D Have you ever noticed the colour of the water in a river or stream after a heavy rainfal

D

Haveyouevernoticedthecolourofthewaterinariverorstr eamafteraheavyrainfall?Whatd oyouthinkcausedthischangeincolour?Itissoilthathasbeenwashedintotheriverfromtheriverbankorfromt}lenearbyfields.

ComponentsofSoil

Soilismadeupofanumberoflayers（层），eachhavingitsowndistinctivecolourandtexture.Theupperlayerisknownasthelitter.Itactslikeablanket.limitingtemperaturechangesandreducingwaterloss.Thetopsoillayerismadeupofsmallparticlesofrockmixedwithrottenplantandanimalmattercalledhumus（腐殖质），whichisblackandgivesthetopsoilitsdarkcolour.Thislayerisusuallyrichinnutr ients，oxygen，andwater.Belowthetopsoilisthesubsoil，alayerthatcontainsmorestonesmixedwithonlysmallamountsoforganicmatter.Thislayerislighterincolourbecauseofthelackofhumus.Beneaththesoilliesalayerofbedrock.

Soilformsfromthebottomup.Overtimebedrockisattackedbyrain,wind，frost,andsnow.Itisgraduallybrokendownintosmallerparticlesinaprocesscalledweathering.Plantsbegintogrow，androttenmaterialsenrichthetopsoil.MostofthesoilinEasternCanada.forexample.Wasformedfromweatheredrockthatwasexposedwhentheicedisappearedl2,000yearsago.

WaterBeneaththeSoil

Surfacewatercollectsandflowsabovethegroundinlakes.ponds.andrivers.onceinthesoilorrock，itiscalledgroundwater.Gravitypullsgroundwaterthroughthesoilinaprocesscalledpercolation（渗透）.Eventuallythewaterreachesalayercalledthewatertable .Underthisisbedrockthroughwhichwatercannotpercolate.

Aswaterpercolatesdownward，itd issolvesorganicmatterandmineralsfromthesoilandcarriesthemtodeeperlayers.Thiscausesaseriousproblembecauseplantsrequirethesenutr ientsforgrowth.

SoilpH

Soilcanbeacidic.neutr al.orbasic.ThepHofthesoilisdeterminedbythenatureoftherock

fromwhichitwasformed.andbythenatureoftheplantsthatgrowandrotinit.

TheacidityofrainandsnowcanlowerthepHofthegroundwaterthatentersthesoil.Byburningfossilfuelssuchascoal，oilandgasoline，humanshavebeencontr ibutingtohigherlevelsofacidityinmanysoils.Whenfossilfuelsareburned.gasesarereleasedintotheairandthenfallbacktoearthasacidrain.Acidsoilincreases出eproblemofcarryingnutr ientstolowersoillevels.Asnutr ientsareremoved，soilislessfertile.Plantsgrowmoreslowlyinacidicsoil，andalsobecomeeasilyattackedbydiseases.

67.Thelayerofsoilthatprovidesnecessarynutr ientsforplantgrowthiscalled.

A.1itterB.topsoilC.humusD.subsoil