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View of Phosphate adsorption characteristics of two soils responding differently to P fertilization




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MaalaloustieteellinenAikakauskirja Vol. 63:363—369, 1991

Phosphate adsorption


of two


responding differently

to P fertilization


Kemira Oy, Espoo Research Centre, Luoteisrinne 2, SF-02270ESPOO, Finland

Abstract. The soil samples of thepresentstudy originatedfrom two field experimentsin which five rates ofP(annually0, 13or 16, 26or32, 47 or 56, 60 or 72kgP/ha) had been appliedfor 11or 12years.The fieldsweresilty claysoils (Cryochrepts) not differing markedly inpH, contentsof clay, organic C, orpoorly crystallineA 1 andFe oxides. Before the field experiment,the quantities ofPextracted withanammonium acetate solution (pH 4.65)were approximately6mg/dm! inboth fields. However,the fields differed considerablyinthere- sponse of the crop toPfertilization. Phosphorus adsorption by the soil sampleswasstudied by shakingthe samplesinsolutions of differentPconcentrations (0—0.5mg/1). SoilI,show- inggreater responsetoPfertilization inthefield, adsorbedPconsiderablymore effectively than did soil 11.Also the quantities of reversibly adsorbedPweresmallerinthe subsamples of soil Iascompared tothose of soilII receivingthesame fertilization. FertilizerPapplied duringthe field experiments had been adsorbed and converted to forms unavailable to plants toalarger extentinsoilI,resultingingreater responsetoPfertilizationinthis soil. The differ- ence inresponse to applied P orinresidual effect ofPfertilization could not be predicted from soil characteristics other than P sorption.

Index words: adsorptionisotherms, mineralsoils,reversiblyadsorbedP


Single extractions with e.g.wateror an am- moniumacetate solution have rathersuccess- fully been used for predicting yield responses toP fertilization of cereal crops in Finland (e.g. Sippolaand Saarela 1986). The results obtained with these methods give anindexfor thereserves of plant-available P at the time

of sampling and for P fertilization require- ment in that year. The residual effect of P fer- tilizationcannot, however, be predicted byor- dinary soil testing methods prior tofertilizer application, because individual soils differ greatly in therate of P fixation. For exam- ple, in 18 mineral soils of England, alarge ap- plication of superphosphate lost half of its fer- tilizer value over a period ofonetosix years



(Larsen et al. 1965). This phenomenon resultsatleasttosomeextentinanunpredict- ableduration of fertilization effect ofadded P.

This study deals with soil samples taken from two long-term field experiments in which,despite the initially samefairly low lev- el of easily solubleP, adifferent response by cereal crops to P fertilization was observed (Yli-Halla


b). In one experiment, no statistically significant responsewas observed until the eighth year, while in the otherone, substantial responsewasmeasured already in the first experimental year. Further, more P was extracted withwater or an ammonium acetatesolutionattheend ofthefieldexperi-

mentsfrom soil samples taken from the field less responsive toP fertilization.Yet, theex- perimental fields didnot differ considerably in thecontentsof clay, organicmatter, poorly crystalline Al and Fe oxides orpH.

In thepresent study, the soils of thesetwo experimentswere examined inmore detail in ordertofindoutanexplanation for the differ- entyield responses toP fertilization in theex- perimental fields. Adsorption-desorption iso- therms for phosphate weredetermined and the soilswereanalyzed for reversibly adsorbed P.

Materials and methods

The soil material consisted of samples tak- enfrom the unlimed plots oftwofield experi- mentsin which fiveratesof P fertilization (0,

13or 16,26or32, 47 or 56, and 60or 72 kg P/ha) had been applied annually for 12 years (soilI;fourblocks,20 samples)orfor 11 years

(soil II; three blocks, 15 samples). The ex- perimental fields, located inVihti, Southern Finland, weresilty clay soils and, according

to the U.S. Soil Taxonomy, tentatively clas- sifiedasCryochrepts. Theexperimental design, soil characteristicsas wellasthe results of the field experiments, in which mainly cereal crops were cultivated, have been reported earlier (Yli-Halla


b). In that previous paper, soils I and II were referred to as ex- periments A andB,respectively. Some proper- tiesof the experimental soils are summarized in Table 1.

