A Rapid Turbidimetric Potassium Test Modified for Use with the Pressurized Hot‐Water Extraction

Abstract

Colorimetric or turbidimetric quantification of soil potassium (K) coupled with pressurized hot‐water (PHW) extraction could provide an inexpensive alternative to standard methods for small‐scale farmers in developing countries. Two of many methods for K analysis, one using sodium tetraphenylborate and the other using sodium cobaltinitrite, were modified for use with PHW extraction and evaluated for the following requirements: readable on the spectrophotometer, minimal equipment requirement, rapid, simple, and comparable in accuracy to proven methods of K analysis. The sodium tetraphenylborate method was unreliable at low K concentrations, did not relate with K extracted using ammonium acetate and analyzed by atomic absorption (AA), required extract filtration and was too expensive in developing countries. Sodium cobaltinitrite was both simple and inexpensive, and results were consistent and reliable. Test results from 38 arid soils from the western United States support the use of sodium cobaltinitrite as an acceptable procedure for K quantification compared to AA analysis (r² of 0.90, p<0.05). When coupled with PHW, K measured using sodium cobaltinitrite was predictably related with ammonium acetate‐AA measured K (r² of 0.67, p<0.05).

Keywords:

• International soil analysis
• potassium soil test
• pressurized hot water
• small‐scale agriculture
• sodium cobaltinitrate
• sodium tetraphenylborate