This study aimed to develop an ICP-AES method to determine phosphorus in fertilizers as an alternative to official AOAC methods. The method was tested on 15 Magruder check fertilizers. Results showed that wavelength 213.618 nm was best for phosphorus determination and that acid and matrix effects were negligible. The ICP-AES method provided comparable accuracy and precision to official methods.

1. YANG ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 85, NO. 6, 2002 1241
AGRICULTURAL MATERIALS
Determination of Phosphorus in Fertilizers by Inductively
Coupled Plasma Atomic Emission Spectrometry
WEI MIN YANG, RHONDA L. BOLES, and THOMAS P. MAWHINNEY1
University of Missouri-Columbia, Experiment Station Chemical Laboratories, Rm 4, Agriculture Bldg, Columbia, MO 65211
An inductively coupled plasma atomic emission tion curve, the weight of the fertilizer samples is adjusted to
spectrometry (ICP-AES) method was developed for control final sample phosphate concentrations.
the determination of phosphorus in fertilizers. To- A major advantage of inductively coupled plasma atomic
tal phosphorus, direct extraction available phos- emission spectrometry (ICP-AES) is its ability to determine
phorus (EDTA), and water-soluble phosphorus, re- several elements simultaneously in a wide range of concentra-
ported as phosphorus pentoxide (P2O5), in tions. For determination of phosphorus, the sensitivity of the
15 Magruder check fertilizers were measured by ICP-AES analysis method exceeds that of the AOAC official
ICP-AES, and the results were compared with methods; in addition, the ICP-AES has a calibration curve of a
those obtained by the AOAC official method. very wide linear range. As a result, no sample weighing adjust-
Five analytical wavelengths of phosphorus, ment is required for the ICP-AES method. Because of these fea-
177.499, 178.287, 213.618, 214.914, and 253.565 nm, tures, the determination of phosphorus by the ICP-AES method
were tested for the determination of phosphorus in has attracted the attention of several scientists.
fertilizers, and their detection limits were obtained. Hamalova et al. (5) determined phosphorus, potassium,
Acid effects of perchloric acid and possible matrix and magnesium in 12 fertilizers by ICP-AES. For phosphorus,
effects of aluminum, calcium, magnesium, potas- the results of ICP-AES were compared with those obtained by
sium, and sodium were negligible for phosphorus the standard gravimetry method. Generally, precision and ac-
determination. Wavelength 213.618 nm was the curacy of the ICP-AES method were satisfactory. Ardis and
best analytical wavelength for phosphorus deter- Baker (6) used an ICP spectrometer to monitor fertilizer plant
mination by all 3 sample preparation methods for effluents for phosphorus, sulfur, and metals. The detection
the selected Magruder fertilizers. The results dem- limit for phosphorus was 0.08 µg/mL at 213.618 nm.
onstrated that the accuracy and precision of the Jones (7, 8) determined the concentrations of major, micro,
ICP-AES method were comparable with those of and trace elements by ICP-AES in fertilizer standards. It was
the official methods. determined that analysis by ICP-AES of fertilizer materials
with wide ranges of elemental contents is a rapid technique
that can yield results comparable with those obtained by
n fertilizers, phosphorus is one of the major nutrients that AOAC protocols.
I is most commonly determined. Accurate analysis of fertil-
izers is necessary both for the quality control of their pro-
duction and for determining their effective use in the field.
This study was undertaken to standardize the ICP-AES
method for determination of phosphorus in several sample
preparation methods in the Magruder fertilizer check samples
Presently, the gravimetric determination with quimociac re- by the use of the same calibration curve.
agent and the spectrophotometric molybdovanadophosphate
METHOD
procedures are the AOAC official reference methods for de-
termination of phosphorus in fertilizers (1–3). The gravimetric Apparatus
procedure is both tedious and time-consuming. The spectro-
photometric molybdovandate phosphate method, on the other (a) Spectrometer.—Atomic emission measurements of
hand, measures only orthophosphate (4). Because of this, fer- phosphorus were made with an ARL 3410+ sequential ICP
tilizer samples that possess phosphate in other forms must be spectrometer (Fisons, Dearborn, MI) with a minitorch.
converted to orthophosphate before determination. Further- (b) Nebulizer and pump.—The sample solutions were
more, to control the citrate interference which is used for di- nebulized by a pneumatic nebulizer (Precision Glassblowing,
rect available extraction, appropriate matrix match is neces- Englewood, CO) with a Gilson peristaltic pump
sary for the calibration standards. Finally, to ensure that the (Worthington, OH). The main operating conditions are listed
measured signals fall within the working range of the calibra- in Table 1.
