The Soil Fertility Clock (SFC) concept is based on the assumption that the critical content (range) of essential nutrients in the soil is adapted to the requirements of the most sensitive plant in the cropping sequence (CS). This provides a key way to effectively control the productivity of fertilizer nitrogen (N_f). The production goals of a farm are set for the maximum crop yield, which is defined by the environmental conditions of the production process. This target can be achieved, provided that the efficiency of N_f approaches 1.0. Nitrogen (in fact, nitrate) is the determining yield-forming factor, but only when it is balanced with the supply of other nutrients (nitrogen-supporting nutrients; N-SNs). The condition for achieving this level of N_f efficiency is the effectiveness of other production factors, including N-SNs, which should be set at ≤1.0. A key source of N-SNs for a plant is the soil zone occupied by the roots. N-SNs should be applied in order to restore their content in the topsoil to the level required by the most sensitive crop in a given CS. Other plants in the CS provide the timeframe for active controlling the distance of the N-SNs from their critical range.

Wheat fertilized with Mg, regardless of the method of application, increases nitrogen fertilizer (N_f) efficiency. This hypothesis was tested in 2013, 2014, and 2015. A two-factorial experiment with three doses of Mg (i.e., 0, 25, and 50 kg ha⁻¹) and two stages of Mg foliar fertilization (without; BBCH 30; 49/50; 30 + 49/50) was carried out. Foliar vs. in-soil Mg fertilization resulted in a comparable grain yield increase (0.5–0.6 t ha⁻¹). The interaction of both fertilization systems increased the yield by 0.85–0.9 t ha⁻¹. The booting/heading phase was optimal for foliar fertilization. Mg accumulation by wheat fertilized with Mg increased by 17% compared to the NPK plot. The recovery of foliar Mg was multiple in relation to its dose. The recovery of the in-soil Mg applied ranged from 10 to 40%. The increase in yield resulted from the effective use of N taken up by wheat. In 2014 and 2015, this amount was 21–25 kg N ha⁻¹. The increase in yield resulted from the extended transfer of N from vegetative wheat parts to grain. Mg applied to wheat, irrespective of the method, increased the efficiency of the N taken up by the crop. Mg fertilization resulted in higher N_f productivity, as indicated by the increased nitrogen apparent efficiency indices.

The increase in the grain density (GD) is the key factor for effectively controlling Nitrogen-Use Efficiency (NUE) in winter wheat. Winter-wheat protection with fungicides and/or foliar fertilization with micronutrients during the critical stages of yield formation affects the grain yield by increasing GD. This hypothesis was verified in a two-factor field experiment, carried out in the 2013/2014, 2014/2015, 2015/2016 growing seasons. A field experiment in a two-factor split-plot design, included three systems of wheat foliar protection (FP): (i) N + Mi (N + macronutrients), (ii) N + P (N + fungicides), (iii) N + Mi + P, and 0, 40, 80, 120, 160, 200, 240 kg N ha⁻¹. The grain yield, despite the significant effect of years, depended on the interaction of FP and N doses. The maximum yield of 8.1, 10.7, 11.1 t ha⁻¹ for the optimal N dose of 79.8, 227.4, and 245.2 kg ha⁻¹, was achieved, respectively, for N + Mi, N + N, N + Mi + P. The wheat-grain yield depended significantly on GD (R² = 0.98, p ≤ 0.001). Wheat protected with fungicides increased GD gradually with the N dose, reaching over 3000 grains per m² when fertilized with 200 kg N ha⁻¹. The lack of fungicide protection, reducing GD, consequently resulted in a greater accumulation of N in straw at the expense of grain. Fungicide protection of wheat through the positive effect on N management by wheat should be considered as an agronomic measure that supports N-Use Efficiency.

Nutrient management by winter oilseed rape (WOSR) during the seed filling period (SFP) is crucial for seed density (SD), which subsequently determines seed yield (Y). This hypothesis was evaluated based on data from field experiments (2008-2010), with six treatments with sequentially added nutrients (0, NP, NPK, NPKMgS1(¹/₃ total MgS rate—spring applied), NPKMgS2 (total MgS rate autumn applied); NPKMgS3 (²/₃-autumn, ¹/₃-spring). Nutrients accumulated in seeds were revealed as the most reliable WOSR trait, determining SD and Y. Yield was defined by the amounts of K and Mg in seeds. The amount nutrient remobilized from vegetative WOSR tissues was sufficient to cover the crop requirements during SFP for N, P, and Zn, but not for Ca, K, Mn, and Cu. The post-flowering K uptake resulted in PD, and SD increase, which subsequently resulted in a concomitant net uptake of Ca and Mg. The excessive accumulation of Ca, as well as Cu, and Mn in seeds due to a shortage of Mg, subsequently resulted in both SD and Y depression. The efficient exploitation of WOSR yielding potential as defined by SD, is possible, provided during the SFP reveals a net K and Mg uptake.

Reliable prediction of winter bread wheat grain yield (GY) and its qualitative parameters (crude protein (CP) and wet gluten (GL) content, wet gluten yield (GLY)) requires evaluation of the plant nutritional status in the Critical Cereal Window (CCW). The reliability of the forecast depends on the dedicated plant characteristics and the correct selection of the diagnostic plant parts. This hypothesis was verified in a one-factor field experiment carried out in the 2013/2014, 2014/2015, and 2015/2016 growing seasons. The field experiment included applying 0, 40, 80, 120, 160, 200, and 240 kg N ha⁻¹. The N, P, K, Ca, Mg, Fe, Mn, Zn, and Cu content in wheat was determined in two growth stages: (i) beginning of booting (BBCH 40) and (ii) full flowering (BBCH 65). The evaluated plant components included the leaves and stem for BBCH 40 and the flag leaf, leaves, stem, and ear of BBCH 65. Grain yields were very high, significantly responding to the increased rates of fertilizer nitrogen (N_f), with a maximum yield of 11.3 t ha⁻¹ achieved in 2014 (N rate of 209 kg N ha⁻¹), 13.7 t ha⁻¹ in 2015, and 8.6 t ha⁻¹ in 2016 (N rate of 240 kg N ha⁻¹). The CP and GL content also increased linearly in accordance with the N_f rates. At the beginning of the booting stage, the GY forecast based on the content of nutrients in the leaves or the stem was 94%. Meanwhile, a slightly higher yield prediction was obtained for leaves during the full flowering stage (95%). The key nutrients comprised K, Ca, and Mn, accounting for 93% of the GY variability. The accuracy of the GL prognosis at BBCH 40, regardless of the plant part, exceeded 99%. Three nutrients, namely, P, Mg, and Zn, explained 98% of the GL variability, and the GLY forecast was high (97%). Both wheat traits depended on Zn, which buffered the action of N and Mg. At the full flowering stage, the highest, yet slightly weaker, predictions of GL and GLY were obtained for leaves (95% and 92%, respectively). At this stage of winter wheat growth, the significant role of Zn and K and the buffering effect of Cu on the action of both nutrients was apparent. The obtained results unequivocally confirm that the game for winter wheat grain yield occurs within the Critical Cereal Window. In addition, the end result depends on the plant’s N supply during this period and the nutritional status of other nutrients. Application of 40–80 kg N ha⁻¹ fertilizer critically impacted the GY and technological quality. Moreover, micronutrients, including Zn and Cu, influence the GY, GL, and GLY considerably. At the beginning of the booting phase (BBCH 40), winter wheat leaves serve as a highly reliable plant component indicator for evaluating nutrient content and quantitative (GY, GLY) and qualitative (GL) characteristics of grain. Moreover, analysis conducted during BBCH 40 allows the farmer to correct the nutritional status of the wheat, taking into account N and other nutrients as necessary.

Indicators of nitrogen economy in winter wheat during vegetative development are a reliable tool for yield prognosis. This hypothesis was verified in a field experiment, carried out in the 2013/2014, 2014/2015, and 2015/2016 seasons. The field experiment, in a two-factor split-plot design, included the following systems of wheat protection (CFP): (i) N + micronutrients, (ii) N + fungicides, (iii) N + micronutrients + fungicides; and N rates: 0, 40, 80, 120, 160, 200, 240 kg N ha⁻¹. The content and accumulation of N in wheat at the beginning of stem elongation and at heading were used for grain density and yield prediction. In the grain-filling phase, the stem N acted as a buffer, stabilizing yield at a high level. The condition for such action was the stem N equilibrium with the ear N at flowering. The N depletion from the leaves during the grain-filling period significantly depended on the grain density. The post-flowering uptake of N by wheat was affected by the grain density, which was affected by the N reserves in the stem. Yield forecast based on pre-flowering indices of nitrogen economy in cereals affects both agronomic decisions aimed at correcting the nutritional status of plants, and farm economics.