N, mainly as nitrate (N-NO3
), also acts a local and systemic signaling molecule involved in the current regulation of the hormonal status and morphology of the plant 
. For this reason, this inorganic N form has recently been termed a plant morphogen 
. Clear evidence for the dominant role of N-NO3
in yield formation is its influence on plant growth, which affects both the shoot and root system architecture 
. The effect of N supply to the plant manifests itself in clear, visible changes in the architecture of the plant’s canopy. Wheat plants grown on an N control plot (i.e., without Nf
supply) presented stunted growth (i.e., dwarf stature), low weight and surface area of leaves, and pale green color. In contrast, plants well-fed with N were characterized by a well-developed shoot, high mass and surface area of leaves, and an intense green color. All of these plants, despite a significant difference in the architecture of shoots, were in the same phase of growth (i.e., booting; BBCH 40–49). This phase is the crucial for the development of yield structure and determines the number of fertile florets 
. Excess N supply to the plant, as shown for maize, results in the establishment of more cobs per plant. However, this does not mean a higher yield of grain. Excessive supply of N also results in excessive growth of non-productive plant parts, leading to a reduction in grain per unit area 
The introduction of new cereal phenotypes in the 1960s, such as varieties with reduced stem length, first for wheat and then for rice, significantly changed the shoot architecture (dwarfism of the shoot). These genetic modifications resulted in an increased harvest index (HI)—that is, the share of grain in the total shoot biomass—at the expense of the stem. Improvement of the harvest index (HI) is the greatest effect of the Green revolution, as it finally led to higher grain yields of cereals, including rice 
. However, the exposition of dwarf genes has also caused a reduction in the root system size of wheat varieties, which is significantly smaller than that of the classic ones 
. As a consequence, the semi-dwarf or dwarf growth mode of modern cereals varieties result in the higher yield, provided that the supply of nutrients (especially Nf
) is high, and that the plants are strongly protected against pathogens 
. It can, therefore, be concluded that the currently grown cereals, due to their high requirements for N on one hand, and their smaller root systems on the other hand, are extremely sensitive to the supply of nutrients responsible for the uptake of N from the soil. One of the proposed solutions aimed at the increase in nitrogen use efficiency (NUE) are new-generation varieties that are capable of developing deep root systems. The proposed ideotype of this root system—referred to as “steep, cheap, and deep”—assumes the effective uptake of water and dissolved nutrients (mainly nitrates) 
. A reorientation of the current breeding approach is urgently required in intensive production systems, where high rates of Nf
are typically applied. It can also provide a good solution in areas with frequent periods of drought, regarding the main phases of plant growth.
3. Nitrogen-Supporting Nutrients
Plant growth and productivity are the result of the action of about 20 elements that must be present in the soil to complete the plant’s life cycle. The biophysical functions of plant-related elements have been well-documented and presented extensively in textbooks and review articles 
. Not all of these elements are considered as nutrients, but all of them have a positive impact on the yield of crop plants 
. A typical example is titanium (Ti), the positive effect of which on many crops has been recently documented 
N, considered especially in the form of nitrate (as discussed in the previous section), is the key nutrient, affecting both the rate of plant growth and the formation of yield components. The key evidence, in addition to that discussed above, is the response of the yield to the application of N–P–K fertilizers in various mutual fertilization systems. The effect of the interactions between N and other basic nutrients has been well-presented in long-term static fertilization experiments 
. Winter rye cultivated on Luvisol in a 7-course crop rotation (including two years of alfalfa) for 40 years yielded on plots without K (NP) or P (NK) only 6% and 5% less than on the NPK plot. The lack of both nutrients (i.e., K and P), as evidenced on a plot fertilized only with N, resulted in a yield drop of only 2.5%. The yield on the absolute control (AC) plot (i.e., the plot without application of any fertilizer, mineral or organic) for 40 years, was 30% lower than that on the NPK plot. Moreover, the same level of yield was recorded on plots fertilized only with P or K. Slightly greater differences between fertilization treatments were recorded for spring barley. Relative yield reductions were: −10%, −7%, and −42%, for NP, NK, and AC, respectively, as compared to NPK. The same yield level as for AC was noted on plots fertilized only with P or K. It must be added that the yields on the N plot were lower by 10%, compared to NPK 
. The above-documented trends of plants grown on the naturally low fertility soil (Luvisol), with respect to different combinations of basic nutrients, have been supported by data from fertile soils, such as Entisol in the Netherlands (calcareous Entisol, containing 30% clay, 10% CaCO3
. The obtained data indicated that a lack of K fertilization over a period of 28 years did not adversely affect the yield of four crops grown in four-course crop rotation (sugar beets, spring barley, potatoes, and winter wheat) over 28 years. The lack of P was much more important, as its lack reduced the yield of sugar beets by 11%, but those of potatoes and spring barley by only 7% 
. These two examples clearly show that the main source of P and K for crops is soil.
All plant nutrients support plant growth and yield formation through their impact on the productivity of N which, to a great extent, depends on the application of Nf
. It can be concluded that it is unrealistic to expect an increase in the yield of a modern variety, as discussed above, without delivering N from external sources, whether natural (e.g., manure) or mineral. For these reasons, all nutrients affecting plant growth and yield can be called “nitrogen-supporting nutrients” (N-SNs). Based on agronomic practice, the whole set of N-SNs can be divided into four groups: (i) basic macronutrients, such as potassium (K) and phosphorus (P); (ii) secondary macronutrients, including magnesium (Mg), sulfur (S), and calcium (Ca); (iii) micronutrients, such as iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (N), molybdenum (Mo), and chloride (Cl); and (iv) the beneficial group, composed of nickel (Ni), sodium (Na), silicon (Si), and titanium (Ti) 
The second important stage in the development of an economically and environmentally sound fertilization system using Nf requires knowledge regarding the patterns of accumulation of N-SNs during the growing season. There are some characteristics “hotpoints” in N-SN in-season patterns that require special attention by the farmer, based on the course of N accumulation. The most important points are:
The higher accumulation of K over N during the whole season of WOSR growth:
Starting with the rosette stage in Spring;
Achieving the maximum K uptake over N at the full flowering stage (K2O:N as 1:1.6);
Declining from flowering to maturity (K2O:N as 1:1).
Slow early growth in P uptake, continued up to the inflorescence stage (BBCH 50), followed by rapid ingrowth, lasting until the full flowering stage (BBCH 65), and then smoothly decreasing up to maturity.
A similar pattern for Mg as for P, but at a much lower level.
A spectacular pattern of Ca accumulation. Its uptake increases sharply at inflorescence, reaching a maximum at the end of the pod growth stage.
The presented patterns of N, P, and K accumulation by WOSR during the growing season, obtained at the end of the 20th century, were very similar to their trends observed in the 1980s 
. The same pattern has been observed for high-yielding rice 
. The effective production of maize depends on the supply of K and N during the period preceding flowering, but requires stabilization of K accumulation up the milk stage 
. It can be concluded that the high yield of the seed crops can be achieved, provided that the K:N ratio is higher than 1.0 during the vegetative phases for seed plant growth. In the light of the available data, the maximum K:N should be revealed for the high-yielding crop just before the end of the linear phase of its growth 
. The seasonal pattern of basic nutrient accumulation by legumes is only slightly different. As observed for soybean, the maximum K was at the late vegetative stages, and the level then decreased smoothly, while the accumulation of N, P, Ca, Mg, and S progressed through to maturity. Moreover, the maximum K:N ratio was 0.6:1 
4. An Efficient System for Management of N-SNs—Principles of the Soil Fertility Clock
4.1. State of K and P Fertility Level and Food Production
The crop production potential of a single field in inherently related to its soil fertility level, presented as the depth of the humus profile and the content of available nutrients 
. Fertile soil creates conditions for the build-up of a large (extensive) root system, which is crucial for the uptake of non-mobile nutrients, such as phosphorus and potassium 
The share of N, P, and K fertilizers in total nutrient uptake by cereals has been assessed as 33% for N, 16% for P, and 19% for K 
. In China, as reported by Ren et al. 
, K fertilizer covers less than 20% (18.5%) of the total K in winter oilseed rape at harvest. Khan et al. 
, who studied 1400 field trials fertilized with K, did not observe any significant impact of the applied K (as KCl) on the yield of basic crops. According to MacDonald et al. 
, 29% of the world area of arable soils shows a deficit, while the remaining part shows a surplus of available P. The significant impact of P fertilizer can be revealed, as a rule, on soils with a low content of available P. The yield loss due to deficiency of P supply (P yield gap) to wheat is 22% (18–28%), 55% for maize (47–66%), and 26% for rice (18–46%). Moreover, the application of P fertilizers reduced this production gap to only 17% for wheat, 46% for maize, and 15% for rice 
. At this point, it is necessary to ask whether the yield gap is actually due to a deficiency of available P, or the ineffectiveness of Nf
due to the imbalance of P and other nutrients.
The key question to be formulated is: what is the appropriate level—or rather, the critical range—of N-SNs content in the soil? In the light of the facts presented above, classic P and K management strategies require significant modifications. The basis for these required corrections is the fact that the plants are grown in a cropping sequence determined by the economic goals of the farm. Not all modern crop sequences follow rational principles (i.e., biologically based crop plant succession) 
. It has been well-documented, in millions of scientific articles, that the deviation of a particular cropping sequence from biological rules leads to a decrease in yield. The classic example is the Rothamsted long-term experiment with winter wheat 
. Wheat followed directly by wheat or grown in monoculture yielded a significantly lower level, compared to that following dicotyledonous plants. Second, the maximum grain yield achieved under non-optimal rotation was, in this example, both lower and, at the same time, required a higher Nf
rate. These results clearly indicate lower unit Nf
productivity due to N immobilization, disturbance in uptake of N-SNs, and stronger pressure by pathogens 
The soil resources of P and K are the main source of nutrients for the cultivated crop. Over-exploitation of available pools of these nutrients leads to the degradation of soil fertility, subsequently resulting in the lower Nf
productivity. Moreover, these processes create a multi-level risk, for the yield, farm economics, and the environment (Photos 2 and 3; 
). The productivity of a particular field is determined by the level of soil fertility, conditioned by water capacity and the content of available N-SNs in the rooting zone of the currently cultivated crop 
. At present, the subsoil resources of N-SNs are not included in typical soil fertility status diagnostic procedures. Moreover, there is a frequently presented opinion regarding their low significance for the plant growth and yield 
. In the light of the published data, the share of P and K from fertilizers for crop plant nutrition is of secondary importance for the maintenance and synchronization of plant needs and nutrient supply from the soil. The logical conclusion to be drawn from these facts is unambiguous: the farmer’s goal for effective management of Nf
is not to use P and K fertilizers or other carriers of these nutrients for direct feeding of the plant, but to coordinate the soil fertility level to maximize Nf
4.2. Management of Soil Fertility—Oriented to Cropping Sequence
Soil productivity is the ability of arable soil to provide the currently grown plant with air, water, and nutrients in the required amounts, mutual proportions, and ratios, ensuring full expression and development of the yield components 
. This general property of arable soils has been indicated as one of the most important objectives listed in the Sustainable Development Goals (SDGs) by the United Nations in the 2030 Agenda for Sustainable Development 
. This global goal can be achieved, but only through two targeted actions. The first is oriented towards stopping the degradation of soil fertility. This action refers to the world regions where soil fertility has been drastically reduced 
. The second requires a significant correction in N-SN management strategies in areas of the world with advanced crop plant productivity. The so-called Old Agricultural Areas of the world will be decisive for food supply to the growing human population in the coming decades.
At present, two main concepts dominate in soil fertility management. The first—called the maintenance approach—is based on the assumption that the main goal of N-SNs is necessary to maintain the content of available nutrients at a certain level, allowing crop growth and yield. Three phases of soil fertility build-up can be distinguished using this approach: (i) build-up, (ii) maintenance, and (iii) draw-down 
. In practice, the recommended rates of nutrients increase with the size of the gap between the maintenance level and the actual soil fertility status for a given nutrient. In most countries, using this fertilization approach, the nutrient doses recommended by agrochemical testing laboratories are consistent with the state of its deficiency. A classic example is the K recommendation in China for WOSR yielding at 3.75 t ha−1 
. The decreasing content of available K (NH4
OAc-K extraction method) resulted in increasing the dose of applied K from 232 kg ha−1
at low K range to 50 kg ha−1
at high K range. The second approach, called sufficiency ranges, is based on the required (i.e., sufficient) level of the given nutrient for the respective crop 
These two fertilization strategies are based on the assumption that low-mobility nutrients are as effective as nitrate nitrogen 
. The coefficient of effective diffusion for the N form is 2.7 × 10−1
. In comparison, this index for K+
ions are about 100-fold lower (1–28 × 10−8
and 6.1 × 10−8
, respectively). For H2
ions, this index is 10,000-fold lower, compared to nitrates 
. Within 7 days of sugar beet growth in July, the N-NO3
resources, but not K, were completely depleted (100%) to a depth of 1.8 m. For K, this level of depletion was reached at a depth of 0.5 m. The most intensive uptake of both nutrients took place in the soil layer (0.0–0.6 m).
The classic concepts of N-SNs do not take into account two crucial facts; that crop plants differ in their sensitivity to the supply of N-SNs:
The efficient management of N-SNs in a soil/plant system should be based on the following principles of crop production:
Annual crop plants should be cultivated in a fixed sequence (i.e., the crop rotation).
Cereals have, as a rule, lower requirements for K, but higher requirements for P, compared to non-cereal crops.
Dicotyledonous plants have higher requirements for K than cereals.
The architecture of the root system of cereals is, as a rule, extensive compared to dicotyledonous plants. Consequently, cereals are less sensitive to the level of P and K fertility.
The distribution of low-mobility nutrients varies with depth.
Plants during the growing season differ significantly in the critical stages of nutrient requirement:
Seed crops show critical periods, in terms of P requirements, during the vegetative (minor one) and reproductive (main one) periods of growth;
All crops are sensitive to K during the linear phase of growth.
The critical period for N requirements by a seed crop is related to stages of seed/grain density formation.
The key yield-forming function of P in all crops is to accelerate the early rate of the plant growth.
The exploitation of P resources by seed crops, accumulated in vegetative parts before flowering, depends on seed/grain density.
The yield-forming function of K in
All of these points should be taken into account by the farmer during the process of development of the fertilization system.
Soil Fertility Clock (SFC) is an approach based on three assumptions which are key to the effective management of Nf:
Critical soil fertility is the value or range of a soil nutrient’s content that is sufficient to provide it in the appropriate amount to the plant most sensitive to its supply in a given crop rotation.
Other, non-sensitive plants in the given crop rotation create the necessary time-frame for recovery of its original critical content.
The content of a specific nutrient cannot be a limiting factor in N uptake and utilization for any crop grown. Its fractional use efficiency, regardless of the actual plant in crop rotation, is ≤1.0.
The critical K level for oilseed rape or any other dicotyledonous crop creates favorable conditions for the succeeding crop; that is, cereals, and most often wheat. The K level will still be high enough to cover the K requirement of wheat. A third crop in a certain cropping sequence—for example, maize—requires the farmer’s attention to adjust the K content. This is necessary only if the K content drops below the medium level. This is very probable when harvest residues are removed from the field. The critical period of K correction, which must be oriented toward the so-called crop rotation critical K level is, in the discussed case, the spring barley growing season. This is a key term in the agronomic clock for determining both the level of K in the soil and determining the fertilization needs for the plant sequence spring barley → winter oil-seed rape. Managing P in crop rotation is a bit more complicated. It requires, regardless of the grown crop, the use of a starting dose of P fertilizer. Basic P fertilization complies with the principles presented for K. An additional component of an effective system regarding the full set of nutrients is foliar fertilization. This method allows for the correction of plant nutritional status—in fact, N action—but only at stages preceding the critical stages of yield formation.