Fertilizer Use Efficiency (FUE) is a measure of the potential of an applied fertilizer to increase its impact on the uptake and utilization of nitrogen (N) present in the soil/plant system. The productivity of N depends on the supply of those nutrients in a well-defined stage of yield formation that are decisive for its uptake and utilization. Traditionally, plant nutritional status is evaluated by using chemical methods. However, nowadays, to correct fertilizer doses, the absorption and reflection of solar radiation is used. Fertilization efficiency can be increased not only by adjusting the fertilizer dose to the plant’s requirements, but also by removing all of the soil factors that constrain nutrient uptake and their transport from soil to root surface. Among them, soil compaction and pH are relatively easy to correct. The goal of new the formulas of N fertilizers is to increase the availability of N by synchronization of its release with the plant demand. The aim of non-nitrogenous fertilizers is to increase the availability of nutrients that control the effectiveness of N present in the soil/plant system. A wide range of actions is required to reduce the amount of N which can pollute ecosystems adjacent to fields.

Attainable, maximum yield:
Yield Gap:
Nitrogen Gap:
where: PFPNf—partial factor productivity of Nf, kg grain/seeds, tubers etc. per kg Nf; Ya—actual yield of a currently grown crop, t ha−1; Nf—the amount of applied fertilizer N, kg ha−1; Yattmax—the maximum attainable yield, t ha−1; cPFPNf—the average of the third quartile (Q3) of the set of PFPNf indices arranged in ascending order, kg grain/seeds, tubers etc. per kg Nf; YG—yield gap, t ha−1; NG—nitrogen gap, kg ha−1 of N.
The NG calculation is important for the farmer for at least three areas of his production activity: (i) the determination of Yattmax, which determines not only the maximum yield for the production area, but also determines the potential requirements of the cultivated crop for N; (ii) the identification of hotspots in N management for a given crop, including an inadequate supply of nutrients other than N; (iii) the set of actions needed to improve the level of soil fertility for a given crop.
where: ANuE—Apparent Nutrient Efficiency, kg yield kg−1 nutrient applied; ANuR—Apparent Nutrient Recovery, %; Yf, Yc—yield on a plot with and without fertilizer, t or kg ha−1; Nr—the rate of a nutrient applied as fertilizer, kg or g ha−1; Nuf, Nuc —the uptake of a tested nutrient on a plot with and without fertilizer, kg, g ha−1.
The recorded values of ANuE and ANuR usually show a decreasing trend, with an increase in the rate of the nutrient applied as fertilizer, which is satisfactory for the researcher. Moreover, the values obtained have a tendency opposite to the soil fertility indexes for a given nutrient [7]. It simply means that FUE is highly dependent on the current soil fertility level, which the farmer needs to know. However, the main disadvantage of these two indices is that the farmer does not have a control plot to assess the actual nutrient productivity in the applied fertilizers. The values of the ANuR indices, evaluated on the global scale, are low and amount to 40–65% for N, 15–25% for P, and 30–50% for K used in fertilizers [9]. At this point it is necessary to pose the question, what is the main source of nutrients for the currently grown crop?
The productivity of nutrients taken up by the crop during one growing season can also be estimated by the partial nutrient balance (PNB) method:
