2. • 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

3. Factors of soil formation
o Parent material (original type of rock-
bedrock)
 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

4. 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%): Partially decayed
plant and animal remains mixed in with soil.
Supplies food for microorganisms
o Air (nitrogen, oxygen, carbon dioxide) (~25%):
Used by soil organisms and plants

5. o Water (~25%): A solution of water
and dissolved minerals and nutrients
o Pore spaces: allows for penetration
spaces
of roots and circulation of air and
water
o Soil organisms: Some decompose
organic matter releasing nutrients
back into the soil and some change
atmosphere nitrogen into nitrogen

6. Soil Profile: as soil develops, it forms separate
soil layers called horizons
o “O” horizon (not a true layer): Fresh or
partly decomposed organic material
(humus) sitting on top of the soil (litter)
and highest concentration of
microorganisms

7. Soil Profile: as soil develops, it forms separate soil
layers called horizons
o “A” horizon: Topsoil, most weathered layer,
horizon
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

9. o “B” horizon: Subsoil (fill dirt), less
horizon
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” horizon: Little biological activity,
horizon
partially weathered parent material, and
more and larger rocks
o “R” horizon (not a true layer of soil):
Bedrock and unweathered parent

11. Soil Erosion is the transportation of soil
Agents
 Water
1. Raindrops hit the soil and dislodge soil
particles
2. 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

12. 3. Gullies drain into streams
4. Streams drain into rivers
5. Rivers flow to the oceans where the
soil is deposited
 Wind, Ice (glaciers), and Human
activities

13. Soil by brainpop