SOILS

What is soil? Soil – relatively thin surface layer of the Earth’s crust consisting of mineral and organic matter

Soil Composition Soil is composed of 4 distinct parts: Mineral particles (45% of “typical” soil) Organic matter (about 5%) Water (about 25%) Air (about 25%)

Importance of Soil Organisms inhabit the soil & depend on it for shelter, food, & water. Plants anchor themselves into the soil, and get their nutrients and water. Humans need plants and, therefore, need soil.

Soil is a renewable resource Soil is a slowly renewed resource Soil formation begins when bedrock is broken down by weathering. Mature soils (soils that have developed over a long time) are arranged in a series of horizontal layers called soil horizons.

Soil Formation Soils form from parent material Parent material (rock) is slowly broken down into smaller particles by biological, chemical, and physical weathering. It takes a long time to form soil. Example: To form 2.5 cm (1 in.) it may take from 200-1000 years.

Physical Weathering Physical breakdown by wind, water, ice, etc.

Chemical Weathering Chemicals interact with rock and break it down. Example: A plant’s roots or animal cells undergo cell respiration and the CO2 produced diffuses into soil, reacts with H2O & forms carbonic acid (H2CO3). This eats parts of the rock away.

Biological Weathering Parent material is broken down by tree roots or lichens. Lichens play a big role in primary succession.

Soil Properties: Texture The percentages (by weight) of different sized particles of sand, silt and clay that it contains.

Soil Properties: Texture, Cont. Grain Size 0.05 to 2mm = sand (the largest soil particles) 0.002 to 0.05mm = silt (about the size of flour) <.002mm = clay (only seen under and electronic microscope)

Soil Properties: Texture, Cont. To tell the difference in soil, take the soil, moisten it, and rub it between your fingers and thumb. Gritty: has a lot of sand Sticky: high clay content and you should be able to roll it into a clump Silt: smooth, like flour.

Soil Structure How soil particles are organized and clumped together. Sand Silt Clay

Friability How easily the soil can be crumbled.

Porosity A measure of the volume of soil and the average distances between the spaces.

Permeability The rate at which water and air moves from upper to lower soil layers. The Distances between those spaces.

Variability Soils (Sand, Silt, & Clay) vary in the size of the particles they contain the amount of space between these particles how rapidly water flows through them.

The size of the rock particle DOES NOT change the porosity! Big spaces, not a lot of them LESS surface area Little spaces but lots of them GREATER surface area EQUAL POROSITY IMPORTANT TO REMEMBER The size of the rock particle DOES NOT change the porosity!

Which size held the most water? 1st trial: look at red yellow green only Demo #1 Which size held the most water?

Soil Texture (descriptions) Sand Silt Clay

Shrink-Swell Potential Some soils, like clays, swell when water gets in them, then they dry and crack. This is bad for house foundations, etc.

Slope Steep slopes often have little or no soil on them because of gravity. Runoff from precipitation tends to erode the slope also. Vegetation?

Depth Some soils are very shallow. It can be only two inches of soil and then you hit rock. Other areas can have soil 36 inches deep or more.

Color Dark soil is rich with lots of organic matter. Light soil (like sand) is not so rich with very little organic matter.

Soil Horizons

Organic Layer (O-horizon) The uppermost layer; it is rich in organic material. Plant litter accumulates in the O-horizon and gradually decays. In desert soils the O-horizon is completely absent In certain rich soils it may be the dominant layer.

Topsoil (A-horizon) It is dark and rich in accumulated organic matter and humus. It has a granular texture and is somewhat nutrient-poor due to the loss of many nutrient minerals to deeper layers and by leaching.

Eluvial(E-horizon) Mineral horizon in upper part of soil Below A-Horizon and above B-Horizon Generally forested areas, light color

Subsoil (B-horizon) The light-colored subsoil beneath the A-horizon; Accumulation of minerals occurs here It is typically rich in iron and aluminum compounds and clay.

Parent Material (C-horizon) Contains weathered pieces of rock and borders the unweathered solid parent material. Most roots do not go down this deep often saturated with groundwater.

Wood sorrel Oak tree Organic debris builds up Lords and ladies Dog violet Rock fragments Grasses and small shrubs Earthworm Millipede Fern Moss and lichen Honey fungus O horizon Mole Leaf litter A horizon Topsoil B horizon Bedrock Subsoil Immature soil Regolith Young soil C horizon Figure 3.23 Natural capital: soil formation and generalized soil profile. Horizons, or layers, vary in number, composition, and thickness, depending on the type of soil. (Used by permission of Macmillan Publishing Company from Derek Elsom, Earth, New York: Macmillan, 1992. Copyright © 1992 by Marshall Editions Developments Limited) Pseudoscorpion Mite Parent material Nematode Root system Actinomycetes Red Earth Mite Fungus Mature soil Bacteria Springtail Fig. 3-23, p. 68

Desert Soil (hot, dry climate) Grassland Soil (semiarid climate) Mosaic of closely packed pebbles, boulders Weak humus-mineral mixture Alkaline, dark, and rich in humus Dry, brown to reddish-brown with variable accumulations of clay, calcium and carbonate, and soluble salts Figure 3.24 Natural capital: soil profiles of the principal soil types typically found in five types of terrestrial ecosystems. Clay, calcium compounds Desert Soil (hot, dry climate) Grassland Soil (semiarid climate) Fig. 3-24a, p. 69

Tropical Rain Forest Soil (humid, tropical climate) Acidic light-colored humus Figure 3.24 Natural capital: soil profiles of the principal soil types typically found in five types of terrestrial ecosystems. Iron and aluminum compounds mixed with clay Tropical Rain Forest Soil (humid, tropical climate) Fig. 3-24b, p. 69

Deciduous Forest Soil (humid, mild climate) Forest litter leaf mold Humus-mineral mixture Light, grayish-brown, silt loam Figure 3.24 Natural capital: soil profiles of the principal soil types typically found in five types of terrestrial ecosystems. Dark brown firm clay Deciduous Forest Soil (humid, mild climate) Fig. 3-24b, p. 69

Coniferous Forest Soil Acid litter and humus Light-colored and acidic Figure 3.24 Natural capital: soil profiles of the principal soil types typically found in five types of terrestrial ecosystems. Humus and iron and aluminum compounds Coniferous Forest Soil (humid, cold climate) Fig. 3-24b, p. 69

Erosion Erosion is the movement of soil components, especially surface litter and topsoil, from one place to another.

Importance of Erosion In undisturbed ecosystems, the roots of plants help anchor the soil, and usually soil is not lost faster then it forms. But, farming, logging, construction, overgrazing by livestock, off-road vehicles, deliberate burning of vegetation etc. destroy plant cover and leave soil vulnerable to erosion. This destroys in a few decades what nature took hundreds to thousands of years to produce.

Soil Erosion & Degradation Figure 13-9

Global Outlook: Soil Erosion Soil is eroding faster than it is forming on more than one-third of the world’s cropland. Figure 13-10