Even though nutrient supply ability of soil has been estimated using extraction method, roots can not absorb nutrients before they reach root surfaces. Soil physics, plant demand and other environmental conditions must be taken into account to evaluate nutrient supply ability of soil. Micro model that was used by Nye and extended by Barber et al., made considerable contribution in the area. But still now, there are some problems that hinders effective utilization of those models. Elongation and development reaction of roots in relation with physical or nutritional conditions of soil, activity of root hair and mycorrhizae, effect of root—induced changes of rooting zone. These are the examples of the hindrances. A macro model that was developed by the author deals behaviours of nitrogen in a paddy soil including mineralization, organization,leaching, denitrification, fertilization, temperature of the soil, absorption by rice plant, development and production of the plant, etc. The model gave some good estimations of experimental outcomes. Many interesting results were obtained by simulation. Utilization of this or other macro models along with micro models will be practical in the area of evaluation of nutrient supply ability and ways of soil management.
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