Spectrochimica Acta Part B 71-72 (2012) 117–122

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Spectrochimica Acta Part B

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Analytical Note

Comparison of inductively coupled plasma mass spectrometry and colorimetricdetermination of total and extractable phosphorus in soils

Krasimir Ivanov a,⁎, Penka Zaprjanova b, Milena Petkova a, Violeta Stefanova c, Veselin Kmetov c,Deyana Georgieva c, Violina Angelova a

a Department of Chemistry, University of Agriculture, Plovdiv, Bulgariab Tobacco and Tobacco Products Institute, Plovdiv, Bulgariac Department of Analytical Chemistry, Plovdiv University “Paisii Hilendarski,” Plovdiv, Bulgaria

⁎ Corresponding author. Fax: +359 32 633157.E-mail address: kivanov1@abv.bg (K. Ivanov).

0584-8547/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.sab.2012.05.013

a b s t r a c t

a r t i c l e i n f o

Article history:Received 21 October 2011Accepted 29 May 2012Available online 13 June 2012

Keywords:Total phosphorusExtractable phosphorousSoil analysisICP-MSColorimetry

The most widely used method for determination of total phosphorus in soils is perchloric acid digestion,followed by a colorimetric assay to measure the concentration of P in solution. The first part of this studycompares an alternative digestion method, using aqua regia (ISO 11466 and EPA Method 3052), withperchloric acid digestion procedure, and also compares inductively coupled plasma mass spectroscopy(ICP-MS) with colorimetry for the measurement of P on the basis of five internationally certified standardsoils and 20 real-life soils with widely different extractability of phosphorus. The phosphorus concentrationwas determined by means of the reduced phosphomolybdenum blue and ICP-MS. The relationship betweenmethods has been examined statistically. Good agreement of the results from colorimetry and ICP-MS wasestablished for all certified soils. The microwave-assisted digestion with aqua regia was comparable, bothin precision and accuracy, with the hot plate aqua regia method. The phosphorus concentration found withthe HF+HClO4 digestion method was in good agreement with the certified mean values, while the superior-ity in extracting phosphorus, when compared to other methods, was obvious.Soil testing for plant-available phosphorus in Bulgaria and many European countries is most commonly con-ducted using acid Ca-lactate extraction (Egner–Riehm test) and alkaline sodium bicarbonate extraction (BDSISO 11263:2002), based on Olsen test, followed by a colorimetric assay to measure the concentration of P insolution. The second part of this study reports the differences between Egner–Riehm test and BDS ISO11263:2002 measured colorimetrically and by ICP-MS. Fifty soils were selected from South Bulgaria to repre-sent a wide range of soil properties. It was established that ICP-MS consistently yielded significantly higher Pconcentrations than the colorimetric method in both extraction tests, and the relative differences weregreatest in soils with lower P concentrations.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

The determination of total (Ptot) and extractable phosphorus(Pextr) in soils is very important in agriculture to determine theamount of phosphate needed for crop production, and in environ-mental studies to evaluate risks associated with elevated levels ofsoil P. The most widely used methods for the determination of totalphosphorus in soils employ sodium carbonate fusion or perchloricacid digestion [1,2]. The fusion method is relatively lengthy and re-quires considerable technical skill [3], which makes it unsuitable forroutine analysis, whereas the perchloric acid digestion is potentiallyhazardous and can give low recovery [2]. In spite of these shortcom-ings, these two methods are recognized standards for determination

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of total phosphorus [4]. This paper compares an alternative digestionmethod, using aqua regia (ISO Standard 11466 [5] and EPA Method3052 [6]), with perchloric acid digestion [7] on the basis of five inter-nationally certified standard soils. The necessity of such study isdetermined by the fact that aqua regia digestion procedure has beenrecognized as a suitable method for estimation of the maximum ele-ment availability to plants [8]. Residual analyte that is not releasedby aqua regia digestion is mostly bound to silicate minerals and con-sidered unimportant for estimating the mobility and behavior of theelement [9].