Isotherms for phosphate adsorption and desorptionwere determined according tothe method by Hartikainen (1982b). Duplicate soil samples (1.0 g) wereequilibrated for 23 hours in 50 ml of solution containing differ- ent concentrations (0.01, 0.02, 0.05, 0.10, 0.20, 0.30, 0.40, or 0.50 mg P/l) of P as KH2P04in centrifuge tubes. No supporting electrolytewas used. After adding the solu- tion,the suspensionswereshaken for 1 h and allowed to stand for 22 h and then shaken again for 10 min. The suspensions were cen- trifuged, and thesupernatant solutions were decantedandfilteredthroughamembrane fil- ter(pore size 0.2 um) and analyzed for P by amolybdate blue method using ascorbic acid as the reducing agent (Anon. 1969). The quantities of P adsorbedordesorbed (y)were plotted against the initial P concentration (x) of the added solution (isotherm A) oragainst the P concentration of thesupernatant solu- tion (isotherm B). The quantities of P ad- sorbed or desorbedwere expressed as milli- grams per kilogram of soil and the solu-

Table 1.Some chemical and physical characteristics oftheexperimentalsoils.

Soil Clay Organic C pH(H20) Al*) Fe*)

Vo Vo mmol/kg

1: Mean 37 3.3 5.4 84 71

Range 27—44 2.6—4.0 5.3—5.6 71 100 64—77

II: Mean 31 3.4 6.0 64 85

Range 25—40 2.1—5.1 5.8—6.1 52—86 76—91

•) extracted with0.05 Mammonium oxalate at pH 3.3.


Table2.Contentsofphosphorus extracted with water (Pw)orwithanacid ammonium acetate solution (Paaac) from subsamples of two soils fertilized with different rates of P.*)

Fertilization Paaac P„


annually mg/dm1 mg/kg

Soil I

0 2.5b 2.5'

13/16 3.4ab 3.6'

26/32 4.8a 5.1b

47/56 5.0a 7.3a

60/72 5.0a 5.5b


0 3.7d 5.1'

13/16 4.7' 6.7d

26/32 S.S'-' 9.4'

47/56 6.4b 12.3b

60/72 7.7a 13.7*

*) The results of each soil and extraction have been tested separately. Means witha common letterarenot dif-

ferent at the95 % level of statistical probability.

tion concentrations as milligrams per litre.

The isotherms had the general formula of y= bx + a.The details of the interpretation of the isotherms have been presented by Har-


Reversibly adsorbed P (Pj) wasdetermined bya method in which P desorbing from the soil is trapped by strips of filter paper impreg- nated with freshly precipitated iron hydrox- ide (Zeeetal. 1987, Yli-Halla


a). Phos-

phorus was also extracted withwater (1:60, w/v) (Hartikainen


a) and with an am- moniumacetate solution (0.5 M ammonium acetate, 0.5 M acetic acid)atpH 4.65 (20:200, v/v)(Vuorinen and Makitie 1955) (Table 2), the results of which have been published earli- er(Yli-Halla




In the isothermA, the quantities of P ad- sorbedordesorbedare plotted against the in- itial concentration of P of the added solution.

Based on this plotting, observations can be made about the effect ofa given P addition, e.g. fertilization, on the quantities of P ad- sorbed or desorbed. The type A isotherms

(Fig. 1) showed that the subsamples ofsoil I adsorbed P somewhat more effectively than did theonesof soil 11. This can also be con- cluded from the larger coefficient b in the equationsofthe A-type isotherms (Table 3).

As anexample, when the solution of 0.5 mg P/l was added (Paddition of25 mg P/kg), the subsamples of soil IInotfertilized with P adsorbed 14.8 mg/kg (59 %) whileas much as 20.2 mg/kg (81 %) was adsorbed by the corrensponding subsample of soil I. From the same addition,61 % and27 °7o was adsorbed by the subsamples fertilized with the highest PrateinsoilsIand 11,respectively. The sub- sample of soil I fertilizedevenwith the highest

Fig.I. TypeAisotherms. Adsorptionordesorptionof phosphate (y)as afunction of the concentration of added Psolution (x) by subsamples of two soils (I and II) into which different rates ofP had been applied annually.


Table3.The equations of phosphate adsorption isotherms of subsamples of two soils receiving different rates of P fertilization.