Reagents
Received October 25, 2001. Accepted by EB June 5, 2002.
1
Author to whom correspondence should be addressed; e-mail: (a) Standard solution of phosphorus(1000 mg/L).—Pre-
mawhinneyT@missouri.edu. pared from dried (2 h at 110°C) KH2PO4 (NIST Standard Ref-

2. 1242 YANG ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 85, NO. 6, 2002
Table 1. ICP-AES instrumental parameters and previously been folded, rinsed, and dried on a
operating conditions polymethylpentene filter. Insert each filter into a numbered
Instrumentation Operating condition 500 mL volumetric flask for collection of filtrate. Wash ac-
tively until ca 250 mL filtrate has been collected. Bring filtrate
to final volume and mix. The phosphorus concentration in the
Instrument ARL 3410+ sequential ICP spectrometer
dissolved samples of 9703 and 9504 are ca 1.0 and 4.0 ppm,
RF generator frequency 27.12 MHz
respectively, and run directly by ICP-AES without dilution.
Plasma torch Minitorch Dilute remaining 13 Magruder dissolved fertilizers samples
Pump Gilson peristaltic pump 10-fold before ICP-AES analysis. The phosphorus concentra-
Nebulizer Meinhard pneumatic nebulizer (Type K) tions in these diluted solutions range from 5.5 to 41.0 ppm.
Forward power 650 W Acid and Matrix Effects
Reflected power <3 W
Phosphorus solutions (5 mg/L) with different concentra-
Viewing height 9.0 mm above load coil
tions of perchloric acid were prepared for acid effects study.
Coolant gas flow rate 10.5 L/min
The final concentration of perchloric acid varied from 0.0 and
Auxiliary gas flow rate 0.8 L/min 0.0001 mol/L to as high as 2.0 mol/L. Phosphorus solutions
Carrier gas flow rate 0.8 L/min (5 mg/L), mixed with different concentrations of possible ma-
Integration time 1s trix elements (Al, Ca, Mg, K, Na), were used for the matrix ef-
Sample uptake rate 2.5 mL/min fects investigations. The individual concentrations of these
5 matrix elements were 50, 100, 150, and 200 mg/L. All the
ICP-AES measurements for acid effects and matrix effects
were triplicate determinations, and relative standard devia-
tions (RSD) were within ± 3.0%.
Results and Discussion
erence Material), and 20 mL perchloric acid was added before
diluting to volume. Acid Effects
(b) Calibration solutions.—Prepared to contain 0.0, 1.0, 5.0,
10.0, 50.0, and 100.0 mg P/L in 4% HClO4. The concentrations After sample preparation as described in the Method sec-
of phosphorus in the samples were calculated from a linear re- tion, the solution of total P2O5 is acidic, containing about
gression equation of the calibration curve. Required correlation 0.4 mol/L HClO4, but the solutions of direct available P2O5
coefficient of this equation was set to not less than 0.9990. and water-soluble P2O5 are both neutral. As a result, it was
necessary to investigate the acid effects of HClO4 on the emis-
Sample Preparation sion signal of phosphorus.
(a) Total P2O5.—Weigh triplicate 0.50 g subsamples into As shown in Table 2 , the emission intensities ratio for all
250 mL boiling flasks. Add 15 mL HNO3 and 10 mL HClO4. 5 phosphorus analytical wavelengths varied from 94 to 105%,
Boil gently on medium-heat hot plate for ca 1 h until solution is while being measured in the presence of 0.0001–2.0 mol/L
colorless, or nearly so, and dense white fumes of HClO4 appear HClO4. Because the dissolved samples were diluted 10-fold
in the flask. After digestion, cool to room temperature, add ca before determination, 4% (v/v) HClO4 was used as the diluent
50 mL deionized water, and boil gently for ca 15 min. Remove in all cases. As a result, the negligible influence of acid effects
sample from hot plate, cool to room temperature, dilute to vol- on phosphorus determination was further minimized in all
ume, stir, and stopper. Dilute dissolved samples 10-fold before ICP-AES assays.
