Evaluation of the performance of commercial soil and plant testing laboratories for analysis of sulphur and nitrogen

Abstract

Ten UK laboratories participated in a study to evaluate variability of analysis for total concentrations of nitrogen (N) and sulphur (S) in plant materials and extractable S in soil samples. Four plant and two soil samples were prepared at IACR-Rothamsted and distributed to the participants. Laboratories were in reasonable agreement for analysis of total N in plant materials, but the variability for total S was considerably higher. Large differences between laboratories were also reported for extractable S in soil. Different analytical methods were used by laboratories for both soil and plant analyses, but this does not explain the large variability in results. Diagnosis of S deficiency based on S analysis may be of questionable validity if S analyses results for plant and soil are not accurate. To improve analytical reliability, more method development and standardisation are needed.

Decreasing sulphur (S) inputs from the atmosphere and fertilisers have led to increasingly widespread S deficiencies in grassland and arable crops in the UK (McGrath et al, 1996). Due to adoption of pollution control measures, sulphur dioxide emissions in the UK have decreased by about 50% since the early 1970's. In other western European and North American countries similar trends have been observed. Furthermore, traditional fertilisers containing considerable amounts of S such as ammonium sulphate and single superphosphate have been largely replaced by high analysis fertilisers containing little or no S.

The problem of increased S deficiency has led to a greater need for plant tissue and soil testing to diagnose whether applications of S fertilisers are necessary. Compared with phosphorus, potassium and nitrogen (N) testing, testing for S is relatively new in the UK and at present only a relatively small number of laboratories are equipped to do this work routinely. It is therefore important to ensure that the laboratories involved in this type of work are using the most appropriate methods of analysis and thereby capable of providing reliable data. Decisions about fertiliser application based on unreliable data could prove costly for arable producers, both in terms of low yield or unnecessary fertiliser use.

To test laboratory performance, an inter-laboratory trial was set up and nine major commercial UK plant and soil analytical laboratories were invited to participate along with IACR-Rothamsted. Laboratories were asked to analyse four selected plant materials for total S and total N and to extract and analyse two soils for available S.