Fertilization kgP/ha

Soil I Soil II

Equation r Equation r


Isotherm A

0 y=44.54x—2.08 0.999***

y=43.82x—2.98 0.999***

y=42.91x—4.11 0.999***

y=42.64x—4.93 0.998***

y=41.76x—4.74 0.999***

y=37.27x—3.79 0.998***

y=35.69x—5.34 0.999***

y=35.99x—7.88 0.999***

y=37.56x—9.00 0.998***

y=35.82x—11.07 0.999***

13/16 26/32 47/56 60/72

Isotherm B

0 y=404.35x—18.91 0.996***

y=379.61x—27.22 0.993***

y=297.1 1x—28.54 0.998***

y=267.45x—29.10 0.998***

y=249.87x—28.25 0.997***

y=124.65x—13.13 0.992***

y= 106.13x—16.00 0.993***

y= 111.1 1x—24.60 0.996***

y=123.39x—30.10 0.986***

y=109.65x—34.07 0.997***

13/16 26/32 47/56 60/72

rate of P for 12 years adsorbed approximate- ly equal quantities of P as did the subsample of soil II notfertilized with P for 11 years.

In eachsoil,phosphate adsorption decreased with increasing P fertilization.

Isotherm B (Fig. 2) allows conclusions to be drawn about P sorption and desorption when the P concentration of soil solution is changed. The slope reflects the P buffer power of the soil. Soil I seemedtobemorebuffered against changes in equilibrium concentration thanwas soil 11. In soil I therewas aslight tendency of the buffer powerto decrease with increasing Pfertilization, while the change in soil II was inconsistent.

From isothermB, the quantity of P adsorp- tion required for the elevation of solution P concentrationto0.2 mg/1wascalculated (Ta- ble 4). This P concentration has been regard- edasbeing sufficient for maximum yield for mostcultivated crops. The calculation demon- strated thegreatertendency for P sorption of soil I. Inversely, the subsamples of soil II receiving the three highest rates of P main- taineda solution concentration even higher than0.2 mg/1. Extrapolation tothelowercon- centrations is questionable because the iso- therm is supposed to converge the y axis (Hartikainen



The equilibrium phosphate concentration (EPC) expresses the concentrationatwhichno net exchangeoccursbetween the soil and the surrounding solution. In soil I, EPC ranged from0.05 mg/1 to 0.11 mg/1 (Table 5), while insoil 11, the rangewaswider, i.e. from0.11

to0.31 mg/1. Accordingtothe paired t-statis- tics, the subsamples of soil I maintained statistically significantly lower EPC values than did theones of soil IIatany level ofP fertilization (t = 4.856**). The smaller changes in EPCs dueto P fertilization in soil I, com- pared tothe range of changes in soil 11,were in agreementwith the higher bufferpower of soil I.However, it should be observed that the EPC values were consistently elevated upon increased P fertilization in both soils.

In ordertofindout the size of labile pool ofP, the quantity of reversibly adsorbed P (Pi) was determined. In that method, P con- centration is maintained at a very low level during the extraction duetoremoval of phos- phate from the solutiontothe surface ofasep- aratesolid iron hydroxide phase, which pro- motesdesorption. The quantity of P;(Table 5)wassomewhatgreaterin the subsamples of soil 11,ascompared with the paired t-statistics with the corresponding samples of soil I(t = 3.557*).


Table4. QuantitiesofPadsorbed(+)ordesorbed (—) at equilibriumsolution concentration of0.2mg P/lin subsamples of twosoils receiving different rates ofPfer- tilization.

Fertilization Padsorbed ordesorbed (mg/kg) kgP/ha at equilibriumconcentration of

annually 0.2 mgP/l

SoilI SoilII

0 62.0 11.8

13/16 48.7 5.2

26/32 27.3 —2.4

47/56 24.4 —5.4

60/72 21.7 —12.1

Table5.Equilibrium phosphateconcentration (EPC) and reversibly adsorbed P(P;) in subsamplesof two soils receivingdifferent rates ofP fertilization.*)

Fertilization EPC Pi

kg P/ha

annually mg/1 mg/kg


0 0.047 b 16.8b

13/16 0.072,b 22.lab

26/32 0.096a 25.3“b

47/56 0.112“ 28.9“

60/72 0.113“ 26.3“b

Soil 11

0 0.105' 21.8'

13/16 0.151bc 26.1'

26/32 0.221abc 33.8b

47/56 0.244*b 38.3“b

60/72 0.311“ 44.7“

*) Theresults of EPC and P,have been tested separa- telywithin each soil. Means witha common letterare notdifferent at the95°lo level of statistical probability.