ICP-AES determination. The phosphorus concentrations in the Matrix Effect
diluted solutions range from 2.5 to 45.0 ppm.
(b) Direct extraction available P2O5 (EDTA).—Weigh Five elements, K, Na, Mg, Al, Ca, were selected as matrix
triplicate 0.50 g subsamples into 250 mL boiling flasks. Add elements in this investigation. Table 3 shows that the phospho-
100 mL ammonium citrate–EDTA solution that has been rus emission intensity ratio at various wavelengths was within
preheated to 65°C. Place flask in 65°C shaker bath. Close the range of 99–104%, clearly demonstrating that matrix inter-
flask tightly with rubber stopper. Vigorously shake flask for ference was minimal for analysis of phosphorus. Notably, con-
1 h at 65°C. Shaking should be vigorous enough to ensure centrations of those elements in diluted dissolved samples were
wetting of sample and flask walls. Remove samples from well below the matrix concentrations presented in Table 3.
bath, and immediately cool to room temperature with 4°C dis- Therefore, matrix effects can be considered negligible from
tilled water. Dilute to volume, and mix thoroughly. Dilute dis- these elements when phosphorus in fertilizers is determined.
solved samples 10-fold before ICP-AES determination. The
Detection Limit
phosphorus concentrations in the diluted solutions range from
2.0 to 45.0 ppm. A blank solution (8%, v/v, HClO4) and a solution of
(c) Water soluble P2O5.—Weigh triplicate 1.00 g P (5 mg/L) were used to measure the detection limit. Detection
subsamples on 11 cm Whatman No.4 filter circles that have limit (3σ) of the wavelengths 177.499, 178.287, 213.618,

3. YANG ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 85, NO. 6, 2002 1243
Table 2. Acid effect of HClO4 on P (5 mg/L) at 5 wavelengths
Phosphorus intensity ratio at different wavelengths (nm), %a
HClO4 concn, mol/L P177.500 P178.287 P213.618 P214.914 P253.565
0.0 100 100 100 100 100
0.0001 103 ± 1.1 101 ± 1.8 103 ± 2.1 102 ± 1.1 100 ± 0.9
0.001 102 ± 0.3 105 ± 0.4 105 ± 0.6 106 ± 0.4 105 ± 0.6
0.01 100 ± 0.5 105 ± 1.0 104 ± 1.3 106 ± 0.8 102 ± 2.3
0.05 96 ± 1.7 102 ± 1.9 101 ± 2.3 100 ± 0.2 98 ± 1.1
0.15 100 ± 1.0 102 ± 1.3 100 ± 0.2 98 ± 2.1 98 ± 0.6
0.25 94 ± 1.2 99 ± 0.1 101 ± 0.9 100 ± 0.7 99 ± 0.4
0.5 96 ± 2.2 98 ± 1.6 99 ± 2.0 98 ± 2.0 97 ± 1.3
1.0 95 ± 1.7 100 ± 2.1 100 ± 1.8 99 ± 1.5 98 ± 1.3
2.0 95 ± 0.8 100 ± 1.9 98 ± 0.7 98 ± 1.1 99 ± 0.8
a
Results are expressed as mean ± standard deviation for triplicate determinations.
