Soil Analysis Techniques – The Need to Combine Precision with Accuracy

Keywords: Soil analysis, Soil sampling, Good laboratory practice, Quality assurance, Quality control, Phosphorus, Potassium, Magnesium, pH.

The laboratory seeks to control both the level of precision and accuracy of a method under a structured quality assurance (QA) and Quality Control (QC) scheme.

Perhaps Taylor (1987) made the most succinct description of a QA/QC programme:

When the uncertainty of a measured value is one-third or less of the permissible tolerance for its use, it can be considered as essentially errorless for that use.

Variability of results can be both random or due to a bias. Typically the error associated with field sampling of soil is much greater than that arising from sample preparation, handling or analysis. Taking into account sources of error associated with the entire process of soil sampling and analysis, the minimum variance for practical decision making might be 0.2 pH_(aq) units, 5 mg/l phosphorus (Olsen’s), 25 mg/l potassium and 10 mg/l magnesium (K and Mg both ammonium nitrate extraction).

C. Collins and A.L. Budden, Natural Resource Management Limited, UK

20 pages, 3 figures, 4 tables, 13 refs.