Simplified Soil Analysis Procedure for Use in Small‐Scale Agriculture

Abstract

Soil analysis for small-scale farms in developing countries is often inconvenient and prohibitively expensive using currently available procedures, yet the information gained from these soil tests could result in economical and environmental benefits. The pressurized hot water (PHW) extraction coupled with colorimetric or turbidimetric analysis shows promise as a viable alternative based on tests done on a limited range of soils. Before this extraction and analysis can be used in developing countries, testing is needed across the range of soils found in these countries. At Brigham Young University (BYU), 228 soils from different areas of Guatemala and Morocco were analyzed for NO 3 -N, phosphorus (P), and potassium (K) using standard methods (water -CTA, Olsen -molybdic acid, and ammonium acetateatomic absorption, respectively). Results were correlated to values obtained from the PHW extraction coupled with colorimetric or turbidimetric analytical procedures. The relationships between the values for these tests were good (r 2 of 0.96, 0.71, and 0.52 for NO 3 -N, P, and K, respectively). Soils from each country were concurrently analyzed for NO 3 -N and P in laboratories in Guatemala and Morocco, and these results were correlated with those from BYU. Positive correlations between BYU values and those from other laboratories were obtained, with the data from the 993 Guatemalan laboratory showing overall closer correlation than the Moroccan laboratory. In an additional study comparing several P extraction methods for Guatemalan soils, relationships between PHW-extracted P and Olsen-, Bray I-, and Mehlich I-extracted P and measured at BYU (r 2 of 0.75, 0.67, and 0.46, respectively) indicate that PHW is a promising alternative P extraction for use even with the highly variable soils of Guatemala. Overall, the data support PHW extraction and accompanying analyses as a less expensive alternative to current soil nutrient extraction and analysis procedures for the soils of Morocco and Guatemala.

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

“…Any variation of color in PHW extracts did not interfere with spectrophotometer readings so filtration was unnecessary, and precipitate formation was stable and sensitive at K levels important to agriculture. The reported data support implementing the sodium cobaltinitrite K analysis procedure with PHW extraction on the arid soils of the western United States and data reported elsewhere (Crane et al, 2006) indicate applicability to acid, neutral, and calcareous soils.…”

“…Preliminary soil tests on 38 alkaline soils comparing K extracted using ammonium acetate with K extracted with PHW and measured with tetraphenylborate were inconsistent and inaccurate (data not shown). Finally, in the two developing countries where the procedure was evaluated, sodium tetraphenylborate was either unavailable or extremely expensive (Crane et al, 2006). Thus, development of the sodium tetraphenylborate procedure was discontinued due to lack of sensitivity at low K concentrations common in agricultural soils, variable precipitate formation, time-consuming charcoal filtration and the expense and unavailability of sodium tetraphenylborate.…”

“…Olson's procedure was chosen because it was simple, rapid, and met the requirement for simple equipment needs (Crane et al, 2006).…”

Pressurized Hot Water Procedures for Use on Small Farms in Developing Countries

“…Phosphorus was determined with the molybdic acid colorimetric procedure outlined by Wantanabe and Olsen (1965). The K analysis used a sodium cobaltinitrite turbidimetric procedure (Crane et al 2005(Crane et al , 2006. For the PHW -cobaltinitrite potassium procedure, a subset of 50 soils, 25 from Guatemala and 25 from Morocco, was used because of insufficient soil.…”

“…Soils were dried and ground in preparation for analysis in their country of origin. Soils from Morocco and Guatemala were extracted and analyzed for NO 3 -N, P, and K using accepted standard methods (Crane et al 2006) as well as using the PHW extraction. The PHW method consisted of a PHW extraction coupled with colorimetric/turbidimetric elemental analysis.…”