ZbornikMaticesrpskezaprirodnenauke / Proceedings for Natural Sciences, Matica SrpskaNovi Sad, 104, 2337, 2003UDC631.416:546.23M a j a S . u v a r d i Facultyof Agriculture, TrgDositejaObradovia8, Novi SadInstituteof FieldandVegetableCrops, MaksimaGorkog30, Novi SadSELENIUMINSOILABSTRACT:Selenium(Se)isanessentialmicroelement, necessaryfornormalfunc-tioningofhumanandanimalorganisms.Itsdeficiencyinfoodandfeedcausesanumberofdiseases.Inhighconcentrations,seleniumistoxicforhumans,animalsandplants.Soilpro-visionwithseleniumaffectsitslevelinfoodandfeedvianutritionchain. However, seleni-umreactivityandbioavailabilitydependsnotonlyonitstotalcontentinsoilbutalsoonitschemical forms. Distributionof thedifferent formsof seleniumdependsonsoil propertiessuch as reaction, aeration, contents of clay and organic matter and microbiological activity.KEYWORDS: selenium, total Se, water-solubleSe, soilINTRODUCTIONThe biological importance of selenium is reflected in the fact that it is es-sential for humans and animals on one side (R a y m a n, 2000), and toxicwhen in high concentration on the other. This controversial element was disco-veredbyBerzelius, aSwedishscientist, in1817, althoughit wasMarcoPoloduringhis travel toAsia, as earlyas the 13thcentury, whoobservedtoxicsymptomsinanimalsfeedingonplantsthat containedahighconcentrationofselenium. Inthefirst halfofthe20thcentury, seleniumwasconsideredexclu-sivelyfromthestandpoint of itstoxicity, until it wasidentifiedasacompo-nentofapreparationwhichpreventedthenecrosisofliverinrats(S c w a r zand F o l t z, 1957). Soon afterwards, selenium was found to play a key role inglutathioneperoxidase, anenzymethat catalyzesthedecompositionofhydro-genperoxidethusprotectingcellsfromoxidativedamage.Seleniumalsoplaysanimportant anti-carcinogenicroleandit neutralizesheavymetalstoxicity.Soil provisionwithseleniumaffectsitslevel infoodandfeedvianutri-tion chain (F i n l e y et al., 2000). However, seleniumreactivity and bio-availabilitydependsnotonlyonitstotalcontentinsoilbutalsoonitschemi-calform. Dependingonoxidationstate, seleniumispresentinsoilasselenide(Se2), elemental selenium(Se0), selenite(SeO32), selenate(SeO42)andor-ganicselenium. Thewater-solubleseleniumfractionisconsideredtobemostavailabletoplants. Its content innormal tilledsoil does not exceed50ppb23(W o r k m a n and S o l t a n p o u r,1980).Althoughseleniumisnotessentialfor plants, theytakeupseleniumandincorporateit intheir aminoacidsandproteins (S h r i f t, 1973). The level of seleniumaccumulationinplants de-pendsontheamount of availableSe, pHvalueandredoxpotential, contentsof sesquioxide, clay and organic matter and the microbiological activity of soil(J u m p and S a b e y, 1989).GEOGRAPHICDISTRIBUTIONOFSELENIUMSeleniumisabundant indifferent partsoftheworldbut itslevel insoilvaries withnative substrate, climatic conditions andvegetationcover (M c -N e a l and B a l i s t r i e r i,1989).IncentralUS,forexample,thereareregi-ons inwhichplants containSe levels 10times higher thanthe toxic level,whileSelevelsinplantsineasternandwesternUSarelow(K u b o t a etal.,1967). IneasternCanada, Seconcentrationsinplant drymatteraremuchbe-low0.1ppm(W i n t e r and G u p t a, 1979), whiletheyare10timeshigherinwesternCanada. SeleniumdeficiencywasobservedinsomepartsofSouthAmerica, seleniumtoxicityinothers(J