Abstract: The concept about crop's effective nutrient area is stated and a model of the relationship between nutrient area of soybean and yield is built.Through optimization and analysis，the nutrient area of soybean is 2837.5cm²，i.e.the effective row width is 53.3cm.The optimum nutrient area is 520.0cm²，i.e.the best row width is 22.8cm.But，in the present production，the ridge width is 67～70cm，longer than the effective row width of soybean，which would waste soil resources.The row width of soybean should be 50 cm under ridge tillage condition or 23cm*23cm under plain tillage condition，which would be benefit for the increase of population yield of soybean.