Soil Management for the Conservation of Soil Nitrogen
It is impossible simply to quantify soil nitrogen fertility. Soil nitrogen fertility should be recognized as the integrated function of various processes in the nitrogen cycle in a soil-plant system, and of the environmental factors affecting them. Nitrogen mineralization is the most important process in the nitrogen cycle. In Japan, nitrogen mineralization is mainly governed by temperature. A method of predicting nitrogen mineralization from temperature has recently been developed in Japan. This method has provided valuable information for the rational application of fertilizers and organic matter. In moist climates, leaching of nitrate is a cause of environmental pollution as well as low efficiency of fertilizer nitrogen. However, some deep-rooted crops can absorb the nitrate which has been leached from the plow layer and has accumulated in the subsoil. The significance of the subsoil in efficient nitrogen use is emphasized. Regarding soil management for conserving nitrogen fertility in Japan, some problems involved in the use of paddy soils for upland crops are discussed. The advantages of minimum tillage in extensive agriculture are also reviewed.