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View of Effect of liming on phosphorus in two soils of different organic matter content: I Changes of native and applied phosphorus in incubation experiment 

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JOURNAL OFTHESCIENTIFICAGRICULTURALSOCIETY OFFINLAND MaataloustieteellinenAikakauskirja

Voi. 55: 345-354, 1983

Effect of liming

on

phosphorus

in two soils

of different organic

matter content

I

Changes of native and applied phosphorus

in

incubation experiment

HELINÄ HARTIKAINEN

Department

of

Agricultural Chemistry, University

of

Helsinki, 00710Hel-

sinki 71

Abstract.The effect ofincreasinglimequantitiesonreactions ofnative and applied Pwasinvestigated in anincubationexperiment performed withtwoacidmineral soils ofpH 4.8 (CaCl2 ).The soil samples differedconsiderablyin thecontent oforganicmatter,whichwasreflectedintheirpH bufferingpower:in thefinesand,richinorganicmatter(6.4%org.C),liming raised thepHlessthaninthemuddyfine sand (3.0 %org. C).

The levelof native water-solublePwas markedlylowered inthe incubated soilsamplestreated with nutrientsalts.Inthemuddyfinesand,the decreasetendedtobe thesmaller,whereasinthe finesandthe

greater,themore intensiveliming was.This heldtrue also of addedP.

Thechanges in CHANGandjACKSON’sPfractionsdid notalonesatisfactorily explain the dissimilar

response of soilP tolime treatments. The fate ofP wasconcluded tobe controlled bythe qualityand quantity ofA 1speciesdiffering intheiraffinityforPsorption.Thechanges inthe solubilityofPare anet result ofprocesses enhancingand ofthosedepressingthesorption tendency. Inthefine sand soilof high initial contentof water-solubleP, the detrimental effect of liming seemed to be attributed to the

abundance of polymerizedA 1theaffinity of which for P retention increasedwith intensifiedliming.

Further,the highpH bufferingpower of this soil reduced theefficiencyof limetoproduce OH'ions able

tocompetewith phosphate forsorptionsites.Inthe muddyfine sandsoil,onthecontrary,the formation

ofsorption-activesiteswasnotequallymarkedand,owingtothe weakerpH buffering, limingraisedthe

OH'concentration moreeffectively.

Introduction

Liming affects the reactions of soil P in many ways. For instance, it increases the concentration of OH- ions able to displace phosphate from oxidesurfacesand toalter the distribution ofsoilP between various fractions used for characterization of differentP compounds (CHANG and JACKSON

1957,KAILA 1961and 1965,HARTIKAINEN 1981 etc.).In acidsoils, it seems,

these reactions contribute to enhancing the solubility ofP in water, even

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though the promoting effect of increasing pH is concluded to be partly dependent on the P status of the soil(HARTIKAINEN 1981).

Onthe other hand,theextentofthe reactions ofPis obviously controlled by the pH buffering powerofthe soilwhich relates tothe contentof organic

matter.Further, organic anions also compete with phosphate for sorptive metal oxide surface. Their affinity to react with metal ions and, thus, to

competewith phosphate is highly pH-dependent.

The purpose of the present incubation experiment was to elucidate the lime-inducedreactions ofnative and added P in twosoils differing markedly in their content of organic carbon. The results wereassumed to demonstrate the role of organic matter and other related factors in controlling the efficiencyoflime toraise pH and toaffect the solubility ofnative and added P in soils.

Materials and methods

The incubationexperiment was carried out with twoacid surface soil samples taken from the experimental farm ofHelsinki University.Characteristics of thesoilsare given inTable 1.The samples

differed mainly intheircontent oforganic carbon andsecondary phosphates. The fine sand soil was

sampledfrom afield usedsuccessfully for sugar beet cultivationin 1976-1979.During thisperioditwas suppliedwithatotal of600kgPperhectare. Beforethat, in 1975,the blockwasmanured and limed. The

muddy fine sandsamplewastakenfromafield whichgrewoats,barley and hayin1976-1979andatthat

timehadreceived a totalof only167kg fertilizerPperhectare.This blockwas limedin 1969.

