Interlaboratory and intralaboratory testing comparison of soil sulfate analysis in Mollisols of the Pampas.

RUSSI, D; F.H. GUTIERREZ BOEM; P. PRYSTUPA ; G. RUBIO

COMMUNICATIONS IN SOIL SCIENCE AND PLANT ANALYSIS

TAYLOR & FRANCIS INC

Lugar: Londres; Año: 2012 vol. 43 p. 2535 - 2543

0010-3624

Sulfur (S) deficiencies in grain and forage crops have been detected in many agri-cultural regions of the world, but soil tests are not commonly used as the basis for Sfertilizer recommendation programs. Errors of measurements of soil sulfate were deter-mined to assess whether the variation among and within soil-testing laboratories couldbe a factor that prevent the adoption of soil testing to assess soil sulfate availability.Subsamples of 10 selected soils (Mollisols) from the Pampas (Argentina) were sent intwo batches to five soil-testing laboratories. Laboratories were unaware of the existenceof subsamples and performed routine sulfate analysis as if these soils came from 60 dif-ferent fields. Soil sulfate ranged from 3.3 to 20.6 mg kg−1. One laboratory reportedsulfate values greater than the other ones, having a mean bias of 4.1 mg kg−1 S sul-fate (SO4). The other four laboratories reported similar sulfate values when soils hadlow sulfate availability (less than 10 mg S kg−1), even when they used different extrac-tants. Considering only these four laboratories, average interlaboratory coefficientsof variations ranged from 6 to 24% for the 10 soils. Within-laboratory mean coeffi-cients of variation (CVs) ranged from 12 to 22%. However, mean absolute errors ofall laboratories were less than 1.2 mg kg−1 S-SO4. Two laboratories reported differ-ent sulfate values for the two batches of shipment (an average difference of 4.7 and3.8 mg kg−1 of S-SO4). Laboratories using different extractants obtained similar results,suggesting that using the same extractant is not a prerequisite to standardize laboratoryresults in these soils. Differences between laboratories in our study were smaller thanin other interlaboratory comparisons for soil sulfate. These differences could be easilydetected and corrected if laboratories participate in an interlaboratory control system.The observed low mean absolute errors suggested that, in general, all laboratoriesachieve acceptable precision when evaluating within the same batch of determina-tions. Differences between batches of shipment (within laboratory error) stressed theimportance of using reference material for internal quality control.Keywords Soil fertility, sulfur, testing methodologies