Trends in Soil Measurement Performance of Australasian Laboratories by Methods and Time

This paper describes Australasian soil-measurement performance using data from inter-laboratory soil proficiency programs of the Australasian Soil and Plant Analysis Council Inc. Rapid assessments focused only on grand median percent robust coefficients of variation (%CVs) from 2004-05 through to 2014-15 inclusive, where grand medians by method were calculated across 12 soil samples annually. The %CV data were subdivided into three groupings (2005-2008, 2009-2012, 2013-2015). For 19 soil tests, CVs declined from 12.8% to 10.6% to 8.8%, suggestive of small improvements in measurement quality with time. Detailed assessments used data from 2009-10 to 2014-15 and included tests regulated for use in “reef catchments” of North-east Australia. Relationships between median-concentrations and associated robust %CVs were initially assessed with power-functions, with each subsequently solved for realistic analyte levels. Predicted trends for each method for the six years were then plotted. From these, soil tests with most variation were Total P, Bray-1P and Acid P. The findings confirm improvements are needed before between-laboratories’ measurement uncertainties for the “reef-preferred” Acid P soil test approach those for Olsen-P, Colwell-P and Mehlich-3P. Also, measurement improvements across the six years for Mehlich-3 P exceeded those of the other empirical soil P tests. Measurements of Walkley-Black Organic C were disappointing in 2009-10 but improved to 2014-15. By 2014-15, soil P tests with lowest to highest predicted robust %CVs were Mehlich-3 , Olsen P, Colwell P, Acid P and Bray-1P, respectively. On this evidence, regulators should be more flexible when specifying preferred diagnostic soil P tests for use on sugarcane farms in “reef catchments”.

Keywords: Diagnostic soil tests; Reef regulations; Mehlich-3; Measure quality; Trend analysis