Abstract: For reducing the long period of shaking for the release of the fixed ammonium by Silva-Bremner method, a new procedure was proposed. The new procedure is that the soil is heated at low-temperature after removal of the exchangeable ammonium and organic N. A manurial loessial soil was sampled to study the effect of acid volume, temperature and heating time on the release of the fixed ammonium by Silva-Bremner method for making comparison. Through this study, optimal conditions were determined for using such a method: treating soil with 5 ml of 5 mol·L-1 HF-1 mol·L-1 HCl and heating at 100 ℃ for 1.5~2 h. Thirty eight soil samples were collected all over China to compare the results between the two methods. Results show that the two techniques gave virtually the same results for the 38 soils with a coefficient of 0.99, and the relative errors was within 5% for the low-temperature heating method. No significant difference was found in the results between the two methods as shown by T-test for individual pairs of each soil.

Key words: Fixed ammonium, Silva-Bremner method, Low-temperature heating method