Colorimetric quantification of the phosphorus based on thephosphoromolybdenum blue complex has always been the mostwidespread analytical method. Recently, many soil test laborato-ries are moving from traditional colorimetric methods to induc-tively coupled plasma optical emission spectrometry (ICP OES)to quantify phosphorus in soil test extracts, due to the advantageof allowing the determination of almost all plant nutrients in one

118 K. Ivanov et al. / Spectrochimica Acta Part B 71-72 (2012) 117–122

analytical process [10]. The estimation of the possibility for a si-multaneous determination of potentially available microelements,heavy metals and some of the essential macro elements, includingphosphorus, is reasonable from both a scientific and a purely eco-nomic point of view. In spite of many reports in this directionduring the last decade, data for using inductively coupled plasmamass spectrometry (ICP-MS) for phosphorus determination ispractically missing. This gave us an opportunity to compare ICP-MS with colorimetry for the measurement of total P.

Soil testing for plant-available phosphorous in Bulgaria [11], manyEuropean countries [12] and Australia [13] is most commonly con-ducted using acid Ca-lactate extraction (Egner–Riehm test) [14] andalkaline sodium bicarbonate extraction (BDS ISO 11263:2002) [15],based on the Olsen test [16], followed by a colorimetric assay to mea-sure the concentration of P in solution. The second part of this studyreports the differences between the results obtained by Egner–Riehm (ER) test and BDS ISO 11263:2002, measured colorimetricallyand by ICP-MS.

The methods used in the study for soil digestion and soil extrac-tion and for phosphorus determination were evaluated according totheir accuracy, recovery of total phosphorus, facility of applicationand speed of performance.

2. Materials and methods

2.1. Total phosphorous

2.1.1. SoilsIn this study, five internationally certified standard soils –

Light Alluvial-deluvial Meadow Soil PS-1, SООМЕТ no. 0001–1999 BG, SOD no. 310a-98; Light Meadow Cinnamonic Soil PS-2,SООМЕТ no. 0002–1999 BG, SOD no. 311a-98; Light Alluvial-deluvial Meadow Soil PS-3, SООМЕТ no. 0003–1999 BG, SOD no.312a-98 (Eurotest Control, Sofia, Bulgaria); NCS DC 77303; andNCS DC 73386 (China National Analysis Center for Iron andSteel, Beijing, China) – were analyzed for total phosphorus. Themain properties of the soils that influence soil phosphorus con-tent are presented in Table S1.

Twenty soil samples were selected from Southern Bulgaria to rep-resent a wide range of soil properties. The soils were air-dried andpassed through a 2 mm sieve, and subsamples were further groundto pass through a 0.25 mm sieve.

2.1.2. Sample digestion

2.1.2.1. Perchloric acid digestion. Soil sample (0.2 g) was placed in a100 ml digestion flask and 3 ml of 60% HClO4 was added. The flaskwas placed on a hot plate at 200±5 °C and digested for 75 min. Re-flux funnels with water cooling were used to reduce evaporation.The digest was cooled, filtered and diluted to 50 ml.

2.1.2.2. Aqua regia digestion. The most widely used aqua regia methodsfor soil digestion – ISO 11466 and EPAMethod 3052 –were used as fol-lows: ISO 11466: 0.5 g of soil sample was placed in a 100 ml digestionflask and 8 ml of aqua regia was added and left to stand for 16 h atroom temperature. The flask was placed on a hot plate and the temper-ature of the reaction mixture was slowly raised until reflux conditionswere reached and maintained for 2 h. Reflux funnels with watercooling were used to reduce evaporation. The digest was cooled, fil-tered and diluted to 50 ml. EPA Method 3052: A microwave digestionsystem (Milestone 1200 MEGA, Italy) with 10 MRD 300 rotor with 10positions, max. pressure of 30 bar and max. power of 1000W wasused. A homogenized sample of 0.2 g dry substance was weighed intoa Teflon beaker and 8 ml of aqua regia was added. The microwave-assisted mineralization program comprised three stages: (i) 5 minnon-pulsed 250W microwave irradiation; (ii) 5 min 400W pulsed

microwave irradiation and (iii) 5 min 600W pulsed microwave irradi-ation. After a ventilation of 1 min the sample was cooled and diluted to50 ml.

