Nitrogen Use Efficiency

Agronomic efficiency asks the only question that matters at the margin: how much extra crop did the nitrogen actually buy? It needs an unfertilised control, which is why the figure is common in trials and rare on farms — and why on-farm nitrogen rates are so often set by habit rather than measurement.

Typical values for cereals run 15–35 kg of grain per kilogram of nitrogen, and the number falls as the rate rises. That decline is the whole economics of the decision. The first 50 kg of nitrogen on a deficient soil might return 40 kg of grain each; the last 50 kg approaching the yield plateau might return 5. The optimum rate is where the last kilogram's return equals its cost, and that point moves every year with prices.

Agronomic efficiency is one of several efficiency measures and they are not interchangeable. Recovery efficiency asks what fraction of the applied nitrogen ends up in the plant — usually 30–50%, which is a sobering figure. Physiological efficiency asks how much grain the plant made per unit of nitrogen it took up. Partial factor productivity, total yield divided by nitrogen applied, is easy to compute because it needs no control, and is correspondingly less informative.

A value far above the normal range usually means the control was severely deficient rather than that the fertiliser was unusually effective, and it does not extrapolate upward. A negative value is real and worth investigating: excess nitrogen causes lodging, delays maturity past a short season, and can burn a crop where it is banded too close to the seed.