• Soil is a layer at the surface of the earth composed of a mixture of weathered rock, organic matter, mineral fragments, water, and air which is capable of supporting the growth of plants

Factors of soil formationFactors of soil formation

o Parent materialParent material (original type of rock) Residual soil: formed from local

bedrock Transported soil: parent material has

been moved to a new location

o Climate: most influential

o Vegetation, Slope, Time, and interaction of all factors

Soil composition (by volume of a healthy soil)

o Weathered rock and mineral matter (dirt)(~45%): Clay and quartz most abundant minerals. Must contain potassium, phosphorus, and nitrates

o Organic matter (humus) (~5%):Organic matter (humus) (~5%): Partially decayed plant and animal remains mixed in with soil. Supplies food for microorganisms

o Air (nitrogen, oxygen, carbon Air (nitrogen, oxygen, carbon dioxide) (~25%):dioxide) (~25%): Used by soil organisms and plants

o WaterWater (~25%): A solution of water and dissolved minerals and nutrients

o Pore spacesPore spaces: allows for penetration of roots and circulation of air and water

o Soil organisms:Soil organisms: Some decompose organic matter releasing nutrients back into the soil and some change atmosphere nitrogen into nitrogen compounds (nitrates)

Soil Profile: as soil develops, it forms separate soil layers called horizons

o ““O” horizonO” horizon (not a true layer): Fresh or partly decomposed organic material (humus) sitting on top of the soil (litter) and highest concentration of microorganisms

Soil Profile: as soil develops, it forms separate soil layers called horizons

o ““A” horizonA” horizon: Topsoil, most weathered layer, most fertile layer, high humus content (nutrients), many microorganisms, dark color due to humus, Zone of leaching (water dissolves minerals and carries them down to a lower layer), the most roots present, and the parent material unidentifiable

o ““B” horizonB” horizon: Subsoil (fill dirt), less root penetration, less biological activity, compact and lighter in color, Zone of accumulation (where leached materials are deposited), many more rocks, and parent material difficult to determine

o ““C” horizonC” horizon: Little biological activity, partially weathered parent material, and more and larger rocks

o ““R” R” horizonhorizon (not a true layer of soil): Bedrock and unweathered parent material

Soil ErosionSoil Erosion is the transportationtransportation of soilAgentsAgents

WaterWater1. Raindrops hit the soil and dislodge

soil particles2. Thin sheets of water (sheet flow)

flowing across the surface carries away dislodged soil particles (sheet erosion)

3. Sheet flows collect in tiny channels called rills

4. Rills drain into gullies

3. Gullies drain into streams4. Streams drain into rivers5. Rivers flow to the oceans where

the soil is deposited

Wind, Ice (glaciers), and Human Wind, Ice (glaciers), and Human activitiesactivities

Special SoilsSpecial Soils

o LateritesLaterites (Brick Soil): Thick red soils form in the wet tropics and subtropics and is the end product of extreme chemical weathering

Special SoilsSpecial Soils

o TundraTundra: Soils near the Soils near the Artic OceanArtic Ocean, , thin soilthin soil with little humus, and with little humus, and supports only small plants, no treessupports only small plants, no trees

Soil by brainpop

1) What is Humus?

2) What is the “Horizon A” layer of soil also known as?

3) What is “bedrock”?