Determination of fixed ammonium in soil

Abstract

1. A method of determining fixed ammonium in soil based on estimation of the difference in the amounts of ammonium released on treatment with N-HF:N-HC1 and with N-KCI has been investigated and compared with the hydrofluoric acid method of Rodrigues and the alkaline distillation method of Barshad.2. Results obtained by the N-HF:N-HCl procedure with profile samples of various soils indicated that 3–8% (average, 5·6%) of the nitrogen in the surface soils and 9–44% (average, 21·5%) of the nitrogen in the subsoils examined was in the form of fixed ammonium. Rodrigues's method gave much higher values and Barshad's method gave much lower values.3. Studies on the forms of nitrogen in soil profiles showed that the proportion of soil nitrogen released by acid hydrolysis as total-N and α-amino-N decreased with depth in the profile, whereas the proportion liberated as ammonium by acid hydrolysis increased with depth. They also showed that a considerable amount of the ammonium released by acid hydrolysis was derived from fixed ammonium in clay minerals.4. It was found that the proportion of soil nitrogen dissolved by N-HF:N-HCl increased with depth in the profile, whereas the proportion dissolved by neutral and alkaline reagents used to extract soil organic matter decreased with depth. On the average, 23·2% of the nitrogen in the surface soils and 47·4% of the nitrogen in the subsoils was dissolved by a single treatment with N-HF:N-HCl at room temperature for 24 hr.5. The reliability and significance of the results obtained by the N-HF:N-HCl method are discussed.