Comparison of Three Digestion Methods for Total Soil Potassium Estimation in Soils of Papua New Guinea Derived from Varying Parent Materials

Abstract

An estimate of total potassium (K) contents of soils is required to assess the difficultly available fractions of soil potassium. Three digestion procedures were evaluated in terms of recovery of elemental K, precision, and simplicity in combination with analysis of K contents by inductively coupled plasma–optical emission spectroscopy (ICP-OES) in soils derived from contrasting parent materials. A hydrofluoric acid–perchloric acid (HClO₄)–nitric acid (HNO₃) mixture extracted the most K from 10 soils studied (4809.9 mg kg⁻¹) followed by aqua regia (3180.89 mg kg⁻¹) and HClO₄–HNO₃ mixtures (1206.75 mg kg⁻¹). On average, aqua regia and HClO₄–HNO₃ digestion methods extracted 66.1% and 25.1% of the K extracted with the HF–HClO₄–HNO₃ mixture, respectively. Precision analysis as percentage of relative standard deviation showed excellent precision (<5%) for the HF–HClO₄–HNO₃ method (1.49%) compared to the HClO₄–HNO₃ mixture (10.72%). The present study concludes that the HF–HClO₄–HNO₃ digestion mixture is superior over both aqua regia and HClO₄–HNO₃ mixtures for total K estimation in the soils of Central Highland provinces of Papua New Guinea especially when ICP-OES is employed for analysis.

Keywords:

• Aqua regia
• digestion methods
• parent materials
• precision
• silicate minerals
• total potassium content

Acknowledgments

Our gratitude goes to the Australian Council for International Agricultural Research (ACIAR) for the scholarship provided to Robin Walter. We also thankfully acknowledge ACIAR Project SMCN/2005/043, under which the soil samples used for this study were collected.