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Chapter 5

soils 2B03 chapter 5.docx

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McMaster University
Earth Sciences
Sherry Smith

2B03 start of chapter 5 oct 2  determine texture by feel: we don't use this specific texture  texture is grain size distribution  can use settling rates in water; stir it up is more dense then water, float thing to determine density, hydrometer will go lower as particles fall  uses stokes law: just need to know why particles fall at different rates due to friction i.e . depends on how thick fluid is (water vs honey)  use a sieve and use that for grain size distribution  different textures have different properties (see behaviour of soils chart  well graded; lots of different sizes, well sorted means all the same size  particle density: mass of solid material/ volume of solid material  2.65 gm/cm3 is density of quartz; starting point for all soils  dry bulk density: mass over volume= mass/ volume of soil+air+water  porosity= n= volume of voids over total volume= (air+water)/(soil+air+water)  soil wetness: how much water in the soil  mass water content= mass of water content/ mass of soil  volumetric water content= volume of water/ volume of soil+ volume of voids  when asked for water content: asking for volumetric content  degree of saturation: volume of water/ volume of air  void ratio= e= volume air+ volume water/ volume soil  air filled porosity: volume air/ volume total 2B03 chapter 5  soil tension = (-) soil pressure, negative numbers and such  fig 5.12 soil water potential is pressure in potential units kilopascals (kPa) (shows range of pore sizes)  volume moisture content= when perfect saturated = porosity  sand porosity ~ 25%, loam ~ 45%, clay is like ~60%  move along diagram more negative pressure  as goes rightward, bigger pores loose water first  sand graph looses water quicker because of its larger pores  curvature of meniscuses is same for each (at each pressure shows size of pore eg at -1000 each has size pores x and under at the point)  textural influences  conductivity vs pressure  conductivity completely changes over pressure  somethings are a million times slower (note the log scale)  clay soil works better at low pressure because in sandy loam there isnt much water left to conduct, clay still has about 50% of tiny pores still filled with water  fig 5.14porous ceramic (looks like marshmellow) as the soil gets water or drier the cermatic caliprates its properties, showing the changes in pressure  use for dry soils, eg to find wilting point of plants, used in irrigation systems  figure 5.17 has little porous tip on the end (always filled with water)  when soil is at -1000, soil will try to pull water out of perfectly sealed instrument, pulling pressure (tension) is recorded  fig 5.15 neutron bulb, neutrons come out of source into the soil, when neutrons hit hydrogen, it hits about 32 it becaomes a thermalized neutron censor (finds slow ones)  wetter soils have more hydrogen therefore will have more, slower netrons will come back faster 
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