Theparticle size compositionof the mineral materialin the experimental soils wasdetermined by a pipettemethod (ELONEN 1971).SoilpH wasmeasured ina0.01 MCaCl2suspension inthe soil-solution ratio of1 to2.5.Thecontent oforganiccarbonwas determinedbyamodifiedWALKEY andBLACKwet

combustion method(GRAHAM 1948)whichwasassumedtocover80 %of the Cinthe soils.Aluminium, ironandmanganesewereextracted with0.05 M NH4-oxalate(pH 3.3) inthe soil-solutionratio of1:10 (w/v)anddeterminedby anatomicabsorption spectrophotometer. Phosphoruswasfractionatedaccord- ing toaslightlymodifiedCHANG and JACKSON (1957)procedure.

Intheexperiment, portions of450gofmoistsoil(corresponding to390g of air-driedmuddyfine sand

and360gof air-dried finesand)wereincubatedinplasticpotswith thefollowing dosages oflime; 0, 0.6, 1.2and2.4g.

Because theincubation experiment was connected withan analogouspot experiment,all the treat-

Table 1. Characteristics ofexperimental soilsamples.

Muddyfine sand Fine sand

Clay% 22.4 22.5

pH (CaCl2) 4.8 4.8

Org. C %ofD.M. 3.0 6.4

Pfractions ppm NH4CI-P 3.4 10.0

NH4F-P 107 321

NaOH-P 286 277

H2S04-P 325 285

Oxal. extr.A 1ppm 1081 1533

Fe 2729 2870

Mn 16.3 12.4

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mentsreceived plant nutrientsasfollows: 100mg NasNH4N0,,20mgMgasMgCI26 H2O,0.5mgB

asH,BOj, 1.5mg CuasCuSO,5H.O,ImgMn asMnSO,4H2O,ImgZnasZnSO,7 H2Oand0.5

ragMoasNa2Mo042 H2O.In addition,half of thepotsreceived 40mgP(correspondingto 104and 114 mgP perkgofmuddyfinesand and finesand,respectively)asK2HP04inwhich they received also 100 mgofK.ThepotsincubatedwithoutPtreatmentreceivedanequivalentquantityofKasKCI.The testwas carriedout with fourreplicates. The soilswere incubated for 16months.

Air-dried subsamples wereanalyzed after incubation periods of4and 16months. Water-solubleP, extractedbyaslightlymodifiedvanderpaauw(1971) andsissingh (1971)method,wasanalyzedbythe ascorbic acid method(ANON. 1969).The exchangeableA 1

was

washedwith fourportionsof1MKCI in

the ratio ofsoil to solution of1:5 (w/v)and determined by amodifiedAlummon method(YUAN and FISKELL 1959).Soil A 1

was

extracted alsowith 1 MNH,OAc solution (according toMcLEAN1965) and determinedbyan atomic absorption spectrophotometer.

Results

The pH values of the incubated soil samples are reported in Table 2. In thefinesand pots incubated withoutlime andK2HP04addition, soilacidity increased by 0.3-0.4 pH units, whereas in the corresponding muddy fine sand pots by 0.5-0.6 pH units. It can be calculated from the titration curve,

obtained by plotting the pH value against the lime dosage, that about 0.45 and 0.50 g of CaCOj per pot, respectively, was required to counterbalance this acidification.

Liming decreased the acidity of the muddy fine sand more effectively than that of the fine sand sample. Further, in the former, the K2HP04

treatment tended somewhat to raise pH. It should be mentioned that immediately after application ofnutrient salts the pHof themuddy finesand and fine sand was 5.4 and 5.2 in the pots treated with P and 4.5 and 4.6 in those withoutP, respectively.

Table 3 shows the incubation treatment to have depressed markedly the level of native water-soluble P in both soil samples. It was, however,

Table2. pHof incubated soil samples*

Incubated for

Lime 4 months 16months

added g - P added - P added

Muddyfinesand(pH 4.8)

0 4.2a 4.3ab 4.3*b 4.4b

0.6 4.9cd 5.1' 4.9' 5.0d'

1.2 5.6'* 5.7* 5.5' 5.6(

2.4 6.3b 6.4bl 6.4h 6.5'

Fine sand(pH4.8)

0 4.4* 4.4ab 4.5bc 4.5‘

0.6 4.9d 4.9d 4.9d 4.9d

1.2 5.3' 5.3' 5.3' 5.3'

2.4 5.9' 5.9* 5.9' 5.9'

*Both soils aretested separately. Themeans followedbya common letterdonot differatP=0.05.

