A new decomposition set which digests nine samples at the same time was developed. Chromium, copper and nickel amounts were analyzed in water hyacinth (Eichhornia crassipes) samples by atomic absorption spectroscopy using only nitric acid or a combination of sulfuric and nitric acid. The results were statistically compared by t- and F- tests. Since there was no significant difference between these two methods, only nitric acid may be used for decomposition of plant samples.