Table 3. Matrix effects of Mg, Ca, K, Al, and Na on P (5 mg/L) emission signals
Phosphorus intensity ratio at different wavelengths (nm), %a
Element Matrix concn, mg/L P177.500 P178.287 P213.618 P214.914 P253.565
— 100 100 100 100 100
Mg 50 102 ± 1.1 102 ± 0.3 101 ± 1.6 101 ± 1.1 103 ± 1.8
100 100 ± 2.1 102 ± 2.0 100 ± 0.2 102 ± 1.3 102 ± 1.2
150 102 ± 0.9 102 ± 1.5 102 ± 0.5 102 ± 1.2 104 ± 1.5
200 102 ± 0.6 103 ± 1.1 102 ± 0.8 103 ± 1.9 104 ± 1.0
Ca 50 101 ± 0.7 103 ± 0.6 103 ± 0.1 102 ± 0.4 100 ± 0.5
100 102 ± 1.4 103 ± 0.4 102 ± 1.1 102 ± 1.1 103 ± 0.9
150 101 ± 2.1 102 ± 2.1 103 ± 1.4 103 ± 0.6 102 ± 1.6
200 103 ± 2.0 102 ± 1.7 101 ± 0.6 104 ± 0.4 103 ± 1.1
K 50 101 ± 0.6 100 ± 1.1 102 ± 0.8 101 ± 2.1 100 ± 2.0
100 100 ± 0.6 102 ± 1.5 101 ± 0.2 101 ± 0.7 101 ± 1.8
150 100 ± 0.9 101 ± 0.7 99 ± 1.5 100 ± 0.1 101 ± 1.7
200 100 ± 0.3 100 ± 0.3 100 ± 1.1 102 ± 0.5 99 ± 1.6
Al 50 102 ± 0.8 101 ± 1.3 102 ± 1.2 103 ± 0.9 100 ± 1.2
100 101 ± 2.1 102 ± 1.3 103 ± 0.5 102 ± 0.3 103 ± 0.2
150 100 ± 1.7 102 ± 1.8 103 ± 0.5 104 ± 1.8 104 ± 0.7
200 100 ± 1.4 103 ± 0.9 104 ± 0.9 103 ± 1.1 103 ± 1.1
Na 50 102 ± 2.0 102 ± 0.5 101 ± 1.3 103 ± 0.3 102 ± 2.0
100 102 ± 2.2 102 ± 1.6 100 ± 1.1 102 ± 1.1 101 ± 2.1
150 102 ± 1.5 103 ± 1.1 102 ± 0.8 102 ± 0.8 104 ± 1.1
200 101 ± 1.1 104 ± 1.4 103 ± 0.2 102 ± 0.2 103 ± 1.5
a
Results are expressed as mean ± standard deviation for triplicate determinations.

4. 1244 YANG ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 85, NO. 6, 2002
Table 4. Linear regression analysis of ICP phosphorus results with average values of Magruder fertilizers
Total P2O5 Direct available P2O5 Water-soluble P2O5
Wavelength, nm Regression line (a Regression line (a Regression line (a
177.499 y = 1.007x – 0.2529 0.9998 y = 1.010x – 0.5845 0.9994 y = 0.8678x + 0.5807 0.9022
178.287 y = 1.013x – 0.2403 0.9997 y = 1.004x + 0.0604 0.9997 y = 0.8797x + 0.3319 0.9082
213.618 y = 1.001x – 0.1247 0.9999 y = 1.002x – 0.0864 0.9999 y = 0.8872x + 0.9076 0.9084
214.914 y = 1.010x – 0.1571 0.9998 y = 1.008x – 0.3590 0.9998 y = 0.8843x + 0.6544 0.9150
253.565 y = 0.9913x – 0.0829 0.9948 y = 0.9875x – 0.5438 0.9984 y = 0.9849x + 0.3170 0.8987
a
Linear regression correlation coefficients.
214.914, and 253.565 nm, which were calculated from the (mono-ammonium phosphate 9803B). The NPK values in
standard deviations of 11 measurements of the blank solution, Table 5 represent the percentages of nitrogen, available phos-
were 40, 42, 23, 55, and 88 µg/L, respectively. phorus (P2O5), and soluble potassium (K2O). Except for
9811A, which is clear liquid fertilizer, the other 14 Magruder
Magruder Fertilizer Analysis by ICP-AES
samples are solid. Triple superphosphate sample
In 1922, E.W. Magruder, chief chemist of the F.S. Royster 0998A (0-46-0) contains only one nutrient: phosphorus; the
Guano Co. (Norfolk, VA) began the fertilizer check sample remaining 14 Magruder samples contain 2 or 3 nutrients: ni-
program that bears his name today. Magruder fertilizer check trogen, phosphorus, potassium. With the 213.618 nm wave-
sample programs are widely used to ensure consistency length used to determine P2O5, the ICP-AES recoveries were
among laboratories and methods. The results can identify sta- compared with the Magruder average standard values (Ta-
tistically different repeatabilities or reproducibilities among ble 5). The recovery was defined as the ratio of the ICP results
different methods or among laboratories using the same to the Magruder average.
