Nitrogen Use Efficiency Analysis

They measure different links in the chain from fertilizer to yield. Recovery efficiency is the fraction of applied nitrogen the crop takes up — how much fertilizer reached the plant. Physiological efficiency is the extra yield produced per unit of extra nitrogen taken up — how well the plant converts absorbed nitrogen to grain. Agronomic efficiency is the extra yield per unit of nitrogen applied — the overall return — and it equals recovery efficiency multiplied by physiological efficiency. That identity is what makes the set diagnostic: a low agronomic efficiency can be blamed on poor uptake, poor conversion, or both.

Because the difference-based indicators isolate the effect of the applied nitrogen, and that requires knowing what the crop would have yielded and taken up from the soil alone. The unfertilized control supplies that baseline; agronomic efficiency subtracts the control yield and recovery efficiency subtracts the control uptake before dividing by the nitrogen rate. Without the control you can only compute rate-based measures like partial factor productivity, which lump the soil's contribution together with the fertilizer's and therefore cannot tell you how efficiently the added nitrogen specifically was used.

It shifts the question from a single fertilizer dose to the whole system over time. By comparing nitrogen removed in harvest against nitrogen supplied as inputs, it reveals whether the system is running a surplus that risks polluting water and air, or a deficit that mines soil nitrogen and erodes future fertility. A balance near one is generally desirable. Because it omits unmeasured inputs such as deposition and fixation, it is explicitly partial, but it captures the long-run sustainability dimension that the per-application efficiency ratios, however precise, do not address.