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 – to 0.05 mm Clay – smaller than mm
Renewable resource Renews very slowly 1 cm takes 15+ years to form
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 pounds for every 2.2 pounds of dry matter Soils with organic waste hold water
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
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
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
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)
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
R horizon Also called bedrock Unweathered rock Beneath all layers of soil Continuous mass of bedrock Hard to dig through
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
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
Cropland Grassland & deciduous forest 1000s of types of soils – 15,000 in the USA!
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 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
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