ecology

the study of the soils

formation of soils

• underlying rock that is the basic source for minerals in soils
• (although wind can blow in minerals from other places)

production of smaller rock particles and can be either accomplished through mechanical or chemical destruction of larger rock

uneven expansion as temperature changes crack rock, expansion of water as it freezes in cracks splits rock, and wind and waves move small particles and make them even smaller

loss of soluble minerals from soil as precipation percolates through the soil to the groundwater and then flows into river, lakes and aquifers

the downward movement of dissolved or suspended materials (i.e nutrients) in the soil due to leaching

deposition of materials out of solution

loss of soils as water carries them downslope, where they can be deposited on botomland or, if they enter a strem, carried downstream where the particles either become part of the stream or river bottom or reach the ocean and settle onto the ocean floor

• organic matter that is no longer recognizable as being from a particular source.
• This is an important soil component and is mostly plant material. Lignified wood
• persists longest in the soil.

• climates, water rapidly percolates through the soil and into the groundwater, where it
• flows into rivers and streams. Soluble soil nutrients are constantly leached out of the
• soils, leaving behind the less soluble ions (Al+ and Fe+++), which precipitate and give the
• soils their color (whitish for Al and red for Fe) and H+ ions, which make the soil acidic.
• Loss of nutrients to leaching results in nutrient‐poor soils.

• precipitation water leaves the soil through the surface rather than through flow of
• groundwater. The minerals dissolved move upward from the groundwater with the
• water and the least soluble (typically CaCO3) are deposited where the precipitate,
• usually in the B horizon, where, if enough precipitate, they can form a hard layer called a
• Caliche

• as calcification except that the precipitate is salt (NaCl) and it occurs in the A layer
• because there is little rainfall to move down through the soil and offset the upward
• movement due to evaporation. Can result in a Salt Crust on the surface of the soil. The
• salt concentration is such that plant growth is inhibited.
• • Irrigation of dry lands can result in salinization where none existed before and
• can poison the land. Irrigation water dissolves salt as it soaks into soil and
• brings the salt to the surface as it evaporates. Huge problem in the US
• southwest, in Australia, and in Northern Africa, major areas of dryland irrigation

• and are not quickly decomposed due to low temperature. Accumulation of humus and
• humic acids causes soils to become acidified. The acidification can become so intense
• even Al and Fe ions are solubilized and the lower portion of the A horizon is left with
• only sand made of silica (glass ‐ SiO2). Soluble nutrients are leached and soil is nutrient
• poor.

• which is very slow. Accumulating organic materials are acidic and the low pH means
• mineral nutrients are lost to leaching. Soils often grey‐black due to incompletely
• oxidized state of iron.

• Leaf litter, unconsolidated (loose) where plant litter on upper side becomes
• humus on lower side as animals, fungi, and bacteria process it

• topsoil ‐ upper layer of mineral soil, usually black due to presence of humus
• from O horizon, often nutrient rich and full of fine, nutrient absorbing roots

• leaves behind insoluble minerals, under the A and on top of the B layer. These particles
• have been leached of soluble nutrients and so are nutrient poor. Found most often in
• soils under wet forests but rarely in grasslands because rainfall is to little to form the
• layer

• subsoil ‐ Illuviation (wash in) layer‐ region where the particles that come
• from the E horizon accumulate. Usually denser in texture (due to small particle
• accumulation) and harder for roots to penetrate

• broken parent material that is beginning to weather (but more slowly than
• minerals in B or A horizon) and so is chemically nearest the parent material

Bedrock ‐ unbroken parent material