2.2. Extractable phosphorus

2.2.1. SoilsFifty soil samples (including samples described in 2.1.1.), the

chemical and physical properties of which varied widely (Table S2),were taken from different locations in Southern Bulgaria to character-ize the difference between ICP-MS and colorimetric P determination.Soil pH ranged from 4.9 to 8.1, oxidized organic matter from 0.9 to5.2%, and clay from 14 to 76%.

2.2.2. Soil test extraction

2.2.2.1. BDS ISO 11263:2002 soil test. Soil sample (2 g) was extractedwith 100 ml 0.5 mol L−1 NaHCO3, adjusted to pH 8.5, by shaking for8 h. Interference from organic matter dissolved in the solution waseliminated by sorbing the organic matter onto activated acid-washed charcoal added to the extract.

2.2.2.2. Egner–Riehm (ER) soil test. Soil sample (5.0 g) was extractedwith 200 ml 0.04 mol L−1 calcium lactate, acidified with hydrochloricacid to pH 3.5, by shaking for 8 h.

2.2.2.3. Procedure of acid peroxydisulphate digestion [17]. Soil extract(50 ml) was transferred to a 100 ml volumetric flask and 0.8 gK2S2O8 and 8 ml 2 mol L−1 H2SO4 were added. The flask was placedon a hot plate at 125 °C and digested for 90 min. The digest wascooled, filtered and diluted to 50 ml.

2.3. Analytical methods

The total phosphorus in soil extracts wasmeasured photometricallyafter adjusting the digests to pH 3±0.5 by two widely used methods –(i) method of Murphy and Riley (MR) [18] and (ii) Phosphate testSpectroquant (Merck KGaA) [19] – using Boeco S-22 UV/VIS Spectro-photometer (Germany) and Spectroquant Pharo 100 spectrometer(Merck KGaA, Darmstadt, Germany) respectively. The results obtainedwere compared with the results of ICP-MS investigation. A Model7700 ICP-MS Agilent (Tokyo, Japan), equipped with Octopole reactioncell (ORC) with two independent gas channels for He and H2 as reac-tion/collision gases, capable of reducing polyatomic spectral interfer-ences, was used.

All reagents were of analytical or suprapure reagent-grade. Double-distilled water was used for the preparation of all solutions. Single ele-ment standard solutions for P, K and Be with concentration 1 g L−1

were bought from CPAchem (CPE Ltd, Stara Zagora, Bulgaria) and Rh10 mg L−1 from Fluka Chemie (Buchs, Switzerland). ICP multi-elementstandard solution VI (Merck, Darmstadt, Germany) was used for deter-mination of other elements together with target analytes. Calibrationstandards were prepared daily after appropriate dilution in 0.05% (v/v)HNO3 (Merck).

For reduction of spectral interferences an optimization of He colli-sion gas flow rate was performed. The analyzed solutions behave asdifferent matrices, depending on the sample treatment (dissolutionwith acids; elution with sodium carbonate or Ca-Lactate). In orderto compensate variable matrix effect 9Be, 10B and 103Rh were testedas potential internal standards (IS). Boron was rejected because it ispresent in the acid digests of soil samples at measurable concentra-tion. Beryllium was selected as the most appropriate internal stan-dard with respect to the stability of the signal ratio (analyte/IS) inall three matrices studied. The method of external calibration with in-ternal standard spiked to calibration standards and sample solutionswas further applied to all analyses.

Table 2Average soil phosphorous concentration and standard deviation (n=4) obtainedfor five CRM by hotplate aqua regia (ISO 11466) and microwave-assisted aqua regiadigestion (EPA Method 3052); all values in mg kg−1.

