seegongsik
Grade 3 (age 8-9)

grade 3 Soil Observation

A handful of playground soil feels gritty, while flowerbed soil is dark and soft. Soil forms over a very long time as rock is broken into tiny pieces by wind, rain, and repeated freezing and thawing, and then mixed with rotted leaves and the remains of small living things. That is why soil differs from place to place in colour, grain size, and how fast water drains through it, and different plants grow well in it. In the first picture, pick each layer you would see if you cut straight down into the ground; in the second, compare playground soil with flowerbed soil.

What is soil made of?

🔬 A close look at soil
①Most of soil is grains of rock broken into tiny pieces
②Mixed in is humus, made of rotted leaves and the broken-down remains of living things
③Soil differs from place to place in colour, grain size, and how it feels in your hand
④Earthworms, small bugs, and tiny microbes too small to see live in the soil

The layers under the ground

🏔️ The layers under the ground
①At the very top is the leaf layer, made of fallen leaves and grass
②Below it, the topsoil is dark and rich because rotted leaves are mixed in, and plant roots mostly spread here
③The subsoil has lots of fine clay and few nutrients
④Further down, half-broken stones and gravel are mixed in, and at the bottom is hard bedrock
⑤The grains get bigger the deeper you go, which shows that rock breaks down into soil from the top

Playground soil and flowerbed soil

🌿 How are the two soils different?
①Playground soil has coarse grains, a light colour, and drains fast
②Flowerbed soil has fine grains, a dark colour, and holds water longer
③Flowerbed soil is dark because it has lots of humus, such as rotted leaves, mixed in
④So plants grow better in flowerbed soil, where there is plenty of water and nutrients

Summary

How soil forms
Rock → Stone → Sand → Soil
Wind and rain, freezing and thawing, and plant roots break rock down over a long time
Soil that plants grow well in
Fine grains + Humus + The right amount of moisture
In places that need to drain fast, like a playground, coarser soil is a better fit
What soil does
Holds roots in place + Gives water and nutrients
It is also a home for the small living things in it
🎯 Key points
①Soil forms over a long time as rock breaks into tiny pieces and humus mixes in
②Soil differs from place to place in colour, grain size, and drainage
③Under the ground, the layers go leaf layer, topsoil, subsoil, gravel layer, and bedrock
④Flowerbed soil has lots of humus so plants grow well, while playground soil drains fast
⑤Soil holds plants in place and gives them water and nutrients

Examples and Unit Test

Example 1
How does hard rock turn into fine soil?
1
Rock is worn by wind and rain, water in its cracks freezes and thaws, and plant roots push in, so it breaks little by little.
2
Rock breaks into stones, stones into sand, and when humus mixes in it becomes soil.
▸ It breaks down over a very long time (rock → stone → sand → soil)
Soil forms so slowly that even 1 cm can take hundreds of years to build up.
Example 2
Between playground soil and flowerbed soil, which one do plants grow better in, and why?
1
Flowerbed soil has fine grains, holds water longer, and has plenty of nutrients from humus such as rotted leaves.
2
Playground soil has coarse grains, drains fast, and has few nutrients, so plants grow better in flowerbed soil.
▸ Flowerbed soil (it has more nutrients and holds water longer)
The darker a soil looks, the more humus it usually has mixed in.
Unit-test style
Which statement about soil is correct?
①Soil has always been the way it is now
②Soil forms when rock breaks down
③Playground soil has more nutrients than flowerbed soil
④Flowerbed soil drains fast, so plants do not grow well in it
⑤Soil has the same colour and grains everywhere
▸ ② Soil forms when rock breaks down
1
Soil forms as rock is broken into tiny pieces by wind, rain, and other forces over a long time.
2
Soil comes from rock (①), flowerbed soil has more nutrients (③), flowerbed soil holds water so plants grow well (④), and soil differs from place to place in colour and grains (⑤).
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