method. In this study, 15 Magruder fertilizer check samples The values of total P2O5 and direct available P2O5 mea-
were selected to study the performance of ICP-AES for the de- sured by ICP-AES compared closely with the reported
termination of phosphorus. Magruder average. Two notable exceptions were observed,
Generally, total P2O5, direct extraction available P2O5 both for water-soluble P2O5 analyses, where significant dis-
(EDTA), modified extraction available P2O5 (non-triple crepancies between the values of ICP results and Magruder
superphosphate), and water-soluble P2O5 are often deter- average were determined. Specifically, these anomalies in-
mined in laboratories. Because the matrixes of direct extrac- volved Magruder samples 9504 and 9811A. The remarkably
tion available P2O5 (EDTA) and modified extraction available large standard deviation of the Magruder average on the solid
P2O5 are similar, this investigation used the sample prepara- fertilizer sample 9504 makes the low recovery understand-
tion procedure of direct extraction available P2O5 (EDTA). able. This is in contrast to the high recovery (329.4%) of phos-
Table 4 summarizes the linear regression analysis of the phorus by ICP-AES for the liquid Magruder fertilizer sam-
ICP results with the Magruder average at various wave- ple 9811A, which is totally water-soluble. This is reasonably
lengths. A main focus of this study was to determine which explained by the fact that all phosphorus in the solution is
wavelength for phosphorus was suitable. For total P2O5 and determined by the ICP spectrometer, whether it is present in its
direct available P2O5, the overall agreement for phosphorus
ortho-, poly-, organic-, or low-oxide forms, and the Magruder
was excellent over the wide concentration range found within
average presented in Table 5 reports only the orthophosphate
the 15 Magruder fertilizers. The intercept was small and not
concentration in the sample. The determined total P2O5 of
significantly different from zero, and the slope was very close
33.47% in this sample by ICP-AES is further substantiated by
to 1.0. This demonstrated that the ICP-AES method possesses
neither constant nor proportional systematic errors compared the fact that sample 9811A’s content as determined by total
with the Magruder average. The best linearity was achieved at P2O5 and direct available P2O5 are in agreement. If sam-
213.618 nm. In addition, 213.618 nm is the most sensitive ple 9811A is excluded from the linear regression (Table 4), the
wavelength for phosphorus. Therefore, among the 5 analytical regression equation of water-soluble P2O5 will be y = 0.9733x +
wavelengths tested, wavelength 213.618 nm was the optimal 0.7855 at 213.618 nm, with a correlation coefficient of 0.9983.
analytical wavelength for determining phosphorus in fertiliz- Analysis of total phosphorus by ICP-AES is simpler and
ers by ICP-AES. faster than the traditional procedures using digestion and spec-
To represent the assorted commercial fertilizer used in the trophotometric determination. Significant advantages of the
agriculture, the contents of the total phosphorus (P2O5) in the ICP-AES method are also demonstrated by its wide dynamic
selected Magruder fertilizer samples varies from several per- range of calibration and high sensitivity. Furthermore, by the
cent, as in organic fertilizers 9703 and 9504, to as high as 52% ICP-AES method, the same amount of fertilizer sample can be

5. Table 5. ICP results of P2O5 recovery on 213.618 nm compared with Magruder average values
Total P2O5, % Direct available P2O5, % Water-soluble P2O5, %
Sample No. NPK Magruder value ICP resultsa Recovery, % Magruder value ICP results Recovery, % Magruder value ICP results Recovery, %
