PREPARATION OF A SOIL REFERENCE SAMPLE AND INITIAL RESULTS ON LABORATORY QUALITY IMPROVEMENT

The mineral constituents of soils were analysed for their agricultural potential by the Soils and Fertilizers Research Institute (SFRI). Until recently the laboratories had no reference materials to validate the tests. This deficit was overcome by creating a reference soil sample. The first step was to collect about 100 kg of surface soil from an unfertilized area adjacent to a rice paddy in the Red River Delta. The soil was air dried, and roots and pebbles were removed before grinding, mixed thoroughly and divided into two 50 kg portions. One portion was sent to Australia where it was circulated as a proficiency sample to 39 agricultural laboratories as part of a certification procedure run by the Australasian Soil and Plant Analysis Council (ASPAC). The inter-laboratory means and coefficients of variation for a wide range of tests were returned to SFRI where they are being used in method validation and quality control. The retained portion was divided into 100 jars each of ~500 g and the nitrogen content (semi-micro Kjeldahl) was used to assess the homogeneity of the material within jars (4 replicates) and between jars using the within-jar means. The mean N content for all jars was 0.067% and the CV was ± 6.22%, and the corresponding ASPAC statistics were 0.066% and ± 12.29%. Other parameters were analyzed and compared with the ASPAC inter-laboratory proficiency results as a part of method validation. For example, there respectively means for SFRI and for ASPAC (in parentheses) were: for the pH of 1 g/5 mL suspension in water 7.96 (8.04); for organic carbon, 0.46% (0.44%); for total-P as P2 O5 , 0.09% (0.08%); and for available K, 4.24 mg/100 g as K2 O (4.7 mg/100 g). However, the agreement for other parameters, such as cation exchange capacity, was poor: 13.2 cmol/kg (10 cmol/kg) and the causes are being investigated. Finally, it is planned to develop additional reference materials to complement the Hanoi reference soil. These materials will strengthen the quality of testing at SFRI and provide objective measures of that quality into the future.