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 Sc heme, and over thirty in the Plant Material Scheme. Standard methods h ave been published for soils, and those for plant material are nearing completion. Duplicate results for different analytes are analyzed sta tistically to evaluate between laboratory and within laboratory standa rd deviation. Outliers are excluded using Cochran's maximum variance t est and Dixon's outlier test, among others. Considerable experience ha s been gained in making the reports understandable and valuable for qu ality assurance.