2.  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

3.  Renewable resource  Renews very slowly  1 cm takes 15+ years to form

4.  Importance  Carbon recycler – 85% of atmospheric CO 2 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

5.  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

6.  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

7.  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

8.  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)

9.  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

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

12.  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

13.  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

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

16.  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

19.  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

20.  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