高级视听说6-7原文

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1、Chapter6HydroponicAquaculture:HowOneSystemWorks　　Thegrowingofplantswithoutsoilhasdevelopedfromexperimentscarriedouttodeterminewhatsubstances(likesoilandwater)makeplantsgrow.Growingplantsinwater(ratherthaninsoil)---inotherwords,hydroponics---datesbackmanymoreyearsthanyoumi

2、ghtthink.ScientistsbelievethathydroponicsoraquacultureisatleastasancientasthepyramidsofEgypt.ScientistsalsoknowthataprimitiveformofaquaculturehasbeenusedintheregionofKashmirforcenturies.Infact,scientistsbelievehydroponicgrowingactuallyprecededsoilgrowing.Theyevenbelieveth

3、atusinghydroponicsasafarmingtoolstartedintheancientcityofBabylonwithitsfamoushanginggardens.Thesehanginggardenswereprobablyoneofthefirstsuccessfulattemptstogrowplantshydroponically.However,returningtomoremoderntimes,researchersattheUniversityoftheVirginIslandshavedevelope

4、dasystemofhydroponicaquaculturethatisbothsimpleandlowcost.Thesystemusesgravitytocreaterecirculatingwatersystemsinwhichfishareraisedandvegetablesaregrown.LetmetakeaminutetoexplaintheprocessofhowthisparticularsystemofhydroponicaquacultureworksontheislandofSt.CroixintheVirgi

5、nIsland.　　Tostartwith,rainwateriscollectedinalarge3,000-gallontank.Thistankislocatedonthehighestpointoftheisland.Thetankissolargethatitmeasuresabout12feetindiameter.Oncethetankisfilledwithrainwater,fishareaddedtothetankandsubsequentlyraisedinalargetank.So,first,theresearc

6、herscollectrainwaterinalargetank,thentheyaddfish;thefishswimaroundandexcretewasteintothewater.Thenextstepintheprocesshappensinthisway.Therainwatercollectedinthelargetankslowlyrunsoutofthebottomofthelargefishtankandintoanothertank.Thisothertankholdsthewastefromthefish.Thew

7、ateristhenfiltered.Afterthewaterisfiltered,itispassedthrougha“bio-filter”thatcontainsbacteria.Thesebacteriaconvertanyharmfulammoniaproducedinthefishwasteintonitrates.Thesenitratesarethenusedtofeedtheplantsinthenextstageoftheprocess.　　So,whathappensnext?Well,afterthewaterh

8、aspassedthroughthisbio-filter,itenterstwo100-foot-longhydroponictanks.Justabovethe100-foot-longt