Thecurrent measurementsshowed that soil I adsorbed Pmoreeffectively and hada higher P buffer power than soil 11. Soil I was very muchbuffered, while soil II represented the average as compared with the clay soils of Hartikainen (1982c). During the last few years of the field experiments from which the soil samples originated, the highest yields in both soils were obtained atthe fertilization level of32 kg P/ha (Yli-Halla


b). The

EPC values of the subsamples taken from these plots were approximately 0.1 mg/1 and

0.2 mg/1 in soils I and 11, respectively, sug- gesting thatalower solution P concentration was sufficient for maximum yield in soil I.

This is in accordance with the conclusion by Olsen and Watanabe (1970) that in soils of higher buffer power (soil 1),alower P con- centration in the soil solutionis needed for a given quantity of P uptake by plants. Thus, the lower EPC values in soil I donot as such imply poorer P supplytothe plants in this soil.

However, in addition to lower P intensities, reflected by the EPC values of the soilsam- ples, thereserveof plant-availableP, as meas- ured by the quantity of reversibly adsorbedP, Fig.2.TypeBisotherms. Relationship betweenPadsorp- tionordesorption (y)and thePconcentration inthe equi- librium solution (x) after addingPto subsamplesof two soils (I and II) into which different rates ofPhad been applied annually.


waslower in the subsamples of soil I. Ascom- bined, these tworesults suggest that a more rapid conversion of added P into forms not availabletoplants had occurred in soil I. This conclusion is in accordance with the results of the field experiments in which much larger responses toP fertilization were obtained in soil I.

Therewere afew differences in the proper- ties of soil I and II which contributetohigher P adsorption in soil I: clay content (37%vs.

31%), pH (5.4vs. 6.0) and oxalate-extractable AI (84 mmol/kgvs. 64 mmol/kg). On the con- trary, the content of oxalate-extractable Fe was greater in soil II (71 mmol/kg vs. 85 mmol/kg). It isnotlikely that thedifferences in P adsorption characteristics aswellaslong- term effect of P fertilizationcan completely be attributedtothese minor differences.

The residual effect of P fertilization is de- pendent on therate of slow long-term fixa- tion processes occurring in thesoil, and the importance of these processes cannot be directly predicted by the determination of phosphate adsorption isotherms. Short-term P adsorption by 24 soils of Australia have been observed tobe in close relationship with P sorption also in the long run and in posi- tive correlation with the yield response by clo- ver to P fertilization in a pot experiment (Barrow 1972). In Sweden,StAhlberg(1982) evensuggested a method basedon the deter- mination of P adsorption for the basis for P fertilization recommendations. Holford (1982) further showed that therecovery of fertilizer Pover aperiod of four years in apot experi- mentwas in close correlation with the results of short-term P adsorption measurements.

Also in thepresent study, there seemedtobe

arelationship between the yield response to P fertilization and P adsorption by the soil.

Even though thecurrentmaterial consisted of samples taken only from two field experi- ments, the observed tendency is likely not a coincidence, because it is well in agreement with the results of other studies mentioned above. The results of thepresent study thus confirm that short-term P adsorption mea- surementsare indeed abletoserve as avalua- ble tool in practical soil fertility research in elucidating the tendency of adsorption of ad- ded P as wellas the probable long-term yield response to P fertilization.

Previous results (Yli-Halla


b) showed

that repeated P fertilization for 11or 12 years hadadifferent effectonthe quantities ofeas- ily soluble P in the two soils studied despite the originallysamePstatusof soils. It should be recognized thatwaterextractionwasin the present soils more accurate in reflecting the residual effect of P fertilization. The present results further showed that the changes in equilibrium phosphate concentration (EPC), attributabletoP fertilization, differed wide- ly from soil tosoil. The difference between thetwosoils couldnotbe predicted from soil properties other than P sorption. Even though it isnotpossible by the results of ordinaryace- tate orwaterextractiontopredict the residu- al effect of Pfertilization,regular soil analy- sis is ofutmostimportance in monitoring the cumulative effect of P status of the soil.By knowing the quantities of P fertilization given and the consequent changes in soil testing results, the farmer is gradually able to get a sound idea of theresidual effect of P fertili- zation in the field concerned.



Anon. 1969. Juoma-jatalousveden tutkimusmenetel- mat. ElintarviketutkijainSeura. 169p. Helsinki.

