Long-term fertilizer nitrogen management—Soil health conundrum

The mental model that fertilizer nitrogen (N) acts as a replacement for N mineralized from soil organic matter (SOM) needs to be revisited. Soil organic matter, the storehouse of N in soil, is one of the most important indicators of soil health. It supplies more N to crop plants than the current-year fertilizer N even when applied at high rates. Limited research shows that the application of fertilizer N above the optimum levels on a long-term basis deteriorates soil health by mineralizing SOM and depleting the soil N pool. Because soil N includes portions of fertilizer N applied in several previous years, year after year application of fertilizer N below the optimum levels will also lead to a gradual decline in soil N pool or soil health. To sustain high crop yield levels, fertilizer N needs to be applied on a long-term basis neither above nor below the optimum levels to ensure that soil health in terms of sustaining supply of soil N is maintained, if not enhanced.