a f f e, 1973). SeleniumdeficiencywasnotedinwesternandsouthernpartsofAustraliaandinNewZealand(W e l s het al., 1981).SevereseleniumdeficiencyregisteredinsoutheasternChinahasbeenas-sociatedwithKeshandisease, adiseasethatoccursalmostexclusivelyinchil-dren.InIndia,toxicitywasobservedintheregionofHarayanaanddeficiencyintheotherpartsofthecountry(D h i l l o n and D h i l l o n ,1991).TotalSecontent isslightlyincreasedinJapanesesoilsascomparedwiththecontentsestablished in other countries (K a n g, 1990). However, because these soils ha-veanacidreactionandbecauseofthehumidclimateofJapan, theconcentra-tionofwater-solubleSeislow, makingonly2.33.85%ofthetotal Secon-tent (K a n g, 1990).InEurope, toxiclevel was registeredinseveral locations inWales andIreland(F l e m i n g, 1962). SymptomsofSedeficiencywereobservedincat-tleinEnglandandScotland(R i m m e r etal.,1990).SuboptimalSecontentswere reported for northern Europe. In Finland, before Se fertilization became aregular practice, wheat contained only 1015 ppb Se (K o i v i s t o i n e n andV a r o, 1981). LowSelevelswereregisteredinPoland(meanvalue0.27mgg1) (B o r o w s k a, 1998) andHungary(G o n d i et al., 1992; H o r v a t het al., 1996).Inour country, preliminarystudiesof soil, cereal andanimal feedsam-ples (K r a j i n o v i , 1983) indicated the presence of low Se levels. Later stu-diesofSecontentingeologicalmaterials(J o v i etal., 1995; D a n g i etal., 1989), soil (J a k o v l j e v i et al., 1995; M a k s i m o v i et al., 1992; u v a r d i et al., 1997) andbiological materials (M a k s i m o v i et al.,1992) confirmedapronouncedSedeficiencyonthisterritory. PerilouslylowSe contents were found in blood serumand hair of two groups of maleexaminees, minersintheMajdanpekcoal stripmineandinhabitantsof are-gioninthevicinityof Belgrade(serum31ppband25ppb, respectively;hair76ppband73ppb, respectively). Accordingtotheseresults, theana-24lyzed populations were comparable to the inhabitants of the countries sufferingasevereSedeficiency, suchasChina, NewZealandandFinland(M a k s i -m o v i et al., 1992).SELENIUMINLITOSPHEREThe primary sources of Se in nature are volcanic rocks and metal sulfidesformedbyvolcanicactivity(NAS, 1974). Althoughsome50mineralscontainSe,itoccursmostfrequentlyinsulfidesofheavymetals(Ag,Cu,Pb,Hg,Ni,etc.),eitherintheformofselenideorsubstitutingthesulfurion(S)inthemi-nerals' crystal grid (A d r i a n o, 1986). Average Se content in the Earth'scrustisconsideredtovaryfrom0.05to0.09mgg1(L a k i n,1972).TotalSecontentvariesfrom0.1to2mgg1inmostsoils(S w a i n e,1955).Increasedamountsof Searefoundinthesoilsformedbythedecompositionof shale,mostlyinaridandsemiaridregions(R o s e n f e l d and B e a t h, 1964).Seleniumcontent inmagmaticrocksseldomexceeds0.05mgg1(K a -b a t a - P e n d i a s,1993).AnextremelylowSecontentwasfoundinmagma-ticrocksfromSerbia(meanvalue46ppbSe)(M a k s i m o v i etal.,1986).Intheserocks, Seis part of sulfides inwhichit substitutes sulphur. So, Secontent depends on the size of sulfide phase. In magmatic rocks, Se concentra-tiondecreasesfrombasicandultrabasictoacidandneutral rocks(D a n g i et al., 1989).In sedimentary rocks, Se is bound to the organic and clay fractions. Low-estSeconcentrationsarerecordedinsandstonesandlimestones(Table1).Se-lenium is most frequently found in