P₂O₅ and K₂O in Fertilizers

Application Note EDXRF1858

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The measurement of phosphorus (P₂O₅) and potassium (K₂O) by empirical calibration is demonstrated. The Fundamental Parameters (FP) is also demonstrated to include the screening of minor elements when no or only a few reference standards are available for calibration using Rigaku Matching Library.


The primary nutrients in fertilizers are nitrogen, phosphorus, and potassium (NPK). While XRF does not measure the nitrogen, it is an ideal tool to measure and control the proper levels of phosphorus (as P₂O₅) and potassium (as K₂O) in the production of fertilizers. Using a one-time calibration, the measurement is direct and simple, allowing for high sample throughput and batch analysis. The Rigaku NEX QC Series are ideal systems for at-line quality control checks, in the QC lab and for researchers alike. Use standalone models with touchscreen interface or add the PC-based QuantEZ software for more detailed analysis using Fundamental Parameters.

NEX QC+ QuantEZ workstationModel: NEX QC+ QuantEZ

Empirical calibrations

A small suite of 7 standards with known values from a referee technique (ICP) was used for calibration. The best-fit regression empirical approach is the most accurate for a given formulation to ensure proper control and balance of the PK components.

Empirical calibrations — P₂O₅

Element: P
Units: %
Sample I.D. Standard value Calculated value
1 7.93 8.06
2 8.96 8.86
3 9.95 9.94
4 11.85 11.71
5 13.92 13.97
6 15.82 16.07
7 17.86 17.69

EDXRF1858 Correlation plot P2O5 in fertilizersCorrelation plot P₂O₅ in fertilizers

Empirical calibrations — K₂O

Element: K
Units: %
Sample I.D. Standard value Calculated value
1 8.23 8.27
2 9.32 9.24
3 10.26 10.35
4 12.31 12.20
5 14.42 14.43
6 16.34 16.45
7 18.55 18.49

EDXRF1858 Correlation plot K2O in fertilizersCorrelation plot K₂O in fertilizers

Repeatability (precision)

To demonstrate effective recovery and repeatability (precision), representative low and high concentration calibration standard samples were analyzed as unknowns. 10 repeat analyses are taken of each sample with results summarized below.

Sample ID: 1       Units: Mass%
Component Standard value QuantEZ value Std. deviation RSD (%)
P₂O₅ 7.93 8.10 0.0050 0.06
K₂O 8.23 8.27 0.0397 0.48

Sample ID: 6       Units: Mass%
Component Standard value QuantEZ value Std. deviation RSD (%)
P₂O₅ 15.82 15.98 0.0344 0.21
K₂O 16.34 16.56 0.0644 0.39

Fundamental parameters (FP)

This demonstration shows the measurement of the PK components and the trace elements S, Zn, and Rb using FP. A standardless semi-quant FP method was developed using the QuantEZ method template for Water. The semi-quant method is excellent for screening and comparative investigation without the need for any assayed reference standards. The FP method was further optimized through the simple creation of Rigaku Matching Library using 3 samples that had been assayed by ICP for P₂O₅, K₂O, S, Zn, and Rb. A Matching Library is easily created by the user and is used in conjunction with the standard FP library in the modeling of a sample matrix and calculation of concentration results. This allows for a quantitative measurement of elemental concentrations without the need for a large suite of known assayed calibration standards.

Two unknown samples were then measured using Rigaku NEX QC+ QuantEZ and ICP, the results are shown here.

Sample ID: Unknown A
Units: P, K (%); S, Zn, Rb (ppm)
Component QuantEZ value ICP value Stat. error
P₂O₅ 18.71 18.60 0.02
K₂O 6.30 6.22 0.03
S 12728 13008 22
Zn 641 680 2
Rb 9.4 0.1

Sample ID: Unknown B
Units: P, K (%); S, Zn, Rb (ppm)
Component QuantEZ value ICP value Stat. error
P₂O₅ 19.36 19.15 0.02
K₂O 2.91 3.14 0.02
S ND* 6
Zn 87.2 0.8
Rb 6.6 0.1


The results shown in these demonstrations show the versatility of the NEX QC Series product line. From stand-alone touchscreen systems for at-line quality checks to the addition of QuantEZ PC-based software, the EDXRF systems from Applied Rigaku Technologies are ideal tools for the production control, screening, and comparative research work in the manufacturing of fertilizers.

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