CRM Hotplate aqua regia Microwave-assisted aqua regia

Colorimetry ICP-MS P Colorimetry ICP-MS P

MR-P Spectroquant P MR-P Spectroquant P

PS – 1 688±18 653±22 683±36 645±20 655±24 660±35PS – 2 453±15 475±18 485±13 482±18 491±20 495±18PS – 3 662±22 680±24 681±30 678±25 663±16 673±3277303 1201±46 1250±48 1235±68 1130±52 1125±52 1180±5477386 394±16 403±22 271±24 410±28 400±25 250±52

119K. Ivanov et al. / Spectrochimica Acta Part B 71-72 (2012) 117–122

Reagent blanks and reference samples were extracted and analyzed(1 per 10 samples) for quality assurance and quality control. The extract-able phosphoruswasmeasuredphotometrically using the phosphate testSpectroquant (Merck) and the results obtained were compared withthose of ICP-MS measurements. Soil pH and organic carbon (Corg) weremeasured according to ISO 10390:1994 (Soil quality – Determinationof pH) and ISO 14235:1998 (Soil quality – Determination of organiccarbon by sulfochromic oxidation) standards.

The comparison of ICP-MS and colorimetric determination of P inthe extracts was performed by regression analysis using the SPSSStatistics Base 17.0 program for Windows. The regression equationchosen was the one with the highest R2 and with coefficients thatwere significantly different from zero.

Themethods used for sample preparation, phosphorus determina-tion and abbreviations are presented in Table 1.

3. Results

3.1. Determination of total phosphorus

3.1.1. Relationship between Hotplate (ISO 11466) and Microwave-assisted aqua regia (EPA Method 3052) digestion

The results obtained from the methods used for five certified stan-dard soils are shown in Table 2.

As presented, the results obtained from 4 measurements for eachsample are similar and there is no significant difference for all sam-ples excluding sample NCS DC 73386. In this case, the colorimetricmeasurement results are essentially identical, while the ICP-MSvalues are significantly lower with both methods of sample digestion.The possible reason for this is the very high iron content in this sam-ple (Table S1), as well as the difficulty of interference eliminationwith ICP-MS. The clarification of this issue requires additional studies.

The degree of phosphorus extraction varies from 69 to 75% forPS and 2 to 100% for NCS DC 73386 (colorimetric determination).Obviously, both variants of aqua regia digestion did not enable total

Table 1Methods for sample preparation and phosphorus determination. Detection limits aremarked in brackets.

Total phosphorus Extractable phosphorus

Sample preparation1. Perchloric acid digestion 1. BDS ISO 11263 soil test2. Aqua regia digestion: 2. Egner–Riehm soil test• Hotplate aqua regia (ISO 11466) 3. Procedure of acid peroxydisulphate

digestion (APD)• Microwave aqua regia (EPAMethod 3052)

Phosphorus determination1. Photometrically: 1. Photometrically:• Method of Murphy and Riley(0.01 mg L−1)

• Method of Murphy and Riley(0.01 mg L−1)

• Phosphate test Spectroquant(0.01 mg L−1)

• Phosphate test Spectroquant(0.01 mg L−1)

2. By ICP-MS (0.0028 mg L−1) 2. By ICP-MS (0.0028 mg L−1)AbbreviationsMR-P – phosphorus, determinedby method of Murphy and Riley

BDS ISO ICP-MS P – phosphorus,determined by ICP‐MS after BDS ISO11263:2002 soil test extraction

Colorimetric P – phosphorus,determined by colorimetry

BDS ISO Colorimetric P – phosphorus,determined by colorimetry after BDS ISO11263:2002 soil test extraction

ICP‐MS P – phosphorus,determined by ICP‐MS

ER Colorimetric P – phosphorus,determined by colorimetry afterEgner–Riehm soil test extraction

HClO4-P – phosphorus,determined after perchloric digestion

ER ICP-MS P – phosphorus, determinedby colorimetry after ICP‐MS afterEgner–Riehm soil test extraction

ISO11466-P – phosphorus, determinedafter aqua regia digestion

APD P – phosphorus, determined afterAPD

phosphorus extraction. The only exception is NCS DC 73386, theresults of which are not convincing and are subject to additionalinvestigation.

3.1.2. Relationship between aqua regia (ISO 11466 and EPA Method3052) and HClO4 digestion

The very similar results for phosphorus obtained by means of ISO11466 and EPA Method 3052 make possible averaging of the resultsin the comparison of the effectiveness of aqua regia and HClO4 diges-tion (Table 3).

It can be seen that four of the five CRM exhibited lower degree ofphosphorus extraction than the certified values. Irrespective of thequantitative determination method for all samples, the degree ofextraction by means of perchloric acid is approximately 10% higher.