phosphates, uranium ore, fossil coal and oilandinshalewithahighcontentoforganicmatter. Soilsformedonsuchsub-stratestypicallyhavehighortoxicSeconcentrations, unlikethesoilsformedonmagmaticrockswhichhavealowSecontent (F l e m i n g, 1980).ThemetamorphicrocksofSerbiahavealowerSecontentthanthemag-matic and sedimentary rocks. An analysis of 89 samples of metamorphic rocksfrom Serbia showed a low Se content of 28 ppb (J o v i et al., 1995), leadingtoaconclusionthat thehightemperatureandpressurepresent duringmeta-morphicprocessestendtoextrudeSefromtheserocks.Tab. 1SeleniuminmainrocktypesmgSeg1(K a b a t a - P e n d i a s, 1993)Igneousrocks mgSeg1Ultrabasic(dunit, peridotite, pyroxenite) 0.020.05Basic(basalt, gabbro) 0.010.05Intermediate(syenite) 0.020.05Intermediatetoacidic(rhyolite, trachyte, dacite) 0.020.05Acidic(granite, gneiss) 0.010.05SedimentaryrocksShale 0.400.60Sandstone 0.050.08Limestone, dolomite 0.030.1025SELENIUMINWATERANDAIRBesideslitosphere, atmosphericprecipitationisanother sourceof seleni-uminsoil. Seleniumisreleasedintotheatmospherefromnatural depositsorduetoanthropogenicinfluences(burningofcoal andotherfuels, releasesformines, oil refineriesandother industries). Soils, plants, microorganisms, ani-malsandvolcanoesreleaseupto0.04ngm3Se(EHC, 1987). Thetotal an-nual anthropogenicemissionofseleniumintheUSAwasestimatedat 4670t( H e r r i n g, 1991). The atmospheric precipitation in Norway and Swedenbringsannually0.51.0gSeha1. Under natural conditions, Secontentsinairandwaterarequitelowlessthan10ngm3inairandafewmgl1ofwater (EHC, 1987). Secontentsinrainandsnowsamplesgatheredindiffe-rent partsof theworldrangedfrom0.03to0.3mgl1(C u t t e r, 1989). Seconcentrationinatmospheric precipitationinSwedenwas below0.1 mgl1( J o h n s s o n, 1991).SELENIUMINSOILSeleniumcontentinsoilishighlyvariable. Itvariesfrom0.1to2mgSeg1in most soils, but it is most frequently between 0.2 and 0.4 mg g1( M c N e a l and B a l i s t r i e r i, 1989). Some soils are lowin selenium,from0.03to0.08mgg1(NAS,1976).Toxicconcentrationsoccurinaridandsemiaridparts of China, inHawaii, Mexico, Columbiaandwesternparts oftheUSAandCanada(M c N e a l and B a l i s t r i e r i, 1989). Soilscontain-inglessthan0.5mgg1oftotalSeareconsideredasdeficientinthiselement(M a y l a n d et al., 1989).Seleniumconcentrationinsoil isdeterminedbyseleniumcontent insoilnativesubstrate, climate, reliefandage(M a y l a n d etal., 1989). Thesefac-torsmaycontributeeithertoseleniumaccumulationduringsoilformingoritsremoval duringor after soil forming. Thesoilsformedonsedimentaryrocksthat containhighamountsoforganicmattertypicallyhavehightotoxicsele-niumconcentrations. Thesoilsformedonmagmaticrocks, whicharepoorinSe, usuallyhavealowSecontent (F l e m i n g, 1980).Besidesnativesubstrate,amountofrainfallplaysanimportantroleinde-terminingseleniumcontent insoil (F l e m i n g, 1980). Inregions withlessthan 500 mm of rain, the soil formed from rocks with a high Se content conta-ins potentially toxic Se concentrations. The same kind of soil formed in humidregions typicallycontains highamounts of Se, but it is boundtoironinapoorlysolublecomplex. Whenthesubstrateislowinselenium, thesoilform-ingonit will havealowSeconcentrationregardlessof climate.SincethetotalSecontentinsoilispredominantlydeterminedbyitscon-tent inthenativesubstrate, it