Have you evernoticed the colour of the water in a river or streamafter a heavyrainfall? What do you thinkcausedthischange in colour? It is soilthat has beenwashedinto the riverfrom the riverbank or from the nearbyfields．
Components of SoilSoil is made up of a number of layers(层)，eachhaving its own distinctivecolour and texture．The upperlayer is known as the litter．It actslike a blanket．limitingtemperaturechanges and reducingwaterloss．The topsoillayer is made up of smallparticles of rockmixedwithrottenplant and animalmattercalledhumus(腐殖质)，which is black and gives the topsoil its darkcolour．Thislayer is usuallyrich in nutrients，oxygen，and water．Below the topsoil is the subsoil，a layerthatcontainsmorestonesmixedwithonlysmallamounts of organicmatter．Thislayer is lighter in colourbecause of the lack of humus．Beneath the soillies a layer of bedrock．
Soilformsfrom the bottom up．Overtimebedrock is attacked by rain, wind，frost, and snow．It is graduallybrokendownintosmallerparticles in a processcalledweathering．Plantsbegin to grow，and rottenmaterialsenrich the topsoil．Most of the soil in EasternCanada．for example．Was formedfromweatheredrockthat was exposedwhen the ice disappeared l2．000 years ago．
WaterBeneath the SoilSurfacewatercollects and flowsabove the ground in lake, pond, and rivers．Once in the soil or rock，it is calledgroundwater．Gravitypullsgroundwaterthrough the soil in a processcalledpercolation(渗透)．Eventually the waterreaches a layercalled the watertable．Underthis is bedrockthroughwhichwatercannotpercolate．
As waterpercolatesdownward，it dissolvesorganicmatter and mineralsfrom the soil and carriesthem to deeperlayers．Thiscauses a seriousproblembecauseplantsrequirethesenutrients for growth.
Soil can be acidic．neutral．or basic．The pH of the soil is determined by the nature of the rockfromwhich it was formed．and by the nature of the plantsthatgrow and rot in it．
The acidity of rain and snow can lower the pH of the groundwaterthatenters the soil．By burningfossilfuelssuch as coal，oil and gasoline，humanshavebeencontributing to higherlevels of acidity in manysoils．Whenfossilfuels are burned．gases are releasedinto the air and thenfallback to earth as acidrain．Acidsoilincreases the problem of carryingnutrients to lowersoillevels．As nutrients are removed，soil is lessfertile．Plantsgrowmoreslowly in acidicsoil，and alsobecomeeasilyattacked by diseases． 【題組】69．We can inferfrom the passagethat the watertablelies ．
(A)between the topsoillayer and the subsoillayer (B)in the subsoillayerabovebedrock (C)between the subsoillayer and bedrock (D)in the bedrocklayerbeneath the subsoil