The Function of Macronutrients in Helping Soybeans to Overcome the Negative Effects of Drought Stress

Abstract

Nutrient deficiencies are a major cause of yield loss under abiotic stress conditions, so proper nutrient management can reduce the negative effects of stress to some extent. Nutrients can alleviate stress by activating resistance genes, enhancing antioxidant enzyme activity, creating osmoprotectants in cells, reducing reactive oxygen species (ROS) activity, increasing cell membrane stability, synthesizing proteins associated with stress tolerance, and increasing chlorophyll content in leaves. The current review highlights changes in soybean metabolic activity caused by drought stress and changes in vital functions caused by the deficiency of primary (N, K, P) and secondary macronutrients (Ca, Mg, S). The role of macronutrients in reducing the adverse effects of water deficit stress is highlighted. Under stressed conditions, appropriate nutrient management options can be implemented to minimize the effects of drought and ensure good yields. Balanced nutrient fertilization helps activate various plant mechanisms to mitigate the effects of abiotic stresses and improve soybean drought resistance/tolerance. Nutrient management is therefore a viable technique for reducing environmental stress and increasing crop productivity.

1. Introduction

Soybean (Glycine max (L.) Merrill) is one of the world’s oldest and most valuable crops. It is at the forefront of the world economy as a major oilseed crop due to its high production potential and the extensive use of its seeds. The unique soybean seeds contain about 40% protein with a favorable amino acid composition and high nutritional value, and about 20% fat, most of which is constituted of unsaturated fatty acids, mainly linoleic, oleic and linolenic. They are also a source of many valuable compounds such as fibre, lecithin, vitamins (mainly E, B1, B2), minerals (K, Ca, Mg, P, Fe, Zn) and antioxidants [1,2]. Soybean is mainly used for oil production, which covers approximately 30% of the world demand for consumer vegetable oil [3]. Soybean meal remaining after oil extraction is used as a high-protein animal feed [4]. Soybean is also used in the pharmaceutical, cosmetic and chemical industries [5]. Although soybean is classified as an oilseed plant, from a botanical point of view, it is a legume, which makes its cultivation beneficial for the soil environment. Due to its symbiosis with the bacteria Bradyrhizobium japonicum, it fixes atmospheric nitrogen, enriching the soil with this component and reducing mineral nitrogen consumption [6]. In addition, it improves the physical properties of the soil due to its extensive root system and increases the biological activity of the soil due to the large amount of nutrient-rich crop residues. This makes it an excellent forecrop for other crops, e.g., wheat and maize [7]. This pro-environmental effect of soybean is of great importance in the context of climate change and reducing carbon footprints [8].

Due to its versatile use in terms of cultivated area, soybean currently ranks fourth in the world after wheat, rice and maize. Over the past 20 years, global soybean seed production has increased almost twice and will exceed 373 million tons in 2021, with a cultivated area of 133.8 million ha [9]. Soybean is mainly grown in North and South America, with the US and Brazil being the largest producers and exporters of soybean in the world. China, in turn, leads the world in soybean seed imports, and European Union countries purchase significant quantities of soybean meal.

According to UN projections, the world’s population will exceed 9 billion in 2050, with an increase in demand for food [10]. In order to ensure food security for the growing population, an increase in plant productivity must be pursued, as the potential for expanding land area is severely limited. Soybean is sensitive to climatic conditions (temperature, day length, rainfall, solar radiation), but also to habitat conditions (soil fertility, pH, drought, excess water, salinity, mineral nutrition) and biotic stresses (pests, weeds, diseases, nematodes). Stress factors disturbs plant growth and development, metabolic processes taking place in the cell and physical and chemical properties of cellular structures, resulting in the reduction in vital processes and inhibition of plant growth, and consequently, a decrease in yield and deterioration of its quality [11,12,13,14,15]. In addition, the formation of root nodules as a result of symbiosis with the bacteria B. japonicum and biologic nitrogen fixation (BNF) is economically and ecologically important, but optimal conditions for this process are difficult to obtain. Stress factors such as drought, soil compaction, low pH or nutrient imbalance can inhibit nodules formation and reduce BNF [16].

According to Lisar et al. [17], soil water deficiency can reduce crop yields by up to 50% in different parts of the world. Climate change is causing summer precipitation in Central and Eastern Europe to decrease and the number of days with extreme temperatures to increase [18]. The recent heat wave and drought in 2018 led to an 8% decrease in cereal production in Europe compared to the previous year, resulting in shortages of livestock feed and a sharp increase in commodity prices (wheat and barley prices increased by 34% and 48%, respectively) [19]. According to Vogel et al. [20], climatic factors during the growing season, including climate and extreme weather events, explain between 20% and 49% of the variance in yield anomalies, depending on the crop. As soybean cultivation has a wide geographic range worldwide, understanding the impact of environmental factors on soybean yield and seed quality under different climatic conditions is of paramount importance to fully exploit the yield potential of this species [21].

