Natural sciences Year 5: Soil: how it forms and what it does
Natural sciences, Year 5 (Portugal): Weathering and living things turn rock into soil; soil has mineral particles, humus, water and air, and it supports plants, stores water and shelters life.
What soil is made of
Soil is the thin, loose layer on top of the rock where plants grow. It has four parts: mineral particles (sand, silt and clay), humus (dark matter from dead plants and animals), water and air. A soil good for crops has, roughly, 45 % mineral particles, 5 % humus, 25 % water and 25 % air (approximate values). Water and air fill the spaces between the particles, and living things live there too.
What soil is for
Soil has several jobs. It holds plants in place and gives them water and mineral nutrients. It stores and filters water. It is home to many living things, such as earthworms, insects, fungi and bacteria, which break dead matter down into humus. Its properties depend on the mix: sandy soil lets water through fast, clayey soil holds water, and humus makes soil darker and richer.
From bare rock to soil
Step 1: on a bare rock, rain water gets into cracks; at night it freezes, expands and widens them (atmospheric agents). Step 2: wind and water break off small fragments. Step 3: lichens and roots grow on and into the rock, and their acids and pressure break it further (biological agents). Step 4: dead plants and animals decay and become humus, which mixes with the fragments. After a very long time, this mixture is soil.
Soil is not just «earth»
It is easy to think soil is just crushed rock, or that it forms quickly. Crushed rock alone is not soil: without humus, water, air and living things it cannot support plants well. Soil also forms very slowly: making a layer a few centimetres thick can take hundreds of years or more. That is why bare, eroded or paved soil is not replaced in a lifetime, and why protecting it matters.
Soil in human activities
Farming, gardening and forestry depend on soil: cereals, vines, olive trees and cork oaks grow in it. Bricks and roof tiles are made of clay from soils, and every building stands on soil or rock. When forests are cut or fires burn, rain can wash the soil away. Which soils are important in your region, and what grows in them?
Test what you learned
Which four parts make up soil?
- a) Mineral particles, humus, water and air
- b) Only sand and clay
- c) Rock fragments and living plants only
Correct answer: a) Mineral particles, humus, water and air — Right: water and air fill the spaces between the particles, and humus comes from dead matter.
A typical crop soil is about 45 % mineral particles, 5 % humus, 25 % water and 25 % air (approximate values). What share of the soil is the space filled by water and air together?
- a) 25 %
- b) 45 %
- c) 5 %
- d) 50 %
Correct answer: d) 50 % — Right: 25 % + 25 % = 50 %, the space between the particles.
Which statement about soil is correct?
- a) It forms in a few days from ground-up rock
- b) Ground-up rock alone is already soil
- c) It forms very slowly and needs humus, water, air and living things as well as rock fragments
Correct answer: c) It forms very slowly and needs humus, water, air and living things as well as rock fragments — Right: that is why bare or paved soil is not replaced in a lifetime.
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