第四章土壤质地和结构

第四章土壤质地和结构第一节土壤三相组成土壤是由固、液、气三相构成的分散系，水、气在土壤中的协调供应，对实现土壤肥力起重要作用。土壤三相的体积比大体为：固相占50%，气相加液相占50%，二者处于动态变化之中。土壤三相之间的关系十分复杂，用公式表示为：1.DryBulkDensity

ρb(theratioofthemassofdriedsoiltoitstotalvolumeincludingsolidsandporestogether)ρb=Ms/Vt=Ms/(Vs+Va+Vw)2.Porosityff=Vf/Vt=(Va+Vw)/(Vs+Va+Vw)3.VoidRatioee=(Va+Vw)/Vs=Vf/(Vt-Vf)generally,evaroesbetween0.3and2.04.SoilWetness.a.MassWetnessww=Mw/Msb.VolumeWetnessθ,θ=Vw/Vt=Vw/(Vs+Vf)

c.Watervolumeratioυw

υw=Vw/Vsd.DegreeofSaturationss=Vw/Vf=Vw/(Va+Vw)5.Air-FilledPorosity

fa

=Va/Vt=Va/(Va+Vs+Vw)AdditionalInterrelations

(1)Relationbetweenporosityandvoidratio:e=f/(1-f)f=e/(1+e)(2)Relationbetweenvolumewetnessanddegreeofsaturation:

θ=sfs=θ/f(3)Relationbetweenporosityandbulkdensity:f=(ρs-ρb)/ρs=1-ρs/ρbρb=(1-f)ρs

(4)Relationbetweenmasswetnessandvolumewetness:θ=wρb/ρww=θρw/ρb

here,ρwisthedensityofwater(Mw/Vw),approximatelyequalto1g/cm3.(5)Relationbetweenvolumewetness,fractionalaircontent,anddegreeofsaturation:fa=f-θ=f(1-s)θ=f-fa

VolumeMassRelationsRelations

Va

Air

MaVf

Vw

Water

Mw

MtVt

Vs

Solids

Ms

第二节土壤结构Soilstructureistheshapethatthesoiltakesbasedonitsphysicalandchemicalproperties.Eachindividualunitofsoilstructureiscalledaped.Takeasampleofundisturbedsoilinyourhand(eitherfromthepitorfromtheshovelorauger).Lookcloselyatthesoilinyourhandandexamineitsstructure.Possiblechoicesofsoilstructureare:粒状结构Granular:Resemblescookiecrumbsandisusuallylessthan0.5cmindiameter.Commonlyfoundinsurfacehorizonswhererootshavebeengrowing.块状结构Blocky:

Irregularblocksthatareusually1.5-5.0cmindiameter

柱状结构Columnar:

Verticalcolumnsofsoilthathaveasalt"cap"atthetop.Foundinsoilsofaridclimates.片状结构Platy:Thin,flatplatesofsoilthatliehorizontally.Usuallyfoundincompactedsoil.

棱柱状结构Prismatic:

Verticalcolumnsofsoilthatmightbeanumberofcmlong.Usuallyfoundinlowerhorizons.

无结构SingleGrained:Soilisbrokenintoindividualparticlesthatdonotsticktogether.Alwaysaccompaniesalooseconsistence.Commonlyfoundinsandysoils.

大块状结构Massive:

Soilhasnovisiblestructure,ishardtobreakapartandappearsinverylargeclods.粒状结构（Granular）块状结构（Blocky）

棱柱状结构（Prismatic）

柱状结构（Columnar）

Columnar

Columnar

大块状结构（Massive）片状结构（Platy）

无结构（Singlegrained）

第三节土壤质地土壤质地指人对于土壤粗细程度的感觉，有时也用机械组成表示，即指土壤中各粒级的土粒所占的质量百分比。土壤质地根据土粒的大小即粒级划分，分为砂粒、粉粒和粘粒，进一步分为砂土组、壤土组、粘土组，及砂土、砂壤、轻壤、中壤、重壤、粘土6个质地名称。Thewayasoil"feels"iscalledthesoiltexture.Soiltexturedependsontheamountofeachsizeofparticleinthesoil.Sand,silt,andclayarenamesthatdescribethesizeofindividualparticlesinthesoil.Sandarethelargestparticlesandtheyfeel"gritty."Siltaremediumsized,andtheyfeelsoft,silkyor"floury."Clayarethesmallestsizedparticles,andtheyfeel"sticky"andtheyarehardtosqueeze.土壤机械组成——StokesLaw式中：υ为土粒沉降速度ρsρw为土粒密度和水的密度η为水的粘滞系数g为重力加速度r为土粒半径球体在介质中作自由落体沉降时重力（F1）是自重F2与浮力Fa之差，F1=F2-Fa=F（粘滞阻力）4/3πr3g（ρs-ρw）=6πrηυ

球体作匀速沉降时S=Vt土壤机械组成的测定及质地名称根据上式，测定已知直径的土壤颗粒沉降于某一定点的深度，计算出所需时间，用吸管法或比重计法，测定出该粒级土粒的重量及百分含量，由下面的三角图给出质地名称。RelativeSizeComparisonofSoilParticles

Plate=Silt

(feelsfloury)(0.02-0.