SOILS

SOIL

Definition – mixture of mineral particles, eroded rock, water, air, organic matter and living organisms Gravel – larger than 2 mm Sand – 0.05 to 2 mm Silt – 0.002 to 0.05 mm Clay – smaller than 0.002 mm

SOIL

Renewable resource Renews very slowly 1 cm takes 15+ years to form

SOIL Importance

Carbon recycler – 85% of atmospheric CO2 comes from dead organism decay

Nutrient recycler – plants need 16 essential nutrients

C H O – from carbon dioxide & water N P K Ca S – from the soil

Water storage – plants use huge amounts of water for growth

400-2000 pounds for every 2.2 pounds of dry matter

Soils with organic waste hold water

SOIL

Importance continued Water filter – removes chemicals, organic

waste, excess nutrients from water Waste disposal – mixing waste into the

soil surface is called “land treatment” EPA developed programs 1/3 of US treated sewage is applied to soil

Use in construction – bricks made from soil

Agriculture – 13.4% of PA soil is harvested cropland

SOIL

Major Layers = Soil horizons Distinct texture &

composition Soil profile = cross-

sectional view of the horizons

Mature soils have at least 3 of the possible horizons

SOIL Horizons

Surface litter layer/O horizon Top layer Freshly fallen, partially decomposed leaves,

twigs, animal waste, etc Brown or black

Topsoil layer/A horizon Porous mixture of humus + inorganic mineral

particles Darker & looser than deeper layers Lots of humus helps topsoil hold water

SOIL

Eluviation horizon / E horizon Eluviation horizon (leeching)

Leeching-- When water moves downward (due to infiltration) it dissolves minerals in upper layers and transports them to lower layers

Made mostly of sand and silt Loses most of its minerals and clay as they

drip through the soil (through the process of eluviation)

SOIL

Horizons continued Subsoil/B horizon

yellowish-brown to brown in color 2-3 feet below surface Plant roots Accumulates clay, iron, aluminum, inorganic

material Parent material (Regolith)/C horizon

inorganic matter Broken down rock (bedrock) No plant roots C horizon lies on bedrock

SOIL

R horizon Also called bedrock Unweathered rock Beneath all layers of soil Continuous mass of bedrock Hard to dig through

SOIL

Horizon special notes Roots = top 2 layers Vegetation anchors the layers so soil can

do its job Top 2 layers

Bacteria, fungi, earthworms, small insects Interact in complex food webs

Organic matter broken down inorganic compounds (soluble in water) Plants then take up nutrients with water

SOIL Horizon special notes

Topsoil color (Horizon A) Dark-brown or black = nitrogen-rich, high in

organic matter Gray, bright yellow, red = low in organic matter &

need nitrogen Pores

Contain different amounts of air (N & O) & water Infiltration – downward movement of water

through soil Leaching – water dissolves various minerals &

organic matter in upper layers & carries them to lower layers

SOIL

Cropland Grassland & deciduous forest 1000s of types of soils – 15,000 in the USA!

SOIL

Soil texture determined by the amounts of sand, silt, clay & gravel Gritty feel = lots of sand Sticky feel = lots of clay Smooth feel = lots of silt

SOIL

Soil porosity – measures the volume of pores or spaces per volume of soil Fine particles to hold water Coarse particles for air spaces

Soil permeability – rate at which water & air move from upper to lower soil layers

SOIL

Infiltration Downward movement of water through soil Due to gravity

Leeching When water moves downward (due to

infiltration) it dissolves minerals in upper layers and transports them to lower layers