For statistical proof of the difference between the effectiveness ofboth methods we used 20 soil samples from Southern Bulgaria, whichwere significantly different regarding physical and chemical proper-ties, and their phosphorus content (Table S3).

The soils selected were either fairly acidic (pH=5.2) or alkaline(pH=8.1) with a mean close to neutral (pH=7.00). The content ofCorg varied from 0.5 to 2.9% and the clay content between 16.4 and70.0%.

The samples were digested following ISO11466 and standardHClO4 acid procedure, described above. The phosphorus contentwas determined colorimetrically by means of Phosphate testSpectroquant (Merck). The results obtained by means of both diges-tion methods, as well as some of the basic characteristics of the stud-ied soil samples are presented in Table S7. The P extracted by HClO4

digestion ranged from 175 to 1626 mg kg−1 with an overall meanof 843.6 mg kg−1, while aqua regia method gave P values rangingfrom 150 to 1475 mg kg−1 with an overall mean of 755 mg kg−1.The difference between P extracted by HClO4 and by aqua regiaranged from 1 to 209 mg kg−1 with a mean value of 89 mg kg−1. Itis obvious that HClO4 extracts showed significantly higher P concen-trations (12.9% for the mean values) and the proportion of HClO4–P/ISO11466–P in all cases is greater than 1. Our results contrast withthose of Gasparatos and Haidouti [7] who reported that HClO4 andaqua regia extract similar amounts of P. The possible reason for thisis that we strictly complied with the requirements of ISO 11466 in

Table 3Average total phosphorus content and standard deviation (n=4) obtained by aquaregia (ISO 11466 and EPA Method 3052) and HClO4 digestion; all values in mg kg−1.

CRM Certifiedconcentrations

Aqua regia (ISO11466+EPA 3052)

HClO4

Colorimetric P ICP‐MS P Colorimetric P ICP‐MS P

PS – 1 860 661±21 669±36 750±45 700±90PS – 2 655 475±18 490±16 605±35 575±65PS – 3 895 671±22 677±31 750±52 700±9277303 1400 1177±54 1208±61 1275±46 1225±10577386 390 402±23 261±38 399±26 347±43

y = 191.96x-0.7333

R2 = 0.5179

0

20

40

60

80

100

120

140

160

180

0 50 100 150 200

BDS ISO Colorimetric P mg kg-1

BD

S IS

O IC

P-M

S -

BD

S IS

OC

olor

imet

ric P

, %

Fig. 1. Relationship between the relative difference of BDS ISO ICP-MS and BDS ISO col-orimetric P, expressed by percentage rise.

120 K. Ivanov et al. / Spectrochimica Acta Part B 71-72 (2012) 117–122

our study, while Gasparatos and Haidouti [7] extended the digestionprocedure to 16 h at 140 °C.

The phosphorus content of 20 soil samples obtained by the twoextracting methods was compared using a t-test, which indicatedthat there is a significant difference between the HClO4 and aquaregia digestion methods.

Fig. S1 shows the comparison between HClO4-P and ISO11466-Pusing linear regression analysis. A significant linear relationship(R=0.986) with slope of 1.043 exists between phosphorus deter-mined by HClO4 and by aqua regia (ISO11466). The data are also com-pared with 1:1 line (dashed line). As it can be seen from the diagram,the slope of the regression is different from 1 and the intercepts aredifferent from zero, clearly indicating that HClO4–P is not similar toISO11466–P.

The most important criteria for recommending digestion andquantification methods for phosphorus determination in soils are ac-curacy, simplicity, fastness and ease of determination. Taking thesecriteria into consideration, the HClO4 digestion method can be rec-ommended, especially for soils with low content of Corg.