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interesting tonotice that in themuddy finesand the decrease tendedto be the smaller and inthe finesand the greater, themore intensive the liming was. In the most heavily limed pots, prolonged incubation caused some further reduction in the solubility of P in water.

Only a minor portion of appliedP wasrecovered inthewaterextractable fraction (Table 3): at four months, 7.9-8.5 % in the muddy fine sand and 11.9-14.9 % in the fine sand sample. In the latter soil, liming seemed to

depress the solubility of added P, too. Besides, a further conversion of applied Pintoa less solubleformtookplace duringthelong-term incubation.

Table3. Extractabilityof soilP andapplied P intowater.

Soil Pppm Recovery of added P%

Lime Incubationtime,months

added g 4 16 4 16

Muddyfinesand

(Original 10.7)

0 5.4“ 5.7* 8.0 5.6

0.6 5.6' 5.8“ 8.5 6.4

1.2 6.3*b 6.3“b 7.9 6.0

2.4 7.3b 6.1* 8.3 6.4

Fine sand (Original42.1)

0 28.9d' 29.5' 14.9 12.2

0.6 28.2d' 26.7“* 13.3 11.0

1.2 25.2' 25.0bc 13.2 10.1

2.4 23.6b 20.7* 11.9 8.7

The fractionation analyses gavesome intimations ofthe fate of native as wellasapplied P in the soils (Table4).Liming seemed toinduce aredistribu-

tion of native P between NFI4F, NaOH and H2S04 extractable forms representing P bound by Al, Fe and Ca, respectively, even if the data obtained did not quantitatively account forthe changes. In the muddy fine sand sample, the NaOH-P was markedly depleted and H2S04-P increased slightly. In the fine sand, on the other hand, the various fractions were

affected toa lesser extent, but some enrichment occured in theNH4F and

NaOH soluble forms.

Thedifferencebetween agiven fractioninthe soils treated and nottreated with P was assumed to represent recovery of applied P. Table 4 suggests liming not tohave affected decisivelythe distribution ofsorbed P. In the fine

sand soil, added P was retained mainly in the fluoride soluble fraction, whereasinthemuddy finesand sample, relatively highquantitiesweresorbed also in the hydroxide extractable one. Further, a surprisingly marked accu-

mulation in the acid soluble form occured in the fine sand soil, also in the unlimed treatment.

The enrichment in native NH4F-P in the limed fine sand soil did not,

however, promote thesolubility of P inwater(cf. Table 3). This rendered it

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necessary toinvestigate the role ofsomeby-reactions induced by the CaC03

treatment. Because theNH4F fraction is assumed torepresent primarily the Al bound resources, attention was paid toreactions ofsoil Al.

Table4. P fractions(ppm) in soils incubated for 16months withorwithoutP.

P extracted sequentially by

Lime NH4CI NH„F NaOH H2S04

added g-P - P - P - P

Muddyfinesand

0 3.8b 6.3d 109b 154' 296c 330' 317* 320*

0.6 2.7* 5.0C 105*b 155c 280b 319d' 318" 321*

1.2 2.5* 5.0' 103" 153' 274b 316d 326“b 325*b

2.4 3.8b 7.5' 110b 156' 261* 308'd 333b 335b

Fine sand

0 11.3' 16.0* 318* 388' 271* 278* 285* 313b

0.6 8.8*b 13.5' 322*b 403d 273* 283*b 284* 322b

1.2 B.o* 12.5d 332b 407d' 277“ 277* 283* 305b

2.4 8.9b 14.7* 336b 420' 286*b 298b 273* 306b

Figure 1 shows the relationshipbetween pHand thecontentof exchange- able Al in theexperimental soils incubated for 16months. Asexpected, KCI displaceableAl sharply diminished as pH increased. In the finesand sample, the initial exchangeable Al resources were lower and decreased less than in the muddy finesandsample. In the lattersoil, the Alconcentration points for samples treated ornot treated withK2HP04 fell on thesame graph, whereas in the finesand soil, application ofP caused anadditional depression in the exchangeable Al fraction.

