Rapid colorimetric methods for analysis of pH, extractable aluminium and Colwell phosphorus in soils

Abstract

Context Analytical procedures and technologies for soil analyses can be prohibitively expensive for small laboratories and researchers in developing countries. There is a need for low cost and high-throughput methods for assaying pH, extractable aluminium and phosphorus when conducting field trials on acid soils. Aims We investigated methods to develop rapid yet inexpensive colorimetric assays for the assay of pH, extractable aluminium and Colwell phosphorus in soil extracts. Methods We developed a colorimetric method to measure soil pH enabling pH to be quantified in a high-throughput assay. Similarly, two existing methods for extractable aluminium and Colwell P were modified for high throughput assays also using microtiter plates. Key results All three methods yielded linear relationships when using absorbance to quantify the parameters with the high throughput methods. Furthermore, there was a strong correlation between pH values of the soil samples obtained with the colorimetric assay and pH values measured with a glass electrode. Conclusions We demonstrated that the rapid assays for all three methods can be implemented to characterise field sites through the mapping of distributions for extractable Al, Colwell P and pH. Implications The high-throughput methods described here will be useful for researchers who conduct field trials to map variations in soil pH, soluble Al and Colwell P. Although the focus of the current work was on acid soils, the colorimetric pH and Colwell P methods can also be applied to non-acid soils.