ICP-MS determination has many advantages, related mainly to thepossibility for simultaneous determination of a large number of ele-ments. The possible matrix effects, especially with samples havinghigh iron content, shall be taken into account. The extremely high di-lution factor used in this method resulted in significantly highervalues of the standard deviation and lower level of measurement ac-curacy. The spectrophotometric method suggested by Merck has sig-nificant advantages, especially when the purpose of the study islimited to phosphorus determination. They are mainly related to ra-pidity and ease of determination. However, in this case, significant di-lution of samples shall be required (dilution factor over 2000), whichmay considerably affect the measurement accuracy. The simulta-neous determination of phosphorus and other significant macro-and microelements in aqua regia extract upon strict compliancewith the requirements of ISO 11466 is risky and may lead to signifi-cant underestimation of phosphorus content in soils.

3.2. Determination of extractable phosphorus

Phosphorus exists in soils in a wide variety of inorganic and organ-ic forms. It is impossible to predict the quantity of plant-available Pfor a given soil, varying with plant and soil characteristics, includingplant root, soil pH, mineralogy and P buffering capacity [13]. Thereare various soil tests for evaluating plant-available P using differentextraction solutions. Mehlich 3 (M3P) [20] is a widely used extractantin the USA due to its versatility. Differences between ICP OES and col-orimetric M3P analysis have been discussed in a number of studies[21–23]. Despite the differences noted between ICP OES and colori-metric M3P determination, fertilizer recommendations based onthese analyses are used by some laboratories without any correctionor field validation [21]. This practice could lead to misapplication ofnutrients and contribute to monetary crop losses or negative environ-mental effects.

The alkaline sodium bicarbonate extraction procedure, based onOlsen P test [16] and ER soil test [14], is currently the most commonlyused available P test in many countries [11–13]. To our knowledgeonly one report is known in the scientific literature, comparing ERand Olsen ICP OES and colorimetric measurements [13]. No studiesare known concerning ICP-MS. This suggests the need for examina-tion of the difference between ICP-MS and colorimetric ER andOlsen plant-available P.

3.2.1. BDS ISO 11263:2002 soil testThe samples were digested following BDS ISO 11263:2002 soil

test, described above. The phosphorus content was determined color-imetrically by means of the molybdate blue method of MR and Phos-phate test Spectroquant (the averaged data are presented in tables

and figures) as well as by ICP-MS. The results obtained by means ofboth analytical methods are presented in Table S4.

Colorimetrically determined phosphorus ranged from 3.8 to170.8 mg kg−1 with an overall mean of 31.3 mg kg−1, while ICP-MSgave P values ranging from 6.1 to 186 mg kg−1 with an overall meanof 36.8 mg kg−1. It is obvious that ICP-MS showed significantly higheramounts of P (17.6% for the mean values), and the difference betweenICP-MS P and colorimetric P ranged from 1.0 to 15.4 mg kg−1 with amean value of 5.5 mg kg−1 and relative difference from 2.4 to 152%with a mean value of 32.3%. These results suggest that significantamounts of non-molybdate reactive P are present in the soils.

The comparison of ICP-MS and colorimetric BDS ISO 11263:2002soil P for all samples is shown in Fig. S2.

Significant linear relationship (R2=0.9896) with a slope of 0.956exists between phosphorus determined by ICP-MS and by colorime-try. The data are also compared with 1:1 line. As it can be seen inthe diagram, the slope of the regression is different from 1 and the in-tercept is different from zero, clearly indicating that ISO ICP-MS–P isnot similar to colorimetric P.

The relative difference between ICP-MS P and colorimetric P de-creased rapidly as colorimetric P increased (Fig. 1), i.e., the relative differ-ence is larger when P content in the soil is lower (10–20 mg kg−1) anddecreases to 10–20% when the P content is higher than 50 mg kg−1.The results are similar to those reported by Pitman et al. [21] forM3P ex-tracts and by Matula [22] for M3P, NH4-acetate and water extracts.

The absolute difference between ICP-MS and colorimetric methodsranges from 0 to 16 mg kg−1 and obviously depends on soil type andP concentration. Generally it increases with increasing P content.

Several authors have found differences between ICP OES and colori-metrically determined available soil phosphorus, strongly dependent onthe soil test. Pitman et al. [21] compared ICP OES versus colorimetric P in6400 M3P extracts and found that ICP gave an average of 9.1 mg kg−1

higher P content than the colorimetric method. On the basis of the re-sults for 59 M3P extracts Mallarino [23] concluded that M3P, measuredby ICP OES, should be considered as a different test from the traditionalcolorimetric assay. Hart and Cornish [13] reached the same conclusionon the basis of investigation of 714 samples taken from 68 different lo-cations within the south-east of New South Wales. According to theseauthors measurement by ICP OES consistently yielded significantlyhigher P concentrations in sodiumbicarbonate soil extracts than the col-orimetric methods (ICP-P=1.122 Col. P+57, R2=0.95, Pb0.001).Sicora et al. [25] are of the opposite opinion. They compared ICP OES–P and colorimetric P in M3P extracts from 2 sets of 1118 and 418 soils,measured in separate laboratories and found slopes of linear regressionvery close to 1:1 and intercepts very close to zero, which indicated thatthe ICP OES soil test P values were very similar to the colorimetric ones.

Some authors attributed the difference between ICP OES and col-orimetric M3P to soil pH [24]. This suggestion was not confirmed byPitman et al. [21] and Mallarino [23]. It is generally accepted that

y = 59.694x-0,596

R2 = 0.6387

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0 100 200 300 400

ER Colorimetric P (mg kg-1)

ER

ICP

-MS

- E

R C

olor

imet

ric P

, %

Fig. 2. Relationship between relative difference of ER ICP-MS and ER colorimetric P,expressed by percentage rise.

121K. Ivanov et al. / Spectrochimica Acta Part B 71-72 (2012) 117–122

the difference in ICP OES P and colorimetric P is due to organic P. ICPOES measures organic forms of P that may not be measured by themolybdenum blue method, which measures predominantly ortho-phosphate. According to this acceptation preliminarily oxidation oforganic P in the soil extracts can increase colorimetric P to the levelof ICP. Rowland and Haygarth [26] recommended acidified per-oxydisulphate digestion (APD) as the best method for the analysisof soil solution. Matula [17] proposed a simplified APD of soil waterextract for determination of total dissolved phosphorus by molybde-num blue colorimetry. Following this idea, we compared ICP-MS Pand APD P in 50 BDS ISO soil extracts. Summary statistics for thesoils are presented in Table S5.

Colorimetrically determined phosphorus ranged from 6.4 to187 mg kg−1 with an overall mean of 36.4 mg kg−1, while ICP-MSgave P values ranged from 6.1 to 186 mg kg−1 with an overall meanof 36.8 mg kg−1. In this case ICP-MS yielded practically the same Pconcentrations as colorimetry. The difference between P determinedusing ICP-MS and P determined using colorimetry ranged in a narrowrange from −5.4 to 4.4 mg kg−1 with a mean value of 0.4 mg kg−1

and a relative difference from −16.5 to 20.7% with a mean value of1.3%.

The comparison of BDS ISO APD P and BDS ISO ICP-MS P is shownin Fig. S3.

Linear regression of the data yielded high R2 value (R2=0.9973),the slope of the regression is close to 1 (0.9879) and the intercept isnegligible (0.0447), clearly indicating that ISO-MS–P is nearly identi-cal with colorimetric P. These results are similar to those reported byMatula [17] who found no difference in the values between the directICP OES P determination in water extract of 79 soils and colorimetricP in the APD. On the basis of the results presented in Table S5 and Fig.S4 it can be concluded that the APD method and subsequent colori-metric detection of phosphorous can be used for determination oftotal phosphorus in the soil extracts. Its advantage over other diges-tion methods is simplicity of realization and substantially less laborand time consumption. The APD method can be recommended forroutine determination of total extractable phosphorus in soils whenthe soil testing laboratory is not equipped with an ICP techniqueand especially when the purpose of the study is limited to phospho-rous determination.

3.2.2. Egner–Riehm soil testThe samples were digested following ER soil test, described above.

The phosphorus content was determined colorimetrically by means ofmolybdate bluemethod ofMR and Phosphate test Spectroquant (the av-eraged data are presented in tables and figures) as well as by ICP-MS.Colorimetrically determined phosphorous in this case varied widelyfrom 4.0 to 333.0 mg kg−1 with an overall mean of 59.5 mg kg−1,while ICP-MS gave P values ranged from 5.8 to 343 mg kg−1 with anoverall mean of 62.5 mg kg−1 (Table S6).

