Comparison of Two Methods for Determination of Soil Fixed Ammonium

Abstract

Twenty-four soils were sampled from northwestern China to study the relations of fixed or nonexchangeable ammonium (NH4) determined by the widely used Silva–Bremner method and the simplest ignition Mogilevkina method with organic matter (OM), total nitrogen (N), and soil particle composition. Results showed that the averaged values of 24 soils determined by the Silva–Bremner method were 10.7% greater than those using the Mogilevkina method, and the standard deviations of the Mogilevkina method were larger than those of the Silva–Bremner method. There was a close correlation in the fixed NH4 between the two methods (R2 = 0.7831, n = 24). The low values obtained by the Mogilevkina method were not due to the heat treatment because there was almost no difference in results between heating for 44 and 72 h. Soil OM had no correlation with the fixed NH4 amounts determined by the two methods due to self-correlations, but total N had significantly weak correlations. The soils sampled from northwestern China were dominant in the 2:1 type of clay minerals, and significant correlations were found for the fixed NH4 amounts determined by the two methods with clay as well as with clay plus silt contents. Correlations (R2) of the fixed NH4 by the Silva–Bremner method were 0.4545 and 0.3145 with clay and clay plus silt contents, respectively, for the initial soils and 0.4765 and 0.446, respectively, for soils after ryegrass planting, which were much greater than those of the Mogilevkina method. Based on the definition of fixed NH4 and the problem that occurred during its determination, two criteria were proposed for evaluation of the methods: (1) the relationship between clay content or clay plus silt contents and fixed NH4 and (2) the correlation of OM content with fixed NH4. By these criteria, the Silva–Bremner method was superior to the Mogilevkina method in terms of reliability and accuracy.