A5: Soil CompositionGrade 6

Challenge Question

• What is the composition of soil? What is it made of?– Mixture of small particles– Rocks, Dirt, and Sand– Water– Air– Organic materials (dead leaves, plants, & animals)

Background: Weathered Rock

• Soil is different in different places of the world• Soil is a mixture of different things, like rocks.• Rocks become tiny by weathering.• Over time, rocks break by water and wind.• Water may react with chemicals in the rock.• Moving air or water may wear away the rock.• Some rocks fall from higher places and break.

Background: Rock Pieces

• Scientists named the different size rocks in soil• Sand is the largest piece of rock in soil (~1mm)• Individual pieces (grains) are easy to see.• Silt is 100x smaller than sand (~0.01mm).• Clay is 10x smaller than silt (~0.001mm).• Sand, silt and clay are the main parts of soil.• Different places have different amounts. For

example, the desert has a lot of sand in it.

Sand Types and Sizes

Soil Particle Sizes

Sand, Silt, and Clay

Sandy Soil vs. Clay Soil

Venn Diagram

• Go to the next blank page in your lab book.• Draw 3 overlapping circles, 2 on top, 1 below.• Label tops “SAND” and “SILT”, bottom “CLAY”.• Write what we just learned about Sand in that

circle, but what it shares in the overlaps.• Repeat for Silt and Clay. Write in pencil.

Venn Diagram Completed SAND SILT Big pieces Bigger Medium size Lots of air than clay

Rock that’s weathered but not organic Smaller than sand CLAY Smallest pieces Little air

Background: Organic Material• Soil has a lot of Decomposed (broken down)

material like fruit, plants, animals, and waste.• These are called Organic matter which means

material from living things.• Bacteria help break down organic matter.• Eventually it breaks down into basic chemicals

called Nutrients, which dissolve in water and are absorbed by plant roots.

Questions

• Why is it important for plants to grow in soil containing organic matter?– Plants use nutrients from decomposing organic

matter to help them grow.• How can a dead plant help another plant live?– Plants remove nutrients from the soil, but when

they die they give the nutrients back for the next plant to live. This is called the Cycle of Nutrients.

Background: Soil Layers/Topsoil

• Soil is made of many layers, each one different• Topsoil is the top soil layer on Earth’s surface.• Its the layer you usually see, about 20cm deep• It is a combination of rock, sand, silt, clay,

organic matter, air, and water.• Plants grow roots in this layer, which is rich

with biological activity (bugs, bacteria, etc.).

Background: Subsoil

• Subsoil gets some chemicals from topsoil as water drips and washes nutrients down.

• There is not much organic material here.

Background: Parent Material

• Parent Material is made of broken bedrock.• Plant roots usually do not get down here.• Very little organic matter is found here.

Background: Bedrock

• Bedrock is hard rock that lies beneath and supports all of the soil layers.

Analysis page A23 #2, 3, 6

On a separate paper answer each in 1 paragraph

2) What happens to dead leaves on the ground?3) Worms help organic matter decompose. In what soil layer do they live? Explain.6) What is wrong with the school garden soil?