In this paper both the absolute and relative contents of fixed NH₄⁺ in soils of soil zones except the alpine and subalpine soil zones of China are presented, the NH₄⁺ fixation capacity and the availability of fixed NH₄⁺ of major soil groups are indicated. The origin of fixed NH₄⁺,the factors affecting the content of fixed NH₄⁺ and the ability for fixing NH₄⁺ of soil, and the significance of NH₄⁺ fixation and defixation in the internal cycle of soil N are disccussed. The content of fixed NH₄⁺ in soils of yellow brown soil zone, as a whole, is the highest, being 257 mg/kg, while that of latosol zone is the lowest, being 48 mg/kg. On average about 15% of N to depth of 20cm for the cultivated soils as a whole of China was estimated to occur as fixed NH₄⁺.Of the native fixed NH₄⁺,dependent on both the kind of parent material (rock) and the degree of weathering of the soil, from 3% to 20% can be utilized by plant. The NH₄⁺ fixation capacity of soils of yellow brown soil zone is also the highest, while soils of latosol zone, lateritic red soil zone, grey brown desert soil zone and brown desert zone as well as some soils in the red soil zone are unable to fix added NH₄⁺.Based on the differences in the amount of fixed NH₄⁺ and its contribution to soil N pool, as well as in the ability for fixing NH₄⁺,soils in China except those in the alpine and subalpine soil zones are grouped into 10 soil regions.