4.soil texture and structure

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4.soil texture and structure

  1. 1. Chapter 4Soil texture and structure
  2. 2. Section 1 Soil three phases Constitution 一、 Soil density and bulk density (一) Soil density ( or Particle density) Soil density is the density of the solid soil particles only; themeasurement does not include water weight or pore (air) space. Thedominant soil minerals- quartz, feldspars, micas, and clay minerals-average approximately 2.65 g/cm3, the “standard” value used incalculations if particle density is not measured. Individual minerals have densities from 2.0 g/cm3 for bauxite(aluminum ore) to 5.3 for hematite (iron ore) or 7.6 for galena (leadore).
  3. 3. 表 4- 1 土壤中常见组分的密度 ( 克/ 厘米 3)石 英 2.60~2.68 赤铁矿 4.90~5.30正长石 2.54~2.57 磁铁矿 5.03~5.18斜长石 2.62~2.76 三水铝石 2.30~2.40白云母 2.77~2.88 高岭石 2.61~2.68黑云母 2.70~3.10 蒙皂石 2.53~2.74角闪石 2.85~3.57 伊利石 2.60~2.90辉 石 3.15~3.90纤铁矿 3.60~4.10 腐殖质 1.40~1.80
  4. 4. (二) Soil bulk density 1. Bulk density, the density for a volume of soil as it existsnaturally, includes any air space and organic materials in thesoil volume. ρb = (mass of dry soil)/(total volume of soil) = Ms/Vt = Ms/(Vs + Vw +Va) A recently cultivated topsoil is likely to have a bulk density in the range from 0.9 to 1.2 g/cm3, while a compact subsoil may have a bulk density of 1.6 g/cm3 or even more.
  5. 5. Uses of soil bulk density : ①To calculate soil porosity(p0): P0 = 1- (Bulk density/soil density) ② To calculate soil weight ③ To calculate storage of soil compositions ④ To calculate total water storage capacity and irrigation (or drainage) quota.2. Bulk density in water = (soil mass over-dry)/(precipitation volume)3. Affect the factors of soil bulk density: Texture, structure, content of organic matter, and variousnature factors and anthropogenic factors.
  6. 6. 二 . Soil three phases constitution and pore( 一 ) 、 Soil three phases constitution and porosity
  7. 7. 小 大孔 孔隙 隙
  8. 8. 1 、 Index three phases constitutionSolid phase (%) = ( solid volume /soil body volume) ×100Liquid phase (%) = ( liquid volume /soil body volume) ×100Gas phase (%) = ( air volume /soil body volume) ×100 2 、 Soil porosity: Soil porosity is the percentage of the volume of pores to the total volume of soil. Soil porosity = 1 - Solid phase (%) = Liquid phase (%) + Gas phase (%)
  9. 9. 3 、 Void ratio The void ratio is the ratio of the volume of pores to the volume of solids. Void ratio = (volume of pores)/(volume of solids)( 二 ) 、 Calculation of three phases constitution and porosity1. Solid ratioSolid ratio = ( bulk density /density ) = ρb/ ρp2. Liquid ratio (volumetric water content)θm = (mass of water /mass of dry soil) = MW/MS θv = (volume of water/ total volume of soil) = Vw/Vt = Vw/(Vs+Vw+Va) = θm × ρb
  10. 10. 3. Gas phase ratioPorosity(P0) =1- solid phase ratio =1- bulk density /density ) = ρb / ρpGas phase ratio = porosity- volumetric water content = P0- θv4. Actual volume ratioSolid phase ratio = actual volume ratio - volumetric watercontentGas phase ratio = 1 - actual volume ratioSoil three phases ratio = solid phase ratio : volumetric water content : gas phase ratio( 三 ). Suitable soil three phases ratio :Solid phase ratio is about 50% ,volumetric water content is 25% to 30% , Gas phase ratio is 15% to 25%.
  11. 11. Section 2 Soil texture 一、 Soil particle and particle fraction (soil separates) ( 一 ) Types of soil particle : • Mineral soil particle and organic soil particle • Single particle and compound particle •Primary particle and second particle ( 二 ) Soil particle fraction (soil separates) Natural soils are comprised of soil particles of varying sizes. The soil particle-size groups, called soil particle fraction (soil separates), are sands (the coarsest), silts, and clays (the smallest).
