Original Articles

The direct determination of phosphorus in citric acid soil extracts by colorimetry and direct-current plasma emission spectroscopy

Published in: South African Journal of Plant and Soil
Volume 12, issue 4, 1995 , pages: 152–157
DOI: 10.1080/02571862.1995.10634356
Author(s): G.R. ThompsonDepartment of Agriculture, Republic of South Africa


The effect of citrate interference on the direct colorimetric determination of P in 1% citric acid soil extracts of a wide range of soils was investigated and compared to that of removing the citric acid by dry ashing. It was found that dilution of the soil extracts to ratios greater than 1:20 caused minimal interference with colour development using the molybdenum blue method. Dilutions of 1:10 and 1:5 caused a 10% and 55% reduction in absorbance values respectively. Despite this interference, the measurement of P in the presence of 1% citric acid is possible if the same concentration of this acid, present in the soil extract, is present in the calibration standards. Generally a 1:10 dilution of soil extracts provides sufficient sensitivity for the determination of P in soils with a P status < 5 mg kg−1 and can accommodate soils with a P status up to 100 mg kg−1. The effect of dry ashing the soil extracts and measuring the P colorimetrically resulted in slightly higher P values, in the order of 2 mg kg−1. Similar results were obtained when the soil extracts were aspirated into a direct-current plasma and the P was measured by emission spectrometry. It was concluded that quantities of P could be present in the soil extracts, either organically or inorganically bound, and were responsible for this difference. These forms of P were not determined by colorimetric procedures but were released by a high-temperature plasma or by dry ashing. For most practical purposes these low values can be ignored.

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