SOILS & SOIL TYPES

What is soil?Soil is • one of the basic resources we

need to survive – like air and water.

• the surface layer of the Earth that contains all the nutrients plants need to grow. The quality of a soil determines what can grow there.

• Home to many plants, animals, and other organisms like bacteria.

• an important filter of water and wastes

What is soil made of?Soil is made up of:• Broken particles of rock and minerals

that have been eroded from rock and changed by the physical and chemical processes of weathering.

• Organic material including living organisms, dead plants and animals, and decomposed plant remains called humus

• Air• Water

How do we describe soils?Texture – the size of the rock and mineral bits in the soil determines the texture.

• Gravel – pieces larger than 2 mm • Sand – pieces between 0.05 mm and 2 mm – feels ‘gritty’• Silt – pieces between 0.002 mm and 0.05 mm – feels like

flour• Clay – pieces smaller than 0.002 mm -- feels sticky when

wet

Too much sand and gravel – create big spaces between the pieces of soil that don’t hold water or nutrients for plants. Plants can’t grow well.

Too much silt – makes good farm land but erodes away easily – is picked up and carried away during floods and blows away during dust storms

Too much clay – makes the soil heavy and dense – not enough space between the tiny particles makes the soil almost like concrete when it’s dry. Plants can’t get air, nutrients, or water and can’t grow.

Loam is the perfect soil. It has equal amounts of sand and silt and a smaller amount of clay. It has enough space between soil pieces for water and air to flow and enough clay to stick together and hold in nutrients.

Why does soil texture matter?

Different soil types develop in different climates due to:

1. Bedrock type2. Amount of

precipitation (rainfall)

3. Temperature4. Time

What factors determine a soil’s texture & composition?

How is soil structured?Soil forms in layers that we call horizons • O-horizon – organic matter, dead

leaves, plant & animal matter• A-horizon – “topsoil” – mineral soil

with nutrients. Most plants root here.

• E-horizon – leaching zone (caliche)• B-horizon – “subsoil” – mineral

particles, clays, salts• C-horizon – weathered & broken

parent tock• D-Horizon – “bedrock” – solid

parent rock

Prairie soils -- dark A-horizon, rich in minerals, form in mid-latitudes & support

grasslands

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Forest soils -- light gray A-horizon, rich in Al & Fe,

temperate humid regions

Tropical soils -- reddish and iron-oxide rich, nutrients are leached

out, form in humid & warm regions.

Tundra soils – Arctic & Antarctic and in high elevations, bottom layers always frozen

(permafrost), very fragile – take thousands of years to form and recover from damage.

Desert soils -- very thin (a few cm), little organic material, rich in CaCO3 nodules &

layers (caliche), form in deserts

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Fun Soil Facts

• An average soil sample is 45 percent minerals, 25 percent water, 25 percent air, and 5 percent organic matter.

• Natural processes take more than 500 years to form 2 cm (less than 1 inch) of topsoil.

• There are about 10 tons of living organisms in 1 acre of soil.

• About 25% of the world’s soil is damaged regularly by unsustainable farming practices.

Fun Soil Facts• Only about 10% of the world’s land is used to grow food

for both human beings and animals.• In one year, earthworms can digest up to 36 tons of soil,

recycling it to make richer soil.• A single shovel full of rich soil can contain more living

species than the entire Amazon Rain Forest above ground.• The roots of plants can move through soil up to a half inch

a day in order to find moisture.• One handful of clay contains more particles than all of the

people on earth.

SUMMARY

Define soil:

What determines a soil’s Texture?

Explain why plants do not grow well in desert soils: