Phosphorus resources, both in phosphate rocks and in the soil, are limited. However, effective food production is not possible without the use of P fertilizers. Recognizing and eliminating or at least ameliorating factors (hot spots) that interfere with the uptake and use of phosphorus (P) by crop plants is of key importance for effective use of both P and nitrogen (N) on the farm. Plants have developed many adaptation mechanisms to their environment, i.e., soil low in available phosphorus. The most important ones include the secretion of organic compounds into the rhizosphere and the association of plant roots with microorganisms. A classic example is mycorrhiza. These mechanisms can be used by the farmer to sequentially select plants in the crop rotation. The uptake of inorganic P (P_i) by plants from the soil is reduced by environmental (temperature and water) and soil factors (low content of available phosphorus, soil acidity, soil compaction). These factors are responsible for the growth and size of the root system. Mitigating these negative effects improves the efficiency of phosphorus uptake from the soil. The second group of critical factors, limiting both root growth and availability of phosphorus, can be effectively controlled using simple measures (for example, lime). Knowing this, the farmer must first control the level of soil fertility in the plant’s effective rooting zone and not only in the topsoil. Secondly, the farmer must multiply the productivity of applied mineral fertilizers used through targeted recycling: crop rotation, crop residues, and manure.

In agricultural practice, in addition to determining the nitrogen (N_f) dose, it is necessary to effectively control its effect on currently grown crops. Meeting these conditions requires not only the use of phosphorus (P) and potassium (K), but also nutrients such as magnesium (Mg) and sulfur (S). This hypothesis was verified in a single-factor field experiment with winter wheat (WW) carried out in the 2015/2016, 2016/2017, and 2017/2018 growing seasons. The experiment consisted of seven variants: absolute control (AC), NP, NPK-MOP (K as Muriate of Potash), NPK-MOP+Ki (Kieserite), NPK-KK (K as Korn–Kali), NPK-KK+Ki, and NPK-KK+Ki+ES (Epsom Salt). The use of K as MOP increased grain yield (GY) by 6.3% compared to NP. In the NPK-KK variant, GY was 13% (+0.84 t ha⁻¹) higher compared to NP. Moreover, GYs in this fertilization variant (FV) were stable over the years (coefficient of variation, CV = 9.4%). In NPK-KK+Ki+ES, the yield increase was the highest and mounted to 17.2% compared to NP, but the variability over the years was also the highest (CV ≈ 20%). The amount of N in grain N (GN) increased progressively from 4% for NPK-MOP to 15% for NPK-KK and 25% for NPK-KK+Ki+ES in comparison to NP. The nitrogen harvest index was highly stable, achieving 72.6 ± 3.1%. All analyzed NUE indices showed a significant response to FVs. The PFP-N_f (partial factor productivity of N_f) indices increased on NPK-MOP by 5.8%, NPK-KK by 12.9%, and NPK-KK+Ki+ES by 17.9% compared to NP. The corresponding N_f recovery of N_f in wheat grain was 47.2%, 55.9%, and 64.4%, but its total recovery by wheat (grain + straw) was 67%, 74.5%, and 87.2%, respectively. In terms of the theoretical and practical value of the tested indexes, two indices, namely, NUP (nitrogen unit productivity) and NUA (nitrogen unit accumulation), proved to be the most useful. From the farmer’s production strategy, FV with K applied in the form of Korn–Kali proved to be the most stable option due to high and stable yield, regardless of weather conditions. The increase in the number of nutritional factors optimizing the action of nitrogen in winter wheat caused the phenomenon known as the “scissors effect”. This phenomenon manifested itself in a progressive increase in nitrogen unit productivity (NUP) combined with a regressive trend in unit nitrogen accumulation (NUA) in the grain versus the balance of soil available Mg (Mg_b). The studies clearly showed that obtaining grain that met the milling requirements was recorded only for NUA above 22 kg N t⁻¹ grain. This was possible only with the most intensive Mg treatment (NPK-KK+Ki and NPK-KK+Ki+ES). The study clearly showed that three of the six FVs fully met the three basic conditions for sustainable crop production: (i) stabilization and even an increase in grain yield; (ii) a decrease in the mass of inorganic N in the soil at harvest, potentially susceptible to leaching; and (iii) stabilization of the soil fertility of P, K, and Mg.

It has been assumed that the long-term impact of a diversified soil use system (SUS) and the continuous application of manure and/or mineral fertilizers (NPK) affects the sustainability of soil fertility components. This influence is manifested through the content and distribution of nutrients, as well as some bioavailable heavy metals in the soil. This hypothesis was verified in 2022 in a long-term field experiment that started in 1957. It consisted of a seven-course crop rotation: potato–spring barley–winter triticale–alfalfa–alfalfa–winter wheat–winter rye and monocultures of these crops plus black fallow. The studies were carried out on three separate fields: black fallow (BF), winter wheat grown in monoculture (WW-MO), and crop rotation (WW-CR). Each of these experimental objects consists of five fertilizer variants (FVs) fertilized in the same way every year: absolute control (AC)—variant without fertilizers for 75 years; farmyard manure—FM; mineral fertilizers—NPK; mixed variant—NPK + FM; mineral fertilizers plus annually applied lime—NPK + L. The second factor was the soil layer: 0.0–0.3 m, 0.3–0.6 m, or 0.6–0.9 m. The obtained results clearly indicate that long-term fertilization with NPK + FM, especially in rotation with legumes, strengthens the eluviation/illuviation processes, decreasing the sustainability of soil fertility. Liming is a factor stabilizing the content and distribution of silt and clay particles in the soil. The key factor determining the content and distribution of micronutrients and heavy metals in the soil was the content of organic carbon (C_org). Its content decreased in the following order: WW-CR (13.2 ± 5.8) ≥ WW-MO (12.3 ± 6.9) > BF (6.6 ± 2.8 g·kg⁻¹). The large variability resulted from a distribution trend with soil depth, which increased as follows: MO ≥ CR > BF. FVs with FM had the highest C_org content. NPK, regardless of the long-term soil use system (SUS), had the lowest content. Among the elements studied, the key one impacting the content of both micronutrients and heavy metals was iron (Fe). The Fe content decreased in the order BL (100%) > WW-MO (90.5%) > WW-CR (85%). The opposite tendency was found for the remaining elements, the content of which was consistent with the content of C_org, which was the highest in CR. The strongest impact of Fe, modified by the SUS, was found for Zn, Pb, and Cd. Despite the differences observed between SUSs, fertilization variants, and soil layers, the content of Fe and Mn was in the medium class, while Zn and Cu were in the high class of availability. The content of Ni was the highest for NPK + FM in WW-CR. The content of Pb was weakly affected by the long-term SUS but showed a strong tendency for accumulation in the topsoil layer. The content of Cd was the highest in BF, where it exceeded the threshold of 0.27 mg·kg⁻¹. The long-term diversified SUS, as the main factor determining the sustainability of soil fertility, makes it possible to indicate the directions of humus accumulation and its distribution in the soil. It turned out to be a key factor, but in cooperation with Fe, it determined the content of micronutrients and bioavailable heavy metals in the soil.

Post-agricultural land differs from typical forest land in physical, chemical and biological features. In addition, the environment of this land type is determined, among other things, by the introduced tree species. These differences may be revealed by the biodiversity and abundance of the soil fauna. We analysed the abundance, species richness and diversity of different instars of mesostigmatid mites inhabiting three different habitat types on post-agricultural land (shaped by pure Pinus sylvestris L., Tilia cordata Mill. and Betula pendula Roth stands). We collected 288 soil samples from eight plots in three stands. The collection was conducted in July and October in two consecutive vegetation seasons (2021 and 2022) for Mesostigmata mites community. Soil characteristics (determination of soil group and analysis of physical and chemical properties of soil and litter) were done in July 2021. In total, 399 individuals (266 females, 50 males and 83 juveniles) were classified into 38 taxa (33 species, five genera). Most individuals belonged to the Parasitidae, Laelapidae and Veigaiidae families. The most abundant species were Hypoaspis aculeifer (Canestini) (21.6% of all recorded mites), Veigaia nemorensis (C.L.Koch) (7.8%) and Trachytes aegrota (C.L.Koch) (7.0%). Abundance, species richness and diversity were shaped by collection month and Fe content in soil. The abundance was influenced by N litter content and was significantly lower in P. sylvestris stand in July (0.57 ± 0.23; mean ± SE) than in P. sylvestris (2.17 ± 0.54) and T. cordata (2.15 ± 0.48) stands in October. Moreover, abundance in P. sylvestris stand in October was higher than in B. pendula stand in July (0.78 ± 0.26). Similarly, species richness was significantly lower in P. sylvestris stand in July than in P. sylvestris and T. cordata stands in October (2.17 ± 0.54 and 2.15 ± 0.48, respectively). Higher Shannon’s diversity of mite communities was reported in P. sylvestris stand in October (0.40 ± 0.10) than in P. sylvestris and B. pendula stands in July (0.12 ± 0.06 and 0.14 ± 0.08, respectively). Large fluctuations of abundance, species richness and diversity of soil mite communities in P. sylvestris and B. pendula stands between collection months give the insights for creating mixed stands on post-agricultural land. It is worth noticing that the wet season creates the most favourable living environment for mesostigmatid mites in P. sylvestris litter.

Protection of high-yielding winter wheat (WW) with fungicides increases the productivity of nitrogen (N) present in the soil–crop system during the growing season. As a consequence of the action of fungicides, the nitrogen gap (NG) reduces. This hypothesis was verified on the basis of data from a field experiment conducted with WW during three growing seasons (2013/2014; 2014/2015, 2015/2016) in Poland. The field experiment included two crop protection systems (CP): (i) CP-0—without fungicides and CP-F—with fungicides and (ii) six N doses increased gradually by 40 kg N ha⁻¹ from 0 to 240 kg N ha⁻¹. The grain yield (GY) of WW treated with fungicides was significantly higher than that of the unprotected. The difference in yields between both CP systems was 17.3% on a plot fertilized with 200 kg N ha⁻¹ (9.13 vs. 11.2 t ha⁻¹). The fungicide yield gap increased progressively with N_f doses from 0.76 t ha⁻¹ in the N_f control plot to 2.17 t ha⁻¹ in the fertilized with 200 kg ha⁻¹. The use of fungicides increased the amount of N in grain (N_gr) from 15 kg N ha⁻¹ in the control N plot to 51 kg N ha⁻¹ in the plot with 200 kg N ha⁻¹. The main source of additional N in grain (N_gr) was inorganic N released from the soil (N_g89) during the WW growing season. The maximum N_g89 values were 64.4 and 83.0 kg N ha⁻¹. These values corresponded to N_f doses of 94.4 and 80.8 kg N ha⁻¹. The N_g89 of 70.1 kg N ha⁻¹ conditioned 100-percentage N_f recovery. As a consequence, the prediction reliability of GY and N_gr was highest when N_g89 was used as a predictor. The net increase in the absolute NG size in response to increasing N input was significantly slower and therefore smaller in fungicide-protected than in unprotected WW. It can be concluded that the use of fungicides due to the increase in inorganic N productivity in the soil–crop system reduces the potential threat of N dispersion into the environment. In the light of the results obtained, it should be concluded that the fungicidal protection of crop plants should be treated as a factor significantly reducing the nitrogen gap and, thus, the yield gap.