Because the fine sand soil, poorerin theexchangeable Al, containedmore

abundantly oxalate soluble Al than the muddy fine sand, the experimental soils can be concluded to differinthe distribution oftheir Alresources into various forms. For more accurate information about active Al reserves, the incubated samples as well as the original ones were extracted with NH4-

acetatewhich,according to McLEAN(1965), attacks in addition toexchange- ablereserves also polymerized Alhydroxides and organicAl complexes.The differencesbetween theNH4-acetateandKCI extractable forms,presented in Table 5, indicate the initial content of the polymerized or complexed Al Table5. Differences (ppm)betweenNH,OAc andKCI solubleA 1inthevarious treatments.*

Muddyfinesand Fine sand

Treatment - Padded - P added

Original soil 85'd 147d

CaCOj 0 g/pot 67*b 64* 122b 121b

0.6 91d 97* 138cd 135°

1.2 83' 81' 126b 119b

2.4 70b 66ab 110* 103*

* Both soilsamples aretestedseparately.

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forms tobe markedly higher in the fine sand sample rich in organic matter.

Further, the data reveal the incubation treatments without lime as well as with moderate and high lime dosages to diminish the difference between these forms.

Discussion

The addition ofnutrient salts immediately increased the ionic strength in soil solution and, thus, decreased the pH values. Obviously, thenitrification

Fig. 1.ContentofexchangeableA 1inincubated soilsas afunction ofsoilacidity.

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ofadded NH4-N during the incubation contributed to a further decreasein soilpH. Also the decrease in the level ofwater-solubleP inall the incubation

treatments can be ascribed to the increased ionic strength, shown in most

soils to depress desorptionofP (RYDENand SYERS 1975,HARTIKAINEN and

YLI-HALLA 1982).

In themuddy finesand soil,water- soluble Pwassomewhat increased as a result of liming, but remained far below the level in the original sample. In the fine sand soil, on the contrary, liming seemed to cause a considerable additional reduction in this P fraction. COLEMANetal. (1960) and MOKWU- NYE (1975) have reported similar exceptions to the general rule thatliming improves the solubility of soil P. Theyascribed these to the hydrolysis and polymerization of exchangeable Al3+ ions upon pHincrease,because freshly polymerized Al compounds more effectivelyretain phosphate than doAl3+

ions.

The hydrolysis of exchangeable monomeric hexaquo-aluminium ion can

be presented schematically as follows:

Al(H2o)r + H2O Al(OH) (H2O)52+ + H 30+

The reaction is driven to theright by consumptionof H3C)+ ions, gradually leading to the complete neutralization of Al ions and to the formation of A1(0H)3(H20)°. The hydrolysis products of trivalent Al ions are known, however, to polymerize rapidly and toform large multicharged units.

The results of the present study, too, imply that the lime-induced redistribution of P between various fractions was, at least to some extent,

accompanied by simultaneous changes in the quantities of sorptive com- pounds. Even ifan enrichment in the Al-bound fraction is considered to enhance thesolubility ofP inwater(HARTIKAINEN 1981 and 1982), this did

not hold true of the fine sand soil. The exchangeable Al did not, however, explain the dissimilar responses of the experimental soils to liming. In the muddy fine sand, where the reduction in exchangeable Al was more considerable than in the fine sand, Pretention did not increase.

On the other hand, organic matter contributes to the relationship be-

tweenpH and exchangeable Al. According toEVANS andKAMPRATH (1970),

soils of high organic matter content involve less exchangeable Al at a given

pH than soils poorerin this material,which indicates complexationreactions.

Also in the present study, the content of polymerized or organically bound (and moresorption active) Al species was higher inthe finesand soilrich in organic matter. Thisgives reason tosuppose thatthey wereresponsible fora decrease in H2O-P as liming was intensified.

Figure 1and the datainTable 5 give someevidence ofthepH-dependent equilibrium between various Alspecies and givesomeindication ofthefate of P. The increasedacidity in the unlimed soils transferred the equilibrium be-

tweenpolymerizedand exchangeable forms to the direction ofthe latterspecies.

In the finesand soils incubatedfor 16 months,this was reflected as a higher solubility ofP inwater(see Table 3).Further, the level of hydroxy-Al inthe soils supplied with 0.6 g of CaCOj was about equal to that in the original

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ones. This may be due to this dosage being required to counterbalance the incubation-induced acidification. The decreasing contents at higher liming intensitiesmay, inturn, indicatea morecompleteneutralizationof hydroxy- Al to compounds no more extractable in NH4-acetate.

If the latter assumptionis valid,Table 5justifiesto suppose thata higher quantity of hydroxy-Al species reached this neutralization stage in the fine sand soil richer in organic matter than in the muddy fine sand sample. The conclusion is supportedby theview presentedby HARGROVE and THOMAS (1982) according to which organicallybound Al is apparently hydrolyzed to a greater extent at a given low pH. Thus, it is theoretically possible that owingtothe greaterpolymerization degreetheaffinity ofAlforPretention is higher in soils of highorganic matter content.

