
  5. A slice through a gentle slope covered with straw mulch, with topsoil, subsoil, weathered rock and bedrock under it, and groundwater below the dashed water table in the weathered rock. A pit shows how far the digging has gone. Dug so far: Bedrock (R horizon). Heavy rain falls on the mulch. Arrows show rain soaking down into the topsoil and a little running along the ground. Rain soaking in: Most. Rain running off into the tank at the foot of the slope: A little. Soil carried into the tank: A little.
    6 · Heavy rain on mulchMulch does the same job: it breaks the raindrops' fall and slows the water, so little soil washes off.1 O: organic layer · 2 A: topsoil · 3 B: subsoil · 4 C: weathered rock · 5 Water table · 6 R: bedrock · 7 Tank for runoff
  6. A slice through a gentle slope of bare soil, with no organic layer: topsoil, subsoil, weathered rock and bedrock, and groundwater below the dashed water table in the weathered rock. A pit shows how far the digging has gone. Dug so far: Bedrock (R horizon). Heavy rain falls on the bare soil. Raindrops splash soil up, two arrows show rain soaking in, and three arrows show water running along the ground into the tank, which holds muddy water over a band of settled soil. Rain soaking in: Some. Rain running off into the tank: A lot. Soil carried into the tank: A lot.
    7 · Heavy rain on bare groundBare ground: raindrops splash soil loose, more water runs off, and it carries topsoil into the tank. That is erosion.1 A: topsoil · 2 B: subsoil · 3 C: weathered rock · 4 Water table · 5 R: bedrock · 6 Tank for runoff

The same rain on each surface

SurfaceRainRain soaking inRain running offSoil carried offIn the first figure
Bare groundHeavy rainSomeA lotA lot
MulchHeavy rainMostA littleA little
GrassHeavy rainMostA littleA little
Bare groundLight rainMostA littleA little
MulchLight rainAlmost allAlmost noneAlmost none
GrassLight rainAlmost allAlmost noneAlmost none

Try it in the Lab

Practicals with real materials, each with its safety card.

With a learner

Three questions to ask

  1. Dig down one layer at a time. How is each layer different from the one above it?
  2. Tip a cup of water onto bare ground and onto grass. Which takes it in faster?
  3. Where near you could rain wash bare soil away, and what would keep it covered?

What to expect

Many learners think soil is the same all the way down, or that plants use up the rain so less reaches the soil.

What to try next

Open a practical in Try it in the Lab, above, to send the same rain over bare, mulched and grassed trays and weigh the soil that washes off.

About this model

What is simplified

  • Not to scale: the layers are drawn much thinner than real ones, and the tank much larger. Real soils differ from place to place: some have no organic layer, and bedrock may lie a few centimetres down or many metres.
  • Each layer is drawn with a sharp edge. In real soil one layer often grades into the next.
  • What the rain does is shown as an order, in words, counts of arrows and the tank's levels, not as measured amounts. Real amounts depend on the soil, the slope, how hard and how long it rains, and how wet the ground already is.
  • Mulch and grass are given the same result. In real trials either can do more, depending on how thick the cover is and how well the grass has grown.
  • Grass on hard, trampled ground lets far less water soak in than the model's grass: packing the ground down closes the spaces the water moves through.
  • The water table is drawn at one depth. A real water table rises and falls with the seasons, and real bedrock can hold water and let it through its cracks.
  • A pit dug below the water table fills with groundwater up to the water table, as the model shows. A real pit this deep needs machinery and support for its walls.
  • Nothing moves in the model: the rain is drawn as drops in the air, and what it does is drawn once, as it ends up.

Review

Demonstration: a simplified model. Checked against its written sources, 26 September 2026. Not reviewed by a qualified teacher.

Curriculum references

ST2-10ES-SST2-SCI-01GE2-2HS2-GEO-01GE4-2GE4-PRI-01AC9S3U02AC9HS4K06AC9S5U02AC9HG7K01

Reference, not a verified alignment.

All demonstrations Practicals in the Lab