1. Soil texture and structure
Chapter 4
Soil texture and structure

2. Soil three phases Constitution
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; the measurement does not include water weight or pore (air) space. The dominant soil minerals- quartz, feldspars, micas, and clay minerals-average approximately 2.65 g/cm3, the “standard” value used in calculations 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 (lead ore).

4. （二）Soil bulk density
1. Bulk density, the density for a volume of soil as it exists naturally, includes any air space and organic materials in the soil 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. 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 various nature factors and anthropogenic factors.

6. 二. Soil three phases constitution and pore
(一)、Soil three phases constitution and porosity

7. 大 孔 隙 小 孔 隙

8. 1、 Index three phases constitution
Solid phase (%) =（solid volume /soil body volume) ×100
Liquid phase (%) =（liquid volume /soil body volume) ×100
Gas 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. (二)、Calculation of three phases constitution and porosity
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 porosity
Solid ratio
Solid ratio =（bulk density /density）= ρb/ ρp
2. 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. (三). Suitable soil three phases ratio：
3. Gas phase ratio
Porosity(P0) =1- solid phase ratio =1- bulk density /density）
= ρb/ ρp
Gas phase ratio = porosity- volumetric water content = P0- θv
4. Actual volume ratio
Solid phase ratio ＝ actual volume ratio－ volumetric water content
Gas phase ratio = 1－ actual volume ratio
Soil 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. Mineral soil particle and organic soil particle
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. Particle Diameter Size
Soil particle diameters range over 4 orders of magnitude
Boulders
Sand
Silt
Clay

13. Kachinsky classification system of soil separates
Physical sandy particle:
1 mm to 0.01 mm.
Physical clay particle:
< 0.01 mm

15. Soil particle Classification system of soil separates ：(mm)
Diameter International USDA Kachinsky Chinese
Boulders > > > >1
Sand
Silt
Clay 〈 〈 〈 〈0.002

16. (一） Soil mechanical composition
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

17. (1) International classification：
The textural triangle is used to determine the soil textural name after the percentages of sand (2-0.02mm), silt ( mm), and clay (<0.002 mm) are determined from a laboratory analysis.
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”.

18. The U.S. Dept. of Agri. (USDA)

19. Soil texture

20. Soil texture

21. Sandy soil Loamy soil Clayed soil
The soil texture is divided by three main groups--sand, silt, and clay in our country.
Sandy soil
Loamy soil
Clayed soil

22. 1. Sand < 2 mm to > 0.02 mm Visible without microscope
三、The fertility characteristics, utilize and improves of different soil textures
(一)The fertility characteristics of different soil textures
1. Sand
< 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.

23. Sand
Feels gritty
Considered non-cohesive – does not stick together in a mass unless it is very wet.

24. 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

25. 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

26. 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
Clay

27. Clay Pores spaces are very small Water holding capacity
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

28. 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.

29. Loam Does not feel gritty Floury feel
Wet loam does not exhibit stickiness / plasticity / malleability

30. 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.

31. Silt is responsible for silting over gravel beds in rivers that are needed by fish for spawning.

32. 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.

33. (二）Utilize and improves of different soil textures
① Foreign soil methods
Soil texture can be changed only by mixing with another soil with a different textural class in small quantities
② Deep ploughing, deep turn over etc.
Adding peat or compost to a mineral soil is not considered changing the texture – since it only adds organic matter not sand, silt or clay!!

34. Section 3 Soil Structure
一、Soil structureness
（一）Concept of soil structure
Soil structure- The combination or arrangement of primary soil particles into secondary units or peds. The secondary units are characterized on the basis of size, shape, and grade (degree of distinctness).
(二) Classes and types of soil structure
Types of structure describe the ped shape with the terms granular, 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 (or very thick).

35. Soil structureness

36. platy, blocky, subangular blocky, prismatic, and columnar
Bad structure：
platy, blocky, subangular blocky, prismatic, and columnar
Good structure:
Granular
二. The genesis of granular structure
(一) The formation process of granular structure
Cohesion aggregation
① Condense
②Cohesion of inorganic matter
③ Cementation and compound of organic matter
④ Complex of organic-mineral
⑤ Earthworms and other small animals

37. 2. The process of incision and modelling
① Root ②Wetting and drying cycle
③ Alternate freezing and thawing ④ Tillage
The arrangement of soil particle

38. (二）Formation process of
soil aggregates
单个土粒
团聚体
微团粒

39. Humus 粉 粒 砂粒 粘粒 砂粒 粉粒

40. Ca2+ 腐 殖 质 土粒 土粒 土粒

41. Fe2+ 腐 殖 质 土粒 土粒 土粒
Fe3+
Al3+

42. (三) Microcosmic mechanism of soil aggregates formation
1. Clay domain theory

44. 2. Isoelectric condense theory

45. (四) Significance of granular structure in soil fertility
1. There are big and small pores in granular structure

46. (五) Management of soil structure
2.Balance to water and air in soil 3.Coordinated to keep and apply fertilizer 4. Suitable tillage 5. Had good plowed layer
(五) Management of soil structure
Apply organic fertilizer
Reasonable crop rotation
Reasonable tillage, water management and soil reclamation
Apply soil structure modifier
The electric current reclamation of saline-alkali soil

47. Section 4 Soil pores and soil body constitution
一、Soil porosity
(一) Calculation of soil porosity
1. 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）

48. 3. Loose and tight arrangement of soil particles
2. Scope of soil porosity
Sandy soil 30－45％
Loam soil %
Clay soil %
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）

49. (二) Grade of soil porosity
1. Capillary porosity

50. 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 mm average diameter. Size of silt particles.
Storage pores-Less than 0.03 mm average diameter. Size of clay particles.

51. 四、 Soil body constitution
(一) plowed layer constitution
(二) Texture profiles

52. (三) Structure profiles
A horizon
2. P horizon
3. W horizon
4. G (gley horizon)

53. （四） Porosity profiles The suitable porosity for plant growth:
Plowed layer soil : total porosity in 50% to 56%,
aeration pores in 8% to 10%.

54. 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?

55. The End