ICP-MS showed higher P contents (5.0% for the mean values), andthe difference between ICP-MS P and colorimetric P ranged from 0.7 to9.8 mg kg−1 with a mean value of 3.1 mg kg−1 and relative differencefrom 2.2 to 44.8% with a mean value of 9.8%. Linear regression of thecombined data yielded a highly significant relationship (R2=0.9996,Pb0.001). The comparison of ICP-MS and colorimetric ER P for all sam-ples is shown in Fig. S4.

The relationship between the relative difference of ER ICP-MS Pand ER colorimetric P shows the same tendencies as with BDS ISOsoil test (Fig. 2). The relative difference between ICP-MS P and color-imetric P decreased as colorimetric P increased. This increase is, how-ever, considerably weaker than with BDS ISO soil test. The absolutedifference between the values of ICP-MS P and colorimetric P is alsoconsiderably lower. However, its increase with increasing P contentis more clearly expressed (R2=0.6387).

Close linear relationship (R2=0.8154) was found between phos-phorus amounts measured by the ER and BDS ISO methods (Fig. S5).

ER extraction yielded significantly more phosphorus than BDS ISOmethod (y=1.6824x+6.116).

Similar linear correlation between ER and Olsen methods wasrecorded also in research in Latvia [12], where the correlation coeffi-cient R was found to be 0.95 (n=145). The same authors found thatin alkaline soils, rich in carbonates, the relationship between the ERand Olsen methods becomes weaker. They conclude that, for the de-termination of available phosphorous in alkaline carbonatic soils,the most suitable is the Olsen method.

4. Conclusions

The most important criteria for recommending digestion andquantification methods for phosphorous determination in soils areaccuracy, simplicity, rapidity and ease of determination. Taking intoconsideration these criteria, HClO4 digestion method can be rec-ommended, especially in soils with low contents of Corg. ICP-MS de-termination has many advantages, related mainly to the possibilityof simultaneous determination of a large number of elements. Thepossible matrix effects should be taken into account. The extremelyhigh dilution factor used in this method resulted in significantlyhigher values of the standard deviation and lower level of measure-ment accuracy. The spectrophotometric method suggested by Merkhas significant advantages, especially when the purpose of the studyis limited to phosphorous determination. They are mainly related torapidity and ease of determination. However, in this case, significantdilution of samples is required (dilution factor over 2000), whichmay considerably affect the measurement accuracy. The simulta-neous determination of phosphorous and other significant macro-and microelements in aqua regia extract upon strict compliancewith the requirements of ISO 11466 is risky and may lead to signifi-cant underestimation of phosphorous content in soils.

The analytical method used to measure P in ER and BDS ISO11263:2002 soil extracts substantially affects the results. In all casesthe ICP-MS determination registers significantly higher P concentra-tions than the traditional colorimetric molybdenum blue method.The relative differences between ICP-MS and colorimetric phospho-rus in ER and BDS ISO 11263:2002 soil extracts strongly depend onthe P concentration while the absolute differences between methodsdepend on both soil type and P concentration. Close linear relation-ship (R2=0.8154) exists between phosphorus concentrations mea-sured by the ER and BDS ISO methods.

The method of APD of ER and BDS ISO 11263:2002 soil extractsand subsequent colorimetric detection of phosphorus can be usedfor determination of total extractable phosphorus. Its advantageover other digestion methods is simplicity of realization and substan-tially lower labor and time consumption.

Taking into account ICP OES results for determination of extract-able phosphorus and our results for ICP-MS P in soil extracts we can

122 K. Ivanov et al. / Spectrochimica Acta Part B 71-72 (2012) 117–122

conclude that the ICP techniques cannot be directly correlated withcolorimetric analysis and need to be developed as a new soil test.

Acknowledgement

This work is supported by Bulgarian Ministry of Education ProjectDO‐02‐87/08 and NSFB project GAMA DO 02–70 11/12/2008.

Appendix A. Supplementary data

The reader can access the data online with the article. Supplemen-tary data to this article can be found online at http://dx.doi.org/10.1016/j.sab.2012.05.013.

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