isunderstandablewhythesoilsincertainregi-onsofourcountryhaveverylowSeconcentrations. Forexample, thesoilsinnorthernPomoravlje have the Se contents from0.12to0.44ppm, withthemeanvalue of 0.24ppm(J a k o v l j e v i et al., 1995), inwesternSerbia119.570.5ppbSe(M a k s i m o v i et al., 1992), inthevalleyof theZeta26River from0.2to0.41ppmSe, withthemeanvalueof 0.28ppmSe(J a -k o v l j e v i et al., 1995)LowSeconcentrationshavealsobeenregisteredintheVojvodinaProv-ince.In46soilsamplesfromtheregionsofSremandBanat,takenfromplotsusedfor vegetableproduction, thecontents of Serangedfrom0.11to0.45ppm, with the mean value of 0.25 ppm Se ( u v a r d i et al., 1997). In sam-plesof chernozem, districcambisol, humogley, humofluvisol, arenosol, solo-netzandsolonchaktakenintheVojvodinaProvince, Secontentsinthesurfa-cesoil layer rangedfrom0.024to0.194mgSeg1( u v a r d i , 2000).SELENIUMDISTRIBUTIONALONGSOILPROFILESeleniumcontent insoil typicallydecreaseswithdepthbecauseit bindswithproteins, fulvicacidsandotherorganiccompoundsthatcontentnitrogen.As organic matter decreases along soil profile, so does total selenium (A b r a m setal.,1990; A l e m i etal.,1991; G u s t a f s s o n and J o h n s s o n,1992).Selenium fixation may also be due to microbiological incorporation into aminoacids and other Se-containing organic compounds (M a o, 1999). In addition tohumusdistribution,SedistributionalongsoilprofileresemblesthoseofFe,Aland clay (L e v e s q u e, 1974; G o n d i et al., 1992; u v a r d i , 2000). Se-leniumbindswithclayandarrangesitselfalongtheprofile.TotalSeincreaseswiththeincreaseofsmallfractionsinthesoil( u v a r d i ,2000).Themostimportant characteristicsofthefinesoil fractionarethelargetotal areaofitsparticles andspecific mineralogical compositionwhichdistinguishes it fromthe coarse fractions. Secondary minerals predominate in the finest fraction,clay, whilethecoarsest fractioniscomposedmainlyof quartz(J a k o v l j e -v i and P a n t o v i ,1991).Forthatreason,thefractionsofcoarseandfinesandhavealowadsorptioncapacity. IonsofAloctahedrallayer, whosesidesarebare, maybeconsideredaspositivelychargedsitesonthesurfaceofclayminerals.IonsoftheAloctahedrallayerofcaolinite,which,incontrasttothemineralsfromthemontmorillonitegroup, arenot coveredwiththetetrahedralsheet, maybepositivelychargednot onlyat thesitesofcrystal fracturesbutalsotheir planesmaybecharged, attractingHSeO3asasubstitutefor OH( B a r - Y o s e f and M e e k, 1987).FACTORSAFFECTINGSELENIUMSOLUBILITYANDAVAILABILITYTOPLANTSOrganicandmineralformsofseleniumarepresentinthesoil(C a r y andA l l a w a y,1969).TheorganicformsofSeresultfrompartialdecompositionof selenium-loving vegetation. Mineral Se occurs as a selenide of metals(Se2), elementary Se (Se0), selenite (HSeO3) and selenate (SeO42). Se parti-tioningis affectedbysoil pHandredoxpotential, content of sesquioxides,clay, organicmatter andmicrobiological activity(E l r a s h i d i et al., 1987;J a y a w e e r a and B i g g a r, 1996).27SesolubilityandavailabilitytoplantsispredominantlyaffectedbypH--valueandredoxpotential(G e e r i n g etal.,1968).Accordingtope+pHdia-gram(Figure1) (E l r a s h i d i et al., 1987), selenideandelementarySearefavored in a reducing environment (pe+pH < 7.5), selenite in a slightly