  12. 12. Particle Diameter Size• Soil particle diameters range over 4 orders of magnitude – Boulders – Sand – Silt – Clay
  13. 13. Kachinsky classification system of soil separates Physical sandy particle: 1 mm to 0.01 mm. Physical clay particle: < 0.01 mm
  14. 14. Soil particleClassification system of soil separates : (mm)Diameter International USDA Kachinsky ChineseBoulders >2 >2 >1 >1Sand 2-0.02 2-0.05 1-0.05 1-0.05Silt 0.02-0.002 0.05-0.002 0.05-0.001 0.05-0.002Clay 〈 0.002 〈 0.002 〈 0.001 〈 0.002
  15. 15. 二、 Soil mechanical composition and texture ( 一) Soil mechanical composition ( 二) Soil texture 1.Concept: Soil texture-The relative proportions of the various soil separates in a soil. 2. Classification of soil texture
  16. 16. (1) International classification : The textural triangle is used to determine the soil texturalname after the percentages of sand (2-0.02mm), silt (0.02-0.002mm), and clay (<0.002 mm) are determined from a laboratoryanalysis. The main criterion of classification :Clay <15% is sand and loamy groups; 15 %- 25 % is clay loam group; >25% is clay group.When soil contains silt >45% , soil will be called “silty” ;When soil contains sand in 55%-85% , called “sandy” ,When soil contains sand >85% , called “ loamy sand”or “sand”.
  17. 17. The U.S. Dept. of Agri. (USDA)
  18. 18. Soil texture
  19. 19. Soil texture
  20. 20. The soil texture is divided by three main groups--sand, silt, and clay in our country. Sandy soil Loamy soil Clayed soil
  21. 21. 三、 The fertility characteristics, utilize and improves of differentsoil textures ( 一 )The fertility characteristics of different soil 1. Sand textures• < 2 mm to > 0.02 mm• Visible without microscope• Rounded or angular in shape• Sand grains usually quartz if sand looks white or many minerals if sand looks brown,• Some sands in soil will be brown, yellow, or red because of Fe and/or Al oxide coatings.
  22. 22. Sand• Feels gritty• Considered non- cohesive – does not stick together in a mass unless it is very wet.
  23. 23. Sand• Low specific surface area• Sand has less nutrients for plants than smaller particles• Voids between sand particles promote free drainage and entry of air• Holds little water and prone to drought
  24. 24. 2. Clay• < 0.002 mm• Flat palettes or tiny flakes• Small clay particles are colloids – If suspended in water will not settle• Large surface area – spoonful = football field
  25. 25. Clay• Wet clay is very sticky and is plastic or it can be molded readily into a shape or rod.• Easily formed into long ribbons• Shrink and swell
  26. 26. Clay• Pores spaces are very small – Movement of water and air very slow• Water holding capacity – Tremendous capacity to adsorb water- not all available for plants.• Soil strength- shrink/swell affects buildings, roads and walls.• Chemical adsorption is large
  27. 27. 3. Loam• < 0.02 mm to > 0.002 mm• Not visible without microscope• Quartz often dominant mineral in loam since other minerals have weathered away.
  28. 28. Loam• Does not feel gritty• Floury feel• Wet loam does not exhibit stickiness / plasticity / malleability
  29. 29. Loam• Smaller size allows rapid weathering of non quartz minerals• Smaller particles – retains more water for plants and have slower drainage than sand.• Easily washed away by flowing water – highly erosive.• Holds more plant nutrients than sand.
  30. 30. • Silt is responsible for silting over gravel beds in rivers that are needed by fish for spawning.
  31. 31. Loess Cliff in China• Silt if often left as a vertical face since in this condition the landscape is less prone to water erosion than if the area was graded to a slope.
  32. 32. ( 二) Utilize and improves of different soil textures① Foreign soil methods Soil texture can be changed only bymixing with another soil with a differenttextural class in small quantities② Deep ploughing, deep turn overetc. Adding peat or compost to a mineral soil isnot considered changing the texture – since itonly adds organic matter not sand, silt orclay!!