Barrow, N.J. 1972.Relationshipbetweenasoil’s abili- tytoadsorb phosphate and the residual effectiveness of superphosphate. Aust. J. Soil Res. 11:57 —63.

Hartikainen, H. 1982a. Water soluble phosphorusin Finnish mineral soils and its dependenceon soil properties. J. Scient. Agric. Soc. Finl. 54: 89 —98.

—1982 b.Relationshipbetweenphosphorusintensityand capacityparameters inFinnish mineral soils. I. In- terpretationand application of phosphorus sorption -desorptionisotherms. J. Scient. Agric. Soc.Finl. 54:




11.Sorption-desorptionisotherms and theirre- lation to soil characteristics. J. Scient. Agric. Soc.

Finl. 54:251—262.

Holford, I.C.R. 1982.Effects of phosphate sorptivity onlong-term plant recovery and effectiveness of fer- tilizer phosphateinsoils. Plant and Soil64: 225 —236.

Larsen,S., Gunary,D.&Sutton, C.D. 1965.The rate of immobilization ofapplied phosphateinrelation to soil properties. Plant and Soil 16: 141—148.

Olsen, S.R.&Watanabe,F.S. 1970.Diffusive supply of

phosphorusinrelation to soil texturevariations. Soil Sci. 110; 318—327.

StAhlbero, S. 1982. Afixation method for estimation of the Prequirement of soils. Acta Agric. Scand. 32;


Sippola,J.&Saarela, I. 1986. Someextraction methods asindicators of need for phosphorus fertilization.

Ann. Agric.Fenn. 25: 265—271.

Vuorinen, M.&Makitie, O. 1955.The method of soil testinginuseinFinland. Agrogeol. Publ. 63.44 p.


Yli-Halla, M. 1989a.Reversibly adsorbedP inminer- al soils of Finland. Commun. Soil Sci. Plant Anal.

20: 695—709.

—1989b. Effect of different rates ofPfertilizationon theyieldand Pstatusof thesoilintwo long-term field experiments.J. Agric. Sci. Finl. 61: 361 —370.

Zee,S.E.A.T.M.vander, Fokkink, L.G.T.&Riemsdijk, W.H. van. 1987.A newtechniquefor assessment of reversiblyadsorbed phosphate. Soil Sci. Soc. Amer.

J.51: 599—604.

Ms received February 12, 1991


Fosforin pidattyminen kahdesta

pitkaaikaisesta fosforilannoituskokeesta otettuihin maanaytteisiin

Markku Yli-Halla

Kemira Oy, Espoontutkimuskeskus,Luoteisrinne2, 02270 ESPOO

Tutkimusta varten otettiin maanaytteitakenttakokeista, joissaoli11 tai 12vuoden ajan ollut viisi eri P-lannoi- tustasoa(vuosittain 0, 13tai 16, 26tai32,47tai56seka 60tai72kgP/ha). Kokeet olivathiuesavimailla,janiis-

saoli aluksi samat maarat happamaan ammoniumasetaat- tiliuokseen uuttuvaa fosforia (n.6mg/1), janiiden omi- naisuudetolivatmuutenkin lahes samanlaiset. Kuitenkin toisessamaassa (maa I) P-lannoitus lisasi satoa paljon enemmankuin toisessa (maa II). Kun kenttakokeiden paattyessaotettujen maanaytteiden fosforinpidatyskykya tutkittiin ravistelemalla maanaytteita (1 g) eri vahvuisis- sa(0—0.5mg/1) fosforiliuoksissa,havaittiin,ettamaan Inaytteetpidattivatfosforia huomattavasti tehokkaam-

minkuinmaanIInaytteet. Myos labiilin fosforin koko- naismaara olimaan Inaytteissa pienempikuin maan II vastaavan lannoituksen saaneissa naytteissa. Selitys maalla 1P-lannoituksella kenttakokeissa saatuihin suuriin sadon- lisayksiin oli siis se, etta tassa maassa lannoitefosfori pidattyi nopeastikasveille kayttokelvottomaan muotoon, kun taasmaassa IIlannoitefosfori sailyi suuremmassa maarin liukoisena. Koska nykyisin maa-analyysimene- telmin ei pystyta luotettavasti arvioimaan P-lannoituksen jalkivaikutuksen suuruuttamaanominaisuuksien perus- teella,onviljavuusanalyysi tehtava kyllin useinmaanP- tilan kehityksen seuraamiseksi.



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