oxidiz-ingenvironment(pe+pH7.515.0), andselenateinanoxidizingenvironment(pe+pH>15.0).Inhumidregionsandacidsoils,theprevailingformisseleni-te, whichisfirmlyadsorbedonsesquioxidesandclaymineralsandthusnotreadilyavailabletoplants.HighannualprecipitationandlowtemperaturetendtoreduceSecontent inplants. Thisisduetoreducingconditionsinthesoiland reduction of selenate (SeO42) into less available forms: selenite (SeO32);elementaryseleniumor selenids (Se2) (G e e r i n g et al., 1968). IncreasedleachingofavailableSeformsalsoleadstoreductioninuptake(J o h n s s o n,1991). Underwell-aeratedconditionsinthealkalinesoilsofsemiaridregions,seleniumis present intheformof selenatewhichis not adsorbed, does notforminsolublesaltsandisreadilyavailabletoplants(C a r y et al., 1967).Although soil reaction plays a major role in determining Se solubility andavailability, itsinfluencelessensasthecontentsofclayandorganicmatterinthesoil increase(J o h n s s o n, 1991). BecauseofSeaffinityforclayminer-als, thesoilshavingincreasedclaycontent aretypicallybetter providedwithSethansandysoils(G i s s e l - N i e l s e n, 1975). Asdescribedintheprevio-uschapter, SebindswiththepositivelychargedionsofAL-octahedralsheets,which occur on the edges of clay minerals, primarily caolinite (B a r - Y o s e f,1987). G i s s e l - N i e l s e n (1971) studiedtheeffect of pHvalueandtex-tureonSeuptakefromloamyandsandysoils.InthefirsttwocuttingsofLo-28Fig. 1Theeffect of redoxonSespeciesinsolution(E l r a s h i d i et al., 1987)liummultiflorumgrownonaslightlyacidsoil,Seuptakedroppedwithincrea-seinclaycontent. Insubsequent cuttings, however, theeffect of soil acidityevidentlyweakenedbecauseSebecamemoreandmoreavailable. Whenthesoil pHexceeded7, theeffect of claydisappearedaltogether. Seadsorptionoccursalsooniron(Fe), aluminium(Al)andmanganese(Mn)oxidesandhy-droxides in acid environments, when these particles become positively charged(B a l i s t r i e r i and C h a o,1990).Namely,aspecificcharacteristicoftheseoxidesisthattheychangetheirelectricchargewithachangeofambientreac-tion. Thechargeisnegativeunderalkalineconditionsandpositiveunderacidconditions.Seleniumalsobindswellwithorganicmatterinacidsoils(C h r i s t e n -s e n et al., 1989; G u s t a f s s o n and J o h n s s o n, 1992). Because of ahighcontent oforganicmatterintopsoil, higherseleniumcontent maybeex-pectedtherethanindeepersoil layers(L e v e s q u e, 1974). Whenintheor-ganicfraction,seleniumeitherformscomplexeswithorganiccompoundsorisbuiltinaminoacidsandproteinsbymicroorganismsandplants(C a r y etal.,1967; H a m d y and G i s s e l - N i e l s e n, 1976). Sefractionationfromthreeprofiles of Swedish podzols showed that considerable amounts of Se were pre-sent inthesoil organicmatter, especiallyintheBshorizonwhichhasahighcapacityfor adsorptionof anions(G u s t a f f s o n and J o h n s s o n, 1992).Microorganismsplayanimportant roleinSetransformationsinthesoil.Theyarecapableof transformingtheabsorbedseleniteintoorganiccompo-undsorinot selenate(D o r a n, 1982). It wasalsofoundthat microorganismstakepartinthetransformationofselenateintolessmobileforms,especiallyinsoilsrichincarbon(A l e m i etal., 1991). Productsofmicrobiologicalactiv-ityarealsovolatileSeorganiccompounds, primarilydimethyl selenide. Thisis a way for seleniumdepletion fromthe soil (F r a n k e n b e r g e r andK a r l s o n, 1989). H a m d y and G i s s e