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Soil Structure, Density, and Porosity

Soil Structure, Density, and Porosity. SUMBER: broome. soil .ncsu.edu/ssc012/Labs/4. ppt ‎. Definition. Soil structure is the arrangement of the primary soil particles (sand, silt, and clay) and other soil materials into discrete aggregates. .

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Soil Structure, Density, and Porosity

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  1. Soil Structure, Density, and Porosity SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  2. Definition • Soil structure is the arrangement of the primary soil particles (sand, silt, and clay) and other soil materials into discrete aggregates. SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  3. Peds • Structural units are called peds, and have distinct boundaries and well-defined planes of weakness between the aggregates. • Peds consist of primary particles bound together by cementing agents like organic matter, clay, and hydrous oxides of iron and aluminum. • Peds can take several shapes. SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  4. Ped Shapes • Granular • Blocky • Prismatic • Columnar • Platy • Single-grained • Massive SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  5. Granular Structure • Resembles cookie crumbs and is usually less than 0.5 cm in diameter. • Commonly found in surface horizons where roots have been growing. http://soil.gsfc.nasa.gov/pvg/granular.gif http://soils.usda.gov/technical/manual/images/fig3-30_large.jpg SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  6. Blocky Structure • Irregular blocks that are usually 1.5 - 5.0 cm in diameter. • Can be subangular or angular blocky. http://soil.gsfc.nasa.gov/pvg/blocky.gif SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎ http://www.dpi.vic.gov.au/dpi/vro/glenimages.nsf/Images/gl167_profile/$File/gl167_profile.jpg

  7. Prismatic Structure • Vertical columns of soil that might be a number of cm long. • Usually found in lower horizons. http://soil.gsfc.nasa.gov/pvg/prismatic.gif http://soils.usda.gov/technical/manual/images/fig3-27_large.jpg SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  8. Columnar Structure • Vertical columns of soil that have a salt "cap" at the top. • Found in soils of arid climates. http://soil.gsfc.nasa.gov/pvg/columnar.gif http://soils.usda.gov/technical/manual/images/fig3-28_large.jpg SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  9. Platy Structure • Thin, flat plates of soil that lie horizontally. • Usually found in compacted soil. http://soil.gsfc.nasa.gov/pvg/platy.gif http://soils.ag.uidaho.edu/soilorders/i/Arid_03.jpg SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  10. Single-grained Structure • Soil is broken into individual particles that do not stick together. • Always accompanies a loose consistence. • Commonly found in sandy soils. http://soil.gsfc.nasa.gov/pvg/singlegrained.gif SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  11. Massive Structure • Soil has no visible structure, is hard to break apart and appears in very large clods. http://soil.gsfc.nasa.gov/pvg/massive.gif http://soils.usda.gov/technical/manual/images/fig3-31_large.jpg SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  12. Grade of Soil Structure • The terms weak, moderate, or strong are used to describe the grade or how stable the peds are and how hard they are to break apart. • What do you think the grade would be for this picture? SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎ http://soils.usda.gov/technical/manual/images/fig3-27_large.jpg

  13. Class of Soil Structure • The size or class of the peds is described as fine, medium, or coarse. SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  14. How does structure affect water movement in soils? • In soils with good structure, the pore space that occurs between peds is relatively large and facilitates water and air movement. • Well-developed structure is very important in clayey soils. • Clayey soils with poor structure restrict water and air movement. SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  15. Degree of Water Movement Structure Water Movement http://ohioline.osu.edu/b905/images/006.jpg SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  16. Altering Soil Structure • Unlike texture, structure can be altered by tillage or traffic. • Tilling soils that are too wet, or compacting soils with heavy equipment can break down the natural structural units. http://www.ny.nrcs.usda.gov/programs/images/tractor-tillin.jpg SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  17. Bulk Density • Soil bulk density is the mass per unit bulk volume of soil that has been dried to a constant weight at 105 °C. SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  18. Bulk Density Example • If we have a soil that weighs 50 grams after being oven-dried and has a volume of 30 cm3, what will be the bulk density? • It would be 50 g/30 cm3 or 1.67 g/ cm3. SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  19. Bulk Density in the Field • Bulk density of organic soils can be very low, such as 0.5 g/cm3. • Clayey soils have higher bulk densities. • Compacted clay soils have very high bulk densities, between 1.6 and 1.8 g/cm3. SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  20. Particle Density and Porosity • Particle density is the mass per unit volume of soil particles. • Particle density is a relatively constant parameter and is sometimes assumed to be 2.65 g/cm3. SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  21. Bulk Density vs. Particle Density http://soil.gsfc.nasa.gov/pvg/pd3.htm SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  22. Porosity • Soil porosity is the percentage of a soil that is pore space or voids. • The average soil has a porosity of about 50%, and the pores are filled with air or water depending on the moisture content. • Sands have larger pores, but less total pore space than clays. • If both bulk density and particle density are known, the total porosity can be calculated using these values. SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  23. Soil Permeability • Soil permeability is the ease with which air, water, or plant roots penetrate or pass through soil. • Soils with large pores that are connected are more permeable. • Rainwater soaks in readily and moves down through the soil profile. • Clayey soils can have greater total porosity than sand and still be less permeable than sand since the pores are small. SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  24. Soil Strength • Soil strength is defined as the amount of force required to move or rearrange soil particles. • Soil strength is an important physical property affecting plant growth and seedling emergence. • It is affected by three main factors; moisture content, soil texture, and bulk density. SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  25. Moisture Content & Soil Strength • Water content is the most important factor determining soil strength. • The lower the soil water content, or the drier the soil, the greater the soil strength. • Soils that are saturated, or waterlogged have weak soil strengths. SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  26. Moisture Content • Which soil would have the greater soil strength? http://www.wy.blm.gov/botany/pics/cob-duckswamp-500wfo.jpg http://www.research.noaa.gov/spotlite/archive/images/drysoil.jpg SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  27. Soil Texture & Soil Strength • Soil texture also affects soil strength. • Soil strength of aggregated soils increases as clay content increases. • Poorly aggregated or single grain soils (sands, loamy sands, sandy loams) usually have the weakest soil strengths unless they are cemented or compacted. • Individual particles of single grain (sandy) soils are easy to rearrange, but these soils are susceptible to compaction, sometimes resulting in the formation of hard pans. SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  28. Bulk Density & Soil Strength • Bulk density is the third factor affecting soil strength. • As bulk density of a given soil increases soil strength also increases. • Remember that soil is composed of solids and pores, and the greater the bulk density the greater the amount of solids, and the smaller the amount of pore space. SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  29. Soil Management Problems • Two examples of management problems caused by increasing soil strength are soil crusts and tillage pans. • A soil crust is a thin soil layer that forms at the soil surface following heavy rains. • Tillage or hard pans are high bulk density (>1.7 g/cm3) layers that occur within the Ap and E horizons. • Compaction caused by traffic of equipment, vehicles, or even foot traffic often increases soil strength to levels that restrict root penetration and plant growth. SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

  30. Penetrometers • The force required to push a rod into the soil is a measure of soil strength. • Penetrometers are devices used to measure the resistance of a soil to penetration to estimate the effect of compaction on growth, and to detect layers of different soil strength. SUMBER: broome.soil.ncsu.edu/ssc012/Labs/4.ppt‎

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