Quality assurance schemes for soil and plant analyses in South Africa

Quality assurance schemes, involving two samples monthly, have been in operation in South Africa for several years. Over forty laboratories from Southern Africa and neighboring states participate in the Soil Scheme, and over thirty in the Plant Material Scheme. Standard methods have been published for soils, and those for plant material are nearing completion. Duplicate results for different analytes are analyzed statistically to evaluate between laboratory and within laboratory standard deviation. Outliers are excluded using Cochran's maximum variance test and Dixon's outlier test, among others. Considerable experience has been gained in making the reports understandable and valuable for quality assurance.