l - N i e l s e n (1976) measured thevolatilizationofaddedseleniteunderlaboratoryconditions. Thevolatilizationwashigherfromasandysoil (about 8%oftheaddedSe)thanfromahumicsoil (about 5%of addedSe).Seavailabilitytoplantsdependsalsooncompetingions,especiallyphos-phates andsulfates. Interactionbetweenseleniumandphosphorus (P) inthesoil has beenobservedbyseveral researchers, but its mechanisms have notbeenfullyrevealed.Inagreenhousetest, S i n g h (1979)examinedtheeffectofaddedSeandPondrymattermassandchemical compositionofBrassicajunceaCos. Inthevariant without Pandwith10ppmSe, theseleniumcon-tentintheplantwas100timeshigherthaninthecontrol. Theincreaseswere258- and336-foldwith50and100ppmP. Similar resultswereobtainedbyS i n g h and M a l h o t r a (1976) whofoundthat theapplicationof 50and100ppmPpositivelyaffectedSeuptakebyTrifoliumalexandrinuminallva-riants of Se level, as well as in the experiment with residual Se. Effect of P onSe availabilitydepends onthe auxiliaryioninthe phosphorus fertilizer, Seformandthepresenceof other ions (K h a t t a k et al., 1991). F l e m i n g(1962)studiedtheeffectofPfromsuperphosphate, Ca(H2PO4)2, onSeuptakein an experiment in pots. All three doses of superphosphate reduced Se uptake,probablyduetotheinfluenceofsulfatethat wasacomponent ofthesuperp-29hosphate. Unlike superphosphate, dicalcium phosphate and monocalcium phos-phateincreasedSecontentingrasses(F l e m i n g,1965).Apossibleexplana-tionisthat P, ifincorporatedintothesoil togetherwithSe, substitutesSeinsomesorptionpoints,thusincreasingSemobilityanduptake(C a r t e r etal.,1972). Another possibleexplanationof thisinteractionisthat PincreasesSeuptakebystimulationcell divisioninplant root. Highcell divisionincreasesthecontact betweentheroot andsoil, whichincreasestheactiveroot surfaceandSeuptake(C a r t e r et al., 1972).Becauseof alargechemical similaritybetweensulfur (S) andseleniumandbecauseof thefact that most ScompoundshaveSeanalogues, it seemsreasonabletoexpect antagonisticinteractionduringuptakeof thesetwoele-ments.SeveralauthorsfoundthatSeconcentrationintheplantdecreasesafteraddingsulfatetothenutritivemedium(W a n etal.,1988; M i k k e l s e n andW a n,1990).Inanexperimentinpotsfilledwithpeatysoil, W i l l i a m s andT h o r n t o n (1972) examined the effects of different S compounds on Se up-take.AllScompoundstested(elementaryS,sulfateS,sulfiteS,sulfideSandmethionine) tended to reduce Se uptake. The situation was opposite in the caseofalkalinesoils,wheretheapplicationofsulfateincreasedSeuptake(C a r t e ret al., 1972).WATER-SOLUBLESELENIUMINSOILTotal Se content in the soil is not always in correlation with its content inplants because Se solubilityis affectedbyseveral factors: soil reaction, thecontentsofsesquioxides, clayandorganicmatter(E l r a s h i d i et al., 1987;J a y a w e e r a and B i g g a r,1996; P e z z a r o s s a,1999).Becauseofthat,when assessing soil provision with selenium, concentration of available Se andfactorsthataffectthedynamicsofSeinthesoilhavetobetakenintoaccountinadditiontothetotalSecontent.Thewater-solubleSefractionisconsidered30Fig. 2Contentsof total andwater-solubleSeintheprofilesof theexaminedsoilsintheVojvodinaProvince( u v a r d i , 2000)tobemostavailabletoplants(J u m p and S a b e y, 1989). Non-adsorbedSeandwater-solubleorganicSearereleasedfromthesoilbyextractionwithboi-lingwater(W