9703 4-2-0 2.85 ± 0.30 2.96 ± 0.03 103.9 2.52 ± 0.21 2.61 ± 0.02 103.4 0.12 ± 0.05 0.12 ± 0.00 100.0
9504 6-2-0 4.37 ± 0.44 4.80 ± 0.02 109.8 3.31 ± 0.45 3.47 ± 0.03 104.8 2.71 ± 2.48 0.47 ± 0.01 17.3
b
9401 10-10-10 10.26 ± 0.09 10.10 ± 0.08 98.4 10.55 ± 0.01 10.26 ± 0.04 97.3 — — —
9808 10-10-10 9.92 ± 0.13 10.05 ± 0.03 101.3 9.55 ± 0.32 9.97 ± 0.11 104.4 6.56 ± 0.20 6.78 ± 0.03 103.4
9309 13-13-13 13.39 ± 0.17 13.54 ± 0.23 101.1 13.38 ± 0.16 13.40 ± 0.14 100.1 — — —
9506 19-19-19 19.68 ± 0.32 19.82 ± 0.31 100.7 19.74 ± 0.25 19.71 ± 0.37 99.8 18.20 ± 0.25 18.34 ± 0.08 100.8
9409 4-22-6 22.24 ± 0.25 21.87 ± 0.17 98.3 22.06 ± 0.19 22.18 ± 0.19 100.5 17.31 ± 0.34 16.55 ± 0.07 95.6
9402 7-23-22 22.85 ± 0.16 22.98 ± 0.18 100.6 22.54 ± 0.43 22.57 ± 0.31 100.1 17.80 ± 0.27 17.75 ± 0.30 99.7
9806 8-24-24 20.80 ± 0.19 20.83 ± 0.20 100.1 20.77 ± 0.20 20.74 ± 0.23 99.9 19.79 ± 0.33 19.89 ± 0.16 100.5
9606B 0-32-31 36.40 ± 0.49 36.85 ± 0.20 101.2 36.41 ± 0.51 36.52 ± 0.17 100.3 12.42 ± 1.57 11.69 ± 0.37 94.1
9811A 10-34-0 34.02 ± 0.59 34.02 ± 0.17 100 33.94 ± 0.31 33.98 ± 0.14 100.1 10.16 ± 4.21 33.47 ± 0.29 329.4
YANG ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 85, NO. 6, 2002 1245
896B 41.75 ± 0.64 42.21 ± 0.37 101.1 — — — — — —
9810A 18-46-0 6.72 ± 0.59 46.88 ± 0.17 99.9 46.56 ± 0.36 46.54 ± 0.17 100 43.52 ± 1.07 43.95 ± 0.28 101.0
0998A 0-46-0 46.31 ± 0.23 46.29 ± 0.36 100.0 46.18 ± 0.18 45.78 ± 0.20 99.1 42.74 ± 0.29 41.23 ± 0.27 96.5
9803B 11-52-0 51.06 ± 0.55 51.02 ± 0.24 99.9 50.85 ± 0.33 51.14 ± 0.39 100.5 44.62 ± 0.94 46.33 ± 0.39 103.8
a
ICP results are expressed as the mean ± standard deviation for triplicate determinations.
b
— = No Magruder value provided with check sample.

6. 1246 YANG ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 85, NO. 6, 2002
weighed regardless of the phosphorus concentration, and no general, the precision and accuracy of ICP-AES determina-
spike experiment is required for low phosphorus content. tions were close to the average values of Magruder standard
The precision and accuracy of the methodology developed check fertilizer samples. Care must be taken in the determina-
were tested by analyzing total P2O5 and direct available P2O5. tion of total P2O5 in fertilizer samples in which phosphorus
For short-term precision testing (7 consecutive determinations), may exist in several forms. By the ICP-AES method, all phos-
phate forms are analyzed, yielding a true total P2O5 value.
the RSD of total P2O5 and direct available P2O5 were in the range
of 0.6–2.2 and 0.8–2.6%, respectively. For long-term precision
References
testing (7 determinations over 7 consecutive days), the RSDs of
total P2O5 and direct available P2O5 were in the range of (1) Official Methods of Analysis (1990) 15th Ed., AOAC,
0.7–3.5 and 0.8–3.9%, respectively. The precision of total P2O5 Arlington, VA, sec. 962.02
and direct available P2O5 determination was also excellent. (2) Dahlgren, S.W. (1962) Z. Anal. Chem. 189, 243–256
(3) Official Methods of Analysis (1990) 15th Ed., AOAC,
Conclusions Arlington, VA, secs. 958.01, 963.03(a)
(4) Fujiwara, K., Mignardi, M.A., Petrucci, G., Smith, B.W., &
Winefordner, J.D. (1989) Spectrosc. Lett. 22, 1125–1140
The ICP-AES method enables the determination of total (5) Hamalova, M., Hodslavska, J., Janos, P., & Kanicky, V.J.
P2O5, direct extraction available P2O5 (EDTA), and wa- (1997) J. AOAC Int. 80, 1151–1155
ter-soluble P2O5 in fertilizers by the same calibration curve. (6) Ardis, J.D., & Baker, A.M. (1997) J. AOAC Int. 72, 857–859
Wavelength 213.618 nm proved to be the best analytical (7) Jones, J.B. (1982) J. Assoc. Off. Anal. Chem. 65, 781–785
wavelength for phosphorus determination in fertilizers. In (8) Jones, J.B. (1983) Spectrochim. Acta 38B, 271–276