2. Soybean Water Requirements and Response to Drought

Water scarcity is one of the most important environmental factors that limits the yield of crop plants, including soybean. The impact of drought stress on crops depends mainly on the species, cultivar, developmental stage and severity of the stress [21]. Soybean tolerates short drought periods well. Genetic adaptations of soybean to survive adverse conditions play a major role, including hairiness on leaves and stems, which reduces excessive transpiration, and a well-developed root system that allows water and nutrients to be taken up from deeper soil layers. Furthermore, studies have shown that under soil water-deficit conditions, the root-to-stem ratio increases [22]. In addition, a heliotropism phenomenon is observed in soybean, whereby the plant heats up less and reduces transpiration under conditions of high temperatures and periodic water shortages. The relative drought tolerance of soybean is evidenced by a study of Tabrizi et al. [23], which showed that reducing the irrigation of soybean by 25% compared to an optimally irrigated control maintained yields of over 90%, with an increase in water productivity from 0.44 to 0.56 kg m⁻³.

Soybean water requirements for maximum production vary from 450 to 700 mm per season, depending on climate, length of the growing season and crop management practices [9]. In soybean cultivation, it is possible to distinguish critical periods associated with increased water requirements. One of these is the germination and emergence period. During germination, the soil water content should not exceed 85% or fall below 50% of the available soil water. During this period, the seeds take up significant amounts of water, increasing their weight by 120%. Lack of water during this period results in poor and thinned emergence and incomplete plant density, which may later result in reduced yield. In a study by Heatherly [24], drought during the soybean seed germination period resulted in a reduction in plant density of about 20%. The effect of drought stress on soybean seed germination rate is greater when there is a higher air temperature, but this is also a genetic trait related to cultivar [25].

During the vegetative phase, biomass accumulation and plant growth occur, depending on the temperature and availability of water and nutrients. The root also grows rapidly and vigorously. The duration of the vegetative growth phase depends on the maturity group of the varieties. Later maturing cultivars show greater biomass production than earlier cultivars due to their longer growing season. Excess or shortage of water during this period delays plant growth, but plants can withstand short periods of drought well. Prolonged drought can result in shorter internodes in the stems, which translates into fewer flowers and pods produced [26].

The appearance of the first flowers marks the beginning of the generative phase. The flowering phase in soybeans lasts a relatively long time, about 20–40 days, but more than 70% of the flowers form within the first 2 weeks. During this period, the root also grows very vigorously and lateral shoots form. The taproot of the soybean can reach a depth of more than 1.5 m. The flowering and pod-setting phase is the second critical period in which water requirements increase significantly [26]. Water shortage during this period results in flower and young pod rejection, but similar symptoms can also be caused by over-irrigation during severe water shortages. The apparent resistance of plants to drought at the flowering and early pod-setting stage is related to the long flowering period of soybean. Mild water stress during the flowering period can be compensated for by soybean setting larger pods from later flowering flowers [27]. The number of seeds per pod and the number of pods are closely dependent on the rate of photosynthesis during this period and the factors that determine it, namely the availability of water, light and CO₂ [21]. Pods generally contain one to three seeds, but in better sites and with a favorable rainfall distribution, they can develop up to four seeds.

During the pod-filling and seed development phase, the root stops growing and the plants reach their maximum height, number of nodes and leaf area. This phase accounts for almost 40% of the total plant growth cycle. In addition to the ongoing products of photosynthesis, previously stored carbohydrates, which account for about 15% of the seed weight, are moved to the seed. Water requirements are greatest at this stage of plant development. Water deficiency during this third critical period directly affects yield, as it reduces seed weight and contributes to earlier maturation, which consequently reduces yield [26]. In addition, insufficient water supply shortens the pod-filling phase by reducing leaf area (by about 40%) and accelerating leaf drop, which is an irreversible process [28]. Studies indicate that drought-induced seed yield loss is greatest during this period and can range from 15 to as much as 50%, depending on the length and severity of the stress and air temperature [29,30]. Water deficit at this stage results in a decrease in the number of pods per plant, number of seeds per pod, 1000-seed weight and consequently seed yield [26,29]. Dornbos and Mullen [25] showed that after severe drought, the proportion of seeds larger than 4.8 mm in diameter decreased by 30–40%, while the proportion of seeds smaller than 3.2 mm in diameter increased by 3–15%.