a n g and S i p o l a, 1990; S e b y et al., 1997; K a n g et al.,1993).Innormal tilledsoils, theconcentrationof thewater-solubleSefractiontypicallydoesnotexceed0.050mgg1Se(W o r k m a n and S o l t a n p o u r,1980).Exceptionallylowconcentrationsofwater-solubleSe,rangingfromtra-cesto0.013mgg1, wereregisteredinthesoilsofSremandBanat usedforvegetableproduction( u v a r d i et al., 1997). Lowconcentrationsof wa-ter-soluble Se were found for seven different soil types in the Vojvodina Prov-ince( u v a r d i , 2000). Theseconcentrations rangedfromtraces to0.014mg Se g1of soil, or from 0.59% to 16.35% of the total Se (Figure 2). In Hun-gary, G o n d i et al. (1992)foundthat themobileSefractiondidnot exceed10%ofthetotal Seinmost ofthetestedsoils, withtheexceptionofseveralprofilesofalkalinesoilsinwhichthisfractionrangedbetween20%and35%of thetotal Se.SELENIUMUPTAKEPlants takeupSeintheformof selenateor selenite, or as organicSe( H a m i l t o n and B e a t h,1963,1964).Probablytheymayalsotakeupat-mospheric Se (A s h e r et al., 1967). Capacityfor Se uptake varies amongplantspecies(H u r d - K a r r e n, 1935). Still, differencesamongplantspeciesweresmallerregardingSeuptakefromlowSesoilsthanregardingSeuptakefromhighSesoils(E h l i g et al., 1968). Furthermore, therateofuptakede-pendsontheformofSeprevailinginthesoiloranutritivemedium(Z h a n get al., 1988). Generally, plants exhibit preference for selenate over selenite( B a n u e l o s and M e e k, 1990). This seems to be due to high adsorption ofselenitebyhydratedoxides, orselenitereductiontoelementarySeorselenide(C a r y et al., 1967; G i s s e l - N i e l s e n et al., 1984). Inanexperiment invegetation pots, G i s s e l - N i e l s e n (1973) found that the Se uptake bybarleywas40timeshigher fromselenatethanfromselenite.CONCLUSIONSeleniumisanessential microelement, necessaryfor normal functioningofhumanandanimal organisms, whosedeficiencyinfoodandfeedcausesanumberofdiseases. Inhighconcentrations, seleniumistoxicforhumans, ani-mals and plants. Soil provision with selenium affects its level in food and feedvianutritionchain. Seleniumbioavailabilitydependsnotonlyonitstotalcon-tent insoil but alsoonitschemical forms.Organic and mineral forms of Se are present in the soil. The organicformsofSeresultfrompartialdecompositionofseleniferousvegetation.Min-eral Se occurs as a selenide of metals (Se2), elementarySe (Se0), selenite(HSeO3)andselenate(SeO42).SepartitioningisaffectedbysoilpHandre-31doxpotential, content ofsesquioxides, clay, organicmatterandmicrobiologi-cal activity.Inhumidregionsandinacidsoils,seleniteisthepredominantform.Itisfirmlyadsorbedonsesquioxidesandclaymineralsandthusnotreadilyavaila-bletoplants. Inwell-aeratedalkalinesoil insemiaridregions, Seisfoundintheformofselenate, whichisnotadsorbed, doesnotforminsolublesaltsandisreadilyavailabletoplants.SinceSesolubilityandavailabilitytoplantsaregovernedbyseveralfac-tors, concentrationof availableSeandfactorsthat affect Sedynamicsinthesoil shouldbeconsideredinadditiontothetotal Secontent inthesoil whenestimatingsoil provisionwithselenium.REFERENCESA b r a m s,M.A., S h e n n a n,C., Z a s o s k i,R.J., B u r a u,R.G.(1990):Seleno-methionineuptakebywheat seedling, Agron. 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