Abstract
The ammonium nitrogen content of soil exposed to dry heat was examined as a possible index of the soil nitrogen fertility for the next cropping season. When heated at below 180〜200℃, the soil ammonium nitrogen content tended to increase with the heating temperature and period of heating. However, the correlation with the mineral nitrogen content of the incubated soil, which corresponds to the soil nitrogen fertility for the next cropping season, increased as the heating temperature and period of heating decreased. The highest correlation (r=0.84) of the ammonium nitrogen content with the mineral nitrogen content of the incubated soil was when the soil was exposed to dry heat at 105℃ for one day. Under these conditions, the heating period is short, and the application of dry heat is the same as that currently used for water-content analysis. Thus, the ammonium nitrogen content of soil dried at 105℃ for one day can provide a simple and rapid estimate of the nitrogen fertility of the soil, which can be used to assess fertilizer requirements for the next cropping season. In addition, the soil nitrate content, which is also important for estimating the nitrogen fertility of upland fields, can be simultaneously determined using an extraction solution from the heat-dried soil, because soil nitrate content is little affected by dry heat at 105℃ for one day.
