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 (r 2 of 0.90, p , 0.05). When coupled with PHW, K measured using sodium cobaltinitrite was predictably related with ammonium acetate-AA measured K (r 2 of 0.67, p , 0.05).

Simplified Soil Analysis Procedure for Use in Small‐Scale Agriculture

“…This is expected, because mass action extraction can readily occur with ammonium acetate but not with PHW. The high correlation between AA and cobaltinitrite observed in this study was confirmed by Crane et al (2005) on 38 arid-zone soils.…”

“…This correlation is poorer than that reported by Crane et al (2005) (r 2 of 0.67; p , 0.0001) using the same procedures for 38 arid-zone soils from the western United States. Hanks, Webb, and Jolley (1997), who used sodium bicarbonate as the standard extractant, also observed better correlations (r 2 of 0.72), but their soils had naturally high K concentrations, whereas the Moroccan and Guatemalan samples included soils with low K levels.…”

“…The results presented are limited to NO 3 -N and P because the procedure to quantify K originally proposed and taught to technicians in the two countries (Crane et al 2005;Sunderman and Sunderman 1958) was abandoned because it was too insensitive at low K levels and was too variable (data not shown). Consequently, the cobaltinitrite procedure for quantifying K used in the comparisons was developed and tested independent of the laboratories in Morocco and Guatemala.…”

“…Consequently, the cobaltinitrite procedure for quantifying K used in the comparisons was developed and tested independent of the laboratories in Morocco and Guatemala. Additional data support use of the cobaltinitrite procedure to quantify K (Crane et al 2005).…”

“…Phosphorus was determined with the molybdic acid colorimetric procedure outlined by Wantanabe and Olsen (1965). The K analysis used a sodium cobaltinitrite turbidimetric procedure (Table 2) (Crane et al 2005). For the PHW -cobaltinitrite potassium procedure, a subset of 50 soils, 25 from Guatemala and 25 from Morocco, was used because of insufficient soil.…”