EVOLUTION OF THE ANALYTICAL QUALITY OF SOIL TESTING LABORATORIES INTEGRATED IN A SAMPLE EXCHANGE PROGRAM

After the introduction of new soil test methods in the state of Sao Pa ulo in 1983, a sample exchange program was organized with the objectiv e of encouraging uniform use of the methods and to improve the analyti cal quality of the results. The program started in 1984 with the labor atories that adopted the new methods and has increased almost linearly during the last years, attaining 50 laboratories from 6 states in 199 2. Statistical criteria were adopted for the rejection of discrepant r esults. A system for ranking laboratories was developed based on error estimation due to inaccuracy and imprecision. Analytical difficulties and most common errors have been discussed during annual meetings of the program. All laboratories involved use the same methods and errors due to inaccuracy and imprecision were, in 1992, reduced respectively to 36% and 44% of the values of 1986. The program contributed to incr ease credibility of soil analysis in Sao Paulo state.