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

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.