Soybean yields vary depending on cultivar, water availability, fertilization and row spacing. Under optimal moisture conditions, 1.5 to 2.5 t of seed per ha is obtained, intensive cultivars yield 2.5 to 3.5 t per ha. The decrease in soybean yield associated with water deficit relative to the maximum yield obtained under optimum conditions can range from 24 to 50% depending on location and time [29,31], and in extreme cases, up to 70% [32]. Therefore, in order to maintain food security, efforts should be made to minimize the harmful effects of drought [33].

3. Impact of Nutrient Management on Soybean Responses to Water Deficit

Stress factors limit the ability of plants to reach their potential yields. By optimizing plant growth and development conditions, the effects of stresses can be mitigated to some extent. Water and nutrients are crucial for proper plant life processes, which directly affect plant growth and yield [34]. Optimal nutrition and proper soil cultivation significantly affect the water cycle in plants, which is an effective method to combat drought. Under low soil nutrient concentrations, plants need to absorb more water to take up the same amount of nutrients for their metabolism than would come from a fertile soil with optimal nutrient supply. On the other hand, under water-deficient soil conditions, plants are unable to take up sufficient nutrients despite an adequate supply of nutrients in the soil, which adversely affects overall plant health, growth, development and yield [33,35].

Of the 17 elements recognized as essential for any living organism, the primary macronutrients nitrogen (N), potassium (K), phosphorus (P) (Figure 1) and the secondary macronutrients calcium (Ca), magnesium (Mg) and sulfur (S) play the most important role in reducing the negative effects of water stress on soybean growth, development and yield (Figure 2).

3.1. Primary Macronutrients

3.1.1. Nitrogen

Nitrogen (N) is one of the most important nutrients that plants need in the greatest quantities, and its lack of availability is a major factor limiting plant growth and development [37]. Nitrogen is involved in many physiological and metabolic processes, is crucial in the structural structure of plants, and is a major component of proteins, enzymes and nucleic acids [38]. It is a component of chlorophyll, cytochromes, phytohormones (e.g., cytokinin, auxins) and some vitamins, and is part of secondary metabolites such as alkaloids, mustard oils, cyanogenic glycosides. It is involved in almost all transformations occurring in plant cells. It is a component of nucleotides (ATP and ADP) responsible for energy storage and transfer in cells. It ensures the plant’s efficient utilization of sunlight and adequate growth dynamics. It is involved in various critical processes such as growth and biomass production [39,40,41]. This element has a decisive impact on both quantity and quality of yield [42,43,44,45,46].

Nitrogen is one of the main components of the photosynthetic apparatus. Plants invest huge amounts of N in their photosynthetic mechanisms [47]. The rate of photosynthesis correlates strongly with tissue N concentration, which can be attributed to the need for large amounts of the CO₂ fixing enzyme, RuBisCo [48]. Providing N to plants at optimal rates positively influences plant adaptive responses to drought, including osmotic adjustment and homeostasis of reactive oxygen species, the content of free proline, soluble sugar and superoxide dismutase activity. Under normal N supply conditions, higher activities of antioxidant enzymes, including superoxide dismutase and glutathione, are observed compared to low N supply, which effectively inhibits ROS accumulation and consequently plant damage caused by drought stress is less [49].

Soybean is a crop with a high N requirement, which is related to the high protein content of the seeds (about 40%) [50,51], in addition to a relatively large nitrogen harvest index (NHI) [52]. The total amount of N assimilated in the plant is strongly correlated with soybean seed yield (approximately 70–90 kg N is required per ton of seed) [16]. In order to meet the high demand for N throughout the life of the plant, soybean has the capacity to store this nutrient in vegetative parts (leaves, stems, petioles, and pods) [52]. Under N-deficient conditions, soybean plants remobilize N from the leaves to the seeds, which induces leaf senescence and reduces photosynthesis, resulting in reduced yield [53]. Islam et al. [54] showed that low nitrogen fertilization (5 mg dm⁻¹) at the R6 to R7 soybean development stage did not inhibit plant aging, although a reduction in leaf greenness index (SPAD) and leaf N content was observed. It has been proven that N deficiency can stimulate leaf senescence, while increasing N availability in the soil can delay or even stop leaf senescence, indicating that N availability may be a key regulating factor of monocarpic senescence in soybean. In rainfall-deficit conditions in Central Europe, a higher application of N combined with early sowing was an effective way to increase soybean productivity the efficiency of nitrogen binding by extending the period of effective N uptake, especially in drought years [55].

Soybean, as a legume, is able to partially meet its N requirements through the process of biological N fixation (BNF)_, which involves the activation of N₂ from the air and its reduction to NH₃, followed in the plant by its incorporation into organic compounds [56,57]. Due to the symbiosis with the rhizobium bacteria B. japonicum, soybean plants cover about 50–70% of their requirements for this nutrient [16,53]. Under conditions of low soil N content, BNF can provide 86 to 90% of the total N requirement of soybean [58,59]. Effective BNF process is evidenced by the presence of red colored leghemoglobin in the nodules [60,61]. Enhancement of the BNF process is observed after seed inoculation with B. japonicum [45,62], and the survival of the bacteria in the soil reaches up to 10 years, depending on edaphic conditions [63].

The BNF process provides a number of economic and environmental benefits by eliminating the need for mineral fertilizers [57]. The intensity of the BNF process in legume plants depends on many factors, including legume performance, environmental conditions, soil nutrients availability, contaminants, bacteria abundance and diversity in soil, and bacteria specificity and infectivity [64]. Abiotic stresses due to high temperatures, water shortages, low soil fertility or low soil pH can be barriers to the efficient BNF process [62,65,66,67]. One of the most important stress factors negatively affecting the BNF process is drought [68]. As soil water content decreases, soybean BNF rate decreases in response to reducing the activity of various physiological processes [69,70,71,72,73]. In turn, reduced nitrogen availability adversely affects cell and tissue development throughout the plant and biomass production [74]. Even minor soil moisture deficiencies can reduce BNF by soybean plants, accelerate N remobilization and reduce N content in vegetative tissues during seed filling [75]. Streeter [76] showed a significant reduction (by 30–40%) in N content in leaves and pods of the effect of drought stress, with the negative effect of drought on nodules function not being the cause of stem growth inhibition. At the end of the drought period, the content of carbohydrates, amino nitrogen, and ureides significantly increased in root nodules of plants subjected to drought stress. The authors conclude that BNF in nodules was reduced under drought conditions because the need for fixed N to promote growth was lower.

According to Freeborn et al. [77], N supply through BNF and soil organic matter mineralization are sufficient for high soybean yields. However, the activity of BNF can be limited by a number of environmental conditions, so in the case of BNF deficiency, the availability of this nutrient through mineral N fertilization should be ensured to soybean to obtain a satisfactory yield [78,79]. In the early developmental stages of soybean, a starter dose of nitrogen of about 30 kg N ha⁻¹ is recommended to overcome N deficiency at a time when the source of N contained in the cotyledons is depleted and plants have not yet formed nodules capable of supplying BNF to the plant. Small amounts of N applied as a starter dose before sowing generally have a positive effect on initial plant growth by improving root growth even before modules formation. Under severe water stress, seed yield of soybeans grown without fertilization and seed inoculation was 44% lower than with a starter dose of 20 kg N ha⁻¹ N and seed inoculation with Azotobacter chroococcum [29]. This shows that a starter dose of N combined with seed inoculation with symbiotic bacteria improves yield, especially under soil water deficit conditions.

Fertilization of soybean with mineral N at later developmental stages is sometimes necessary to achieve a satisfactory yield when BNF is unable to provide sufficient nitrogen to the plants. However, it is important to remember that additional N application generally reduces nodules and BNF, although this is also influenced by other factors such as the dose and form of N, the timing of application or the developmental stage of the plant [62,67,80,81,82,83]. In the study of Purcell and King [79], no beneficial effect of supplementary N application on seed yield was shown under optimum soil moisture conditions, while such an effect was observed under drought conditions due to lower flower and young pod fall. This demonstrates the high sensitivity of rhizobium to water deficiency, but also proves that good plant N nutrition increases drought tolerance. In a study by Minguez and Sau [72], soybeans grown under water stress for 20 days at the five to six trifoliate leaves stage (V6–V7) developed a larger root system after mineral N fertilization and had a lower transpiration rate per unit leaf area than unfertilized soybeans. According to the authors, the N-feeding treatment triggered stress-avoidance-directed mechanisms in the plants: reduced transpiration rate, closure of stomata at higher leaf water potentials and delayed onset of osmotic regulation. Shoot development in severely stressed plants was inhibited, with root development and reduced shoot-to-root ratio observed in N-fertilized soybean. Also, Basal and Szabó [84] report that under drought conditions, when BNF is inhibited, additional N doses can be crucial. The authors showed that drought (non-irrigated regime) negatively affected plant height, leaf area index (LAI), relative chlorophyll content (SPAD) and, to a lesser extent, normalized difference vegetation index (NDVI). Increasing nitrogen fertilization to 105 kg N ha⁻¹ generally had a positive effect on the development of these traits at the fourth node (V4), full bloom (R2), full pod (R4), and full seed (R6) stages, regardless of whether the soybean plants were subjected to drought stress or not. Also, other authors confirm a decrease in LAI due to water deficit during the seed-filling phase, a phenomenon that can be mitigated by N fertilization and/or inoculation with B. japonicum bacteria [62,85].

Soybean cultivation even under unfavorable environmental conditions can be successful with appropriate soil preparation and fertilization. Cordeiro and Echer [62] confirmed that soybean