Validation of a modified Egner–Riehm–Domingo (AL) method for determination of plant-available phosphorus in soil

Determination of plant-available phosphorus in soil is fundamental to agrochemical soil testing,
fertilization planning, and high yields in all crops. The Egner–Riehm–Domingo (AL) method, extracting
soil with ammonium lactate–acetic acid solution, is widely used for this purpose. However, because a
modified version of the AL procedure was adopted for routine use rather than the original standardized
method, its analytical performance required formal validation before implementation. The objective of
this work was to validate the modified AL method for determining plant-available phosphorus measured
by UV-Vis spectrophotometry in soil samples, in order to confirm its suitability for routine laboratory
use. Validation followed a standard analytical-quality framework, evaluating limit of detection (LOD),
limit of quantification (LOQ), linear and working range, precision (repeatability and reproducibility),
recovery, accuracy, and measurement uncertainty (MU) using BIPEA reference soil material and certified
reference standards. Calibration curves were constructed from at least five KH₂ PO₄ standard solutions,
each measured in replicate, with correlation coefficients exceeding 0.995. LOD and LOQ were derived
from blank measurements combined with the calibration equation. Repeatability and reproducibility were
assessed from ten parallel measurements of control samples and periodic re-preparations, respectively.
Recovery was tested using certified reference standards and accuracy was verified through interlaboratory
proficiency testing (z-score). Measurement uncertainty was calculated at a 95% confidence level,
incorporating precision, bias, and instrument/reagent uncertainty components. The LOD and LOQ were
0.2 and 1.0 mg/100 g P₂ O₅ , respectively, with a linear/working range of 1.0–100 mg/100 g P₂ O₅
(extendable to 400 mg/100 g via tenfold dilution), repeatability and reproducibility RSD below 10%,
recovery within 90–110%, z-score ≤2.0, and a relative measurement uncertainty of ±2.74%. The method
satisfied all pre-defined acceptance criteria for accuracy, precision, working range, and measurement
uncertainty, indicating that the modified AL procedure, combined with UV-Vis spectrophotometry,
provides reliable and reproducible results across a practically relevant concentration range, with the
dilution step effectively extending applicability to higher-content soils without compromising linearity.
In conclusion, the modified AL method for plant-available phosphorus in soils was successfully validated
according to internal laboratory quality criteria and satisfies the requirements for accuracy, precision, and
measurement uncertainty.