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Chapter 14 Soil Soil Formation Parent Material  subsoil  topsoil Parent material goes through process of “weathering” -mechanical weathering –freeze-thaw.

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Presentation on theme: "Chapter 14 Soil Soil Formation Parent Material  subsoil  topsoil Parent material goes through process of “weathering” -mechanical weathering –freeze-thaw."— Presentation transcript:

1 Chapter 14 Soil Soil Formation Parent Material  subsoil  topsoil Parent material goes through process of “weathering” -mechanical weathering –freeze-thaw –roots –Friction –wind and water move small particles, exposing new material.

2 Chapter 14 -chemical weathering –oxidation –hydrolysis –acid content -primary succession –Lichens –increase humus content (and lowers pH) –Enhance chemical/physical weathering

3 Chapter 14 -earthworms –500,000 per 2.4 acre (1 hectare) –10 tons per year –Mix soil allow pore space for water and air -fungi and bacteria  decompose organic material and reduce size of organic particles. These physical, chemical and biological process allow soil formation: 1 centimeter per 15 years at best More often 1 cm = hundreds of years

4 Chapter 14 Soil Properties Texture: -gravel(> 2 mm) -sand(0.05 to 2 mm) -silt(0.002 to 0.05 mm) -clay(< 0.002 mm) Texture (combination of sand, silt and clay) determines how much air and water the soil can contain. loam: good drainage, but enough retention of nutrients—20% clay, 40% each of silt/sand

5 Chapter 14 Structure: -“clumpiness” and “friability” -Sands don’t clump, but clays do. -Soil texture and moisture content determine friability. Good soils for agriculture: moderate friability good drainage enough air after drainage enough water for plants

6 Chapter 14 Soil Profile Distinct layers known as “horizons”. A horizon: thickness varies, most life forms, nutrients and organic matter in upper A zones - less in lower A zones. E Horizon B/w A and B in heavily leached soils/few nutrients. Dense forests typically B horizon: less organic material, fewer organisms, and nutrients leached out of A horizon.

7 Chapter 14 C horizon: weathered parent material, no organic material, little or no nutrients. influences soil pH and texture. R Horizon Unaltered bedrock below C

8 Chapter 14 Two basic soil types: 1. Grassland soils -deep A horizon -low rainfall, little leaching -thin B horizon, which has little mineral and organic material 2. Forest soils -Much thinner A horizon -more rainfall, leaching more material to B horizon -much leaching of clay particles can lead to “hardpan” layer.

9 Chapter 14 Desert -very little rain results in poorly developed horizons. -little plant growth results in low organic matter. Tropical -high temperatures and humidity allows rapid decomposition of organics. -much leaching -rapid erosion or “baking”

10 Chapter 14 Topography -in flat areas, as soil is built, it stays in place. -on steeper slopes, soil is transported downhill until it reaches a flat area (typically a floodplain). Can categorize many soils within the two basic types, based on numerous factors: –15,000 soil types in North America

11 Chapter 14 Erosion Movement of soil by wind or water. In the north central Texas area, more than 15 tons per acre per year is considered “critical”. Most of this soil ends up streams and rivers, to be deposited in lakes and oceans. Mississippi River moves 325,000,000 mTons per year.

12 Chapter 14 Costs: -most productive layers (A horizons) lost first -farmers must add more fertilizers -streams’ bottoms become covered with silt, destroying much habitat -sediments must be dredged if in shipping lanes -Carries away more soil than produced each year Bottom line—mining soil—finite resources if used this way

13 Chapter 14 In U.S., about 50% of lands are capable of raising crops (21% currently, 26% in pasture). Only 2% of lands are not susceptible to excessive erosion. Worldwide, only 35% of lands are capable of raising crops (11% currently, 24% in pasture).

14 Chapter 14 Soil Conservation Efforts Contour Farming: tilling at right angles to slope of land. Can reduce erosion by 50%, and adds more water to soil for crops. Strip Farming: intertwining plots of contour tilled row crops with sown grain crops. Under right conditions, can be even more effective than contour farming alone.

15 Chapter 14 Terracing: level areas carved out of steeply sloped lands. Flat areas used for crops, while “walls” of terrace are protected. Quite costly, high maintenance, and not very feasible for highly mechanized farms. Waterways: channels designed to move water with little or no gully erosion. Windbreaks: (1) layers of vegetation left on the soil when not cultivated. (2) trees, shrubs or fences placed perpendicular to prevailing wind direction.

16 Chapter 14 Reduced and Conservation Tillage: (1) soil is plowed: buries weeds, adds nutrients, lifts deep nutrients, exposes dark soils which will warm more quickly. (2) field is disked or harrowed: breaks up clods, kills remaining weeds, prepares soil for seeds. (3) a temporary plant is often grown to reduce erosion.  every pass costs money and leaves soil exposed.

17 Chapter 14 Variations: -Mulch tillage (tilling entire surface just prior to planting) -strip tillage (only till narrow strip where seeds will be planted) -no-till (special planters that “inject” seeds into soil).  less time, but more herbicides

18 Chapter 14 Other benefits of reduced tillage: Less erosion (saves valuable soil, clearer streams and rivers, less dredging). Winter food and cover for wildlife. Allows row crops to be grown on hillsides where previously they could not be grown. Fuel is saved, soil less compacted with fewer trips. Can often plant a second crop immediately after first crop.


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