  33. 33. Section 3 Soil Structure 一、 Soil structureness(一) Concept of soil structure Soil structure- The combination or arrangement ofprimary soil particles into secondary units or peds. Thesecondary units are characterized on the basis of size,shape, and grade (degree of distinctness).( 二 ) Classes and types of soil structureTypes of structure describe the ped shape with the termsgranular, crumb, platy, blocky, subangular blocky,prismatic, and columnar. Structure classes are the ped sizes such as very fine,fine, medium, coarse (or thick), and very coarse (orvery thick).
  34. 34. Soil structureness
  35. 35. Bad structure : platy, blocky, subangular blocky, prismatic, and columnar Good structure: Granular二 . The genesis of granular structure( 一 ) The formation process of granular structure1. Cohesion aggregation① Condense②Cohesion of inorganic matter③ Cementation and compound of organic matter④ Complex of organic-mineral⑤ Earthworms and other small animals
  36. 36. 2. The process of incision and modelling① Root ② Wetting and drying cycle③ Alternate freezing and thawing ④ TillageThe arrangement of soil particle
  37. 37. ( 二) Formation process ofsoil aggregates 单个土粒 团聚体微团粒
  38. 38. Humus粉 砂粒粒 粘粒粉粒 砂粒
  39. 39. Ca2+ 腐 土粒 殖 质土粒 土粒
  40. 40. Fe2+ 土粒 腐 殖 质土粒 土粒 Fe3+ Al3+
  41. 41. ( 三 ) Microcosmic mechanism of soilaggregates formation 1. Clay domain theory
  42. 42. 2. Isoelectric condense theory
  43. 43. ( 四 ) Significance of granular structure in soil fertility1. There are big and small pores in granular structure
  44. 44. 2.Balance to water and air in soil3.Coordinated to keep and apply fertilizer4. Suitable tillage5. Had good plowed layer( 五 ) Management of soil structure1. Apply organic fertilizer2. Reasonable crop rotation3. Reasonable tillage, water management and soil reclamation4. Apply soil structure modifier5. The electric current reclamation of saline-alkali soil
  45. 45. Section 4 Soil pores and soil body constitution 一、 Soil porosity ( 一 ) Calculation of soil porosity1. Soil porosity = pore volume/soil volume=(soil volume – soil particle volume)/soil volume= 1- ( soil particle volume/ soil volume )= 1-[ ( soil particle volume×soil weight ) / ( soil volume× soil weight ) ]= 1-[ ( soil weight/ soil volume ) × ( soil particle volume/ soil weight ) ]=1-[ ( soil weight/ soil volume ) / ( soil weight / soil particle volume ) ]= ( 1-bulk density/ density )
  46. 46. 2. Scope of soil porosity Sandy soil 30 - 45 % Loam soil 40-50% Clay soil 45-60% Peaty soil 〉 80 %3. Loose and tight arrangement of soil particles 方体排列的孔度为 47.46% (左) 三斜六面体排列的孔度为 24.51 %(右)
  47. 47. ( 二 ) Grade of soil porosity 1. Capillary porosity
  48. 48. 2. Classification of porosity (Studied by yourself)3. Aeration porosity二 . The porosity model of granular structure (Studied by yourself)三 . Equivalent pore diameter (effective aperture)Aeration pores- Greater than 0.1 mm average diameter.Size of medium sands.Capillary pores- 0.1-0.03 mm average diameter. Size of siltparticles.Storage pores-Less than 0.03 mm average diameter. Size ofclay particles.
  49. 49. 四、 Soil body constitution( 一 ) plowed layer constitution( 二 ) Texture profiles
  50. 50. ( 三 ) Structure profiles1. A horizon2. P horizon3. W horizon4. G (gley horizon)
  51. 51. (四)  Porosity profiles The suitable porosity for plant growth: Plowed layer soil : total porosity in 50% to 56%, aeration pores in 8% to 10%.
  52. 52. 二、思考题1 、团粒结构在土壤肥力方面作用和意义有哪些 ?2 、为什么说粒状——团粒状结构是农业生产上比较理想的结构 ? 培育良好结构的有效途径是什么 ?

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