Nitrogen Use Efficiency in Maize over Sixteen Years of Unbalanced Fertilization with Nitrogen and Potassium

Adequate fertilization with nitrogen (N) and potassium (K) is essential to sustain crop productivity and minimize nitrogen losses to the environment. However, the increasing imbalance in fertilizer use in Poland, with low potassium inputs, may impair long-term soil fertility and nitrogen use efficiency. The aim of this study was to evaluate the effects of long-term potassium omission on maize productivity and nitrogen use efficiency under varying nitrogen fertilization rates. A 16-year field experiment (2003–2018) was conducted in two different regions of Poland (Grabów and Baborówko), on soils with medium to low available potassium content. Maize response to potassium fertilization cessation was evaluated at increasing nitrogen rates (50–250 kg N ha−1). Grain yield, nitrogen uptake (Yn), nitrogen surplus (Ns), and nitrogen use efficiency (NUE) were assessed according to the EU Nitrogen Expert Panel (EUNEP) approach. Potassium omission had little effect on maize yield and NUE indices. At nitrogen rates commonly applied in agricultural practice in Poland (~100 kg N ha−1), NUE strongly exceeded 100%. The other NUE indices—nitrogen surplus and nitrogen uptake remained optimal (<80 kg N ha−1, and >80 kg N ha−1, respectively), regardless of potassium treatment. These results indicate a low risk of nitrogen losses under moderate nitrogen inputs, even without potassium fertilization. However, given the declining NUE trend on soils with low potassium content, a reduction in nitrogen use efficiency can be expected, particularly under high nitrogen application rates and continued unbalanced potassium fertilization.