In thefine sand soil, the addition ofK2HP04 decreased the exchangeable Al, which indicates some interactionbetween Al and P. Because this sample

wasrelatively poor in exchangeable Al3+ and rich innative water-soluble P,

the application of P possibly led to the solubility product ofan aluminium phosphate to be exceeded and to its precipitation. ROBARGE and COREY (1979) observedin theircation resinstudyasimilar decreaseinthepercentage ofAl3+ ions at higher P additions but not at low ones. The quite marked accumulation ofadded P seen in theH2S04 solublefraction may be aresult of the inaccuracy of the fractionation method and the formation of some discrete compounds. Therefore, further studies are needed to elucidate the interaction between P and exchangeable Al3+ ions in soils.

The results discussed above illustrate the pH-dependent relationship between P retention tendency, soil

A 1 and

organic matter. In addition,

organic matter and Al may also indirectly affect the fate of P. Al-organic

matter complexes are considered responsible for theweak-acid behaviour of the organic matter in acid soils and an important source of pH buffering (COLEMAN and THOMAS 1967,KAILA 1971,BLOOM et al. 1979). Recently,

HARGROVE and THOMAS(1982) have proved thepKavalues forthe organic

mattersamples to increase considerably(theacid strength to decrease)as the Al contentincreases. Thus,inthe fine sand soilof high contentof hydroxy-

A 1

species and high pH buffering power, liming did not effectivelyraise the pH nor produce OH~ ions enough tocompete successfully withphosphate ions for sorption sites. Ontheother hand,inthe muddy fine sand,poorer in thehydroxy-Aland oflowerpH bufferingpower, liming increased theOH' ion concentration sufficientlyto cause a displacement of sorbed phosphate.

Even if the comparison of two soil samples does not justify categorical conclusions,the results suggest thequantityand quality of

A 1

species and the factorsinvolvedto be of importance incontrollingtheefficiency of limingto

regulate the changes in native and added P in soils.

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References

ANON. 1969.Juoma-ja talousvedentutkimusmenetelmät.Elintarviketutkijain Seura. 169p. Helsinki.

BLOOM,P. R., McBRIDE, M. B.& WEAVER, R. M. 1979.Aluminumorganic matter in acid soils:

bufferingand solution aluminumactivity.Soil Sei. Amer.J.43: 488—493.

CHANG, S. C.&JACKSON,M. L. 1957.Fractionation ofphosphorus. Soil Sci. 84: 133-144.

COLEMAN,N. T.&THOMAS, G.W. 1967.The basicchemistryof soilacidity,p. I—4linSoilacidity and liming.Agronomy 12.

THORUP, J.T.&JACKSON,W. A. 1960.Phosphate-sorption reactionsthat involve exchange- able Al. Soil Sci.90; 1-7.

ELONEN,P. 1971.Particle-sizeanalysisof soil. ActaAgr,Fenn. 122; 1-122.

EVANS, G.E. &KAMPRATH, E. J. 1970.Lime response asrelated topercent Al saturation,solution Al,and organicmatter content. Soil Sei. Soc. Amer. Proc. 34; 893-896.

GRAHAM,E. R. 1948. Determination of soil organic matterbymeans ofaphotoelectric colorimeter.

Soil Sci. 65; 181-183.

HARGROVE,W. L.&THOMAS, G.W. 1982.TitrationpropertiesofAl-organic matter.Soil Sci.134:

216-225.

HARTIKAINEN, H. 1981.Effectof decreasing acidityonthe extractability ofinorganicsoil phospho-

rus. J.Scient.Agric. Soc.Finl. 53; 16-26.

1982.Water solublephosphorus inFinnish mineral soils and its dependenceonsoilproperties. J.

Scient. Agric.Soc.Finl. 54: 89-98.

&YLI-HALLA, M. 1982.Chloride and sulphate solutionsasextractantsfor soilP. IEffect ofionic

speciesand ionicstrength onP desorption.J.Scient.Agric. Soc.Finl. 54: 287-296.

KAILA, A. 1961.Effect of incubation andlimingonthephosphorusfractionsinsoil.J.Scient.Agric.Soc.

Finl. 33: 185-193.

1965.Somephosphorus testvalues and fractions ofinorganic phosphorus in soils.J.Scient.Agric.

Soc.Finl. 37: 175-185.

1971.Aluminium and acidity inFinnish soils.J.Scient.Agric. Soc.Finl. 43; 11-19.

McLEAN, E.O. 1965.Aluminum,p.978-1010 inMethods of soilanalysis. Agronomy 9(2).

MOKWUNYE, U. 1975.The influence ofpH onthe adsorptionofphosphate bysoils from theGuinea

and Sudan savannah zonesofNigeria.Soil Sei. Soc. Amer. Proc. 39: 1100-1102.

PAAUW, F.vander, 1971.Aneffectivewaterextraction method for the determination ofplant-available soil phosphorus.Plant andSoil 34: 467—481.

ROBARGE,W. P.&COREY,R.B, 1979.Adsorption ofphosphate by hydroxy-aluminum specieson a cation exchangeresin. Soil Sei. Soc. Amer.J.43: 481—487.

RYDEN,J. C.&SYERS,J.K. 1975.Rationalization ofionic strengthand cationeffectsonphosphate sorption by soils.J.Soil Sci.26: 395-406.

SISSINGFf,H. A. 1971. Analytical techniqueofPw method,used for theassesment ofthephosphate

statusofarablesoilsinthe Netherlands.Plant and Soil34: 483—486.

YUAN, T. L.& FISKELL,J.G. A. 1959.Aluminum determination. Aluminum studies. Soil andplant analysis of aluminum by modification oftheAluminon method. J.Agr.Food Chem.7: 115-117.

Ms received May6, 1983

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SELOSTUS

Kalkituksen vaikutus fosforiin kahdessa erityyppisessä hietamaassa

I Luontaisen ja maahan lisätyn fosforin muutokset muhituskokeessa

Helinä Hartikainen

Helsinginyliopiston maanviljelyskemian laitos, 00710 Helsinki71

Kahdella happamalla maanäytteellä (pHCaCl24.8) tehdyssä muhituskokeessa selvitettiin nousevien kalsiittikalkkimäärien vaikutusta maan luontaisen sekä maahan lisätyn fosforin reaktioihin. Maanäytteet poikkesivat selvästi toisistaan orgaanisen aineksen suhteen, mikä heijastui niiden pH-puskurikyvyssä: kalkitus nosti runsaasti orgaanista ainesta(6.4% org. C) sisältäneen karkean hiedan pH:ta vähemmän kuin liejuisen hiedan(3.0% org. C).

Ravinnesuolojen lisäys pienensi merkittävästi vesiliukoisen fosforin määrää molemmissa maanäytteissä. Liejuisen hiedan kalkituissa koejäsenissä väheneminen ei kuitenkaan näyttänyt olevan yhtä voimakasta kuin kalkitsemattomassa. Karkeassa hiedassa kalkitus sen sijaan edelleen selvästi vähensi sekä luontaisenettämaahan lisätyn fosforin uuttumista.

Fosforin fraktioissa tapahtuneet muutokset eivät selittäneet riittävästi kalkituksen erilaista vaikutusta maanäytteiden fosforinuuttuvuuteen,joka näytti pikemminkin riippuvan kalkituk-

sen vaikutuksestamaan sorptioaktiivisen aluminiuminmäärään. Alunperin runsaasti vesiliu- koista fosforia sisältäneessä karkeassa hiedassa kalkituksen fosforin liukoisuutta vähentävä vaikutus näytti aiheutuvan siitä, että tässä näytteessä oli runsaasti orgaaniseen ainekseen sitoutunutta tai polymeroitunuttaaluminiumia, jonka fosforinsitomistaipumus lisääntyi kal- kittaessa. Lisäksi tehokkaan pH-puskuroinnin vuoksi OH-ionien pitoisuus ei ilmeisesti

noussut riittävästi, jottane olisivat pystyneet merkittävästi kilpailemaan fosfaatin kanssa sorptiopaikoista. Liejuisessa hiedassa aluminiumin muodostaman uuden sorptiopinnan synty-

minen ei ollut yhtä merkittävää. Lisäksi heikomman pH:n puskuroinnin vuoksi kalkitus pystyi tässä näytteessänostamaan tehokkaammin OH-ionien pitoisuutta ja siten lisäämään fosforin liukoisuutta.

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