Chapter 4Soil texture and structure

Section 1Soil 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).

表 4-1 土壤中常见组分的密度 (克/厘米 3)

石 英

正长石

斜长石

白云母

黑云母

角闪石

辉 石

纤铁矿

2.60～2.68

2.54～2.57

2.62～2.76

2.77～2.88

2.70～3.10

2.85～3.57

3.15～3.90

3.60～4.10

赤铁矿

磁铁矿

三水铝石

高岭石

蒙皂石

伊利石

腐殖质

4.90～5.30

5.03～5.18

2.30～2.40

2.61～2.68

2.53～2.74

2.60～2.90

1.40～1.80

（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.

Uses of soil bulk density：

①To calculate soil porosity(p0):

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.

P0 = 1- (Bulk density/soil density)

(一 )、 Soil three phases constitution and porosity

二 . Soil three phases constitution and pore

小孔隙

大孔隙

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 (%)

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 ratioSolid 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

3、 Void 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%.

(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).

一、 Soil particle and particle fraction (soil separates)

Section 2 Soil texture

Particle Diameter Size• Soil particle diameters

range over 4 orders of magnitude– Boulders– Sand– Silt – Clay

Physical sandy particle: 1 mm to 0.01 mm.

Physical clay particle: < 0.01 mm

Kachinsky classification system of soil separates

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

( 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

二、 Soil mechanical composition and texture

(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”.

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

Soil texture

Soil texture

Clayed soil

Loamy soil

Sandy soil

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

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.

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

(一 )The fertility characteristics of different soil textures

Sand

• Feels gritty • Considered non-

cohesive – does not stick together in a mass unless it is very wet.

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

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

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

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

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.

Loam

• Does not feel gritty• Floury feel • Wet loam does not

exhibit stickiness / plasticity / malleability

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.

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

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.

(二）Utilize and improves of different soil textures① Foreign soil methods Soil texture can be changed only by Soil texture can be changed only by mixing with another soil with a different mixing with another soil with a different textural class in small quantitiestextural 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!!

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).

Section 3 Soil Structure

(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).

Soil structureness

Bad structure： platy, blocky, subangular blocky, prismatic,

and columnarGood 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

① Root Wetting and drying cycle ② ③ Alternate freezing and thawing Tillage④

2. The process of incision and modelling

The arrangement of soil particle

单个土粒

团聚体

微团粒

(二） Formation process of

soil aggregates

砂粒

砂粒粉粒

粉粒

粘粒

Humus

土粒

土粒

土粒

Ca2+

腐殖质

土粒

土粒

土粒

Fe2+

腐殖质

Fe3+

Al3+

1. Clay domain theory

(三 ) Microcosmic mechanism of soil aggregates formation

2. Isoelectric condense theory

(四 ) Significance of granular structure in soil fertility

1. There are big and small pores in granular structure

2.Balance to water and air in soil3.Coordinated to keep and apply fertilizer4. 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

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）

Section 4 Soil pores and soil body constitution

一、 Soil porosity

(一 ) Calculation of soil porosity

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）

(二 ) Grade of soil porosity

1. Capillary porosity

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.

四、 Soil body constitution

(一 ) plowed layer constitution

(二 ) Texture profiles

(三 ) Structure profiles

1. A horizon

2. P horizon

3. W horizon

4. G (gley horizon)

（四） Porosity profiles

The suitable porosity for plant growth:Plowed layer soil : total porosity in 50% to 56%, aeration pores in 8% to 10%.

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 ?

二、思考题