The first step in assessing nitrogen fertilizer’s impact on the environment relies on its productivity. Several series of indices have been developed to describe nitrogen use efficiency (NUE). Their usability as potential agro-environmental tools depends, among other things, on the degree of responsiveness to crop nutritional status at its maturity. Selected NUE indices were evaluated based on grain yield, the number of kernels per con (NKC), and general crop indicators such as i) total nutrient uptake (TNU), ii) nutrient harvest index (NHI), and iii) unit nutrient uptake (UNU). This concept was validated using original experimental data on maize response to increasing nitrogen rates (0, 100, 150, and 200 kg ha^-1) on the background of long-term potassium fertilizing systems, differing in soil K fertility level (medium, high) and K fertilizer application (K0, K+). The most promising indices of diagnostic values to describe both agronomic and environmental impacts of nitrogen fertilizer were the partial factor productivity of nitrogen fertilizer (PFP_FN) and apparent nitrogen recovery (NR). The first one, directly describing productivity of the unit of applied nitrogen fertilizer, showed high sensitivity to those nutrients, which significantly defined maize yield. In the studied case it refers to potassium, whose low supply during the critical window resulted in a significant decrease in the number of kernels per cob. Consequently, the reduced capacity of the maize physiological sink during the grain-filling period resulted in the apparent excess of phosphorus and magnesium. This conclusion is corroborated by an N/P ratio of 7.4-10.8:1 and an N/K ratio of 7.4-10.8:1. This was the key reason for limiting productivity of nitrogen fertilizer. The NR index was the slightly poorer indicator of nitrogen fertilizer management, but also responded significantly both to natural and experimentally induced factors that are decisive for production variability. Other frequently used NUE indices did not allow for making both a simple and reliable evaluation of the nitrogen fertilizer productivity under different management systems.