1. Chapter 4
Soil texture and structure

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; 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).

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. （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. 小 大
孔 孔
隙 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. 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
1. 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. 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. 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 >2 >2 >1 >1
Sand 2-0.02 2-0.05 1-0.05 1-0.05
Silt 0.02-0.002 0.05-0.002 0.05-0.001 0.05-0.002
Clay 〈 0.002 〈 0.002 〈 0.001 〈 0.002

16. 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 (0.02-0.002
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. The soil texture is divided by three main groups--
sand, silt, and clay in our country.
Sandy soil
Loamy soil
Clayed soil

22. 三、 The fertility characteristics, utilize and improves of different
soil 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.

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. 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

27. 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

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. 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
1. 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 ② Wetting 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 砂粒
粒
粘粒
粉粒 砂粒

40. Ca2+
腐
土粒
殖
质
土粒
土粒

41. Fe2+
土粒 腐
殖
质
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. 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
1. Apply organic fertilizer
2. Reasonable crop rotation
3. Reasonable tillage, water management and soil
reclamation
4. Apply soil structure modifier
5. 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. 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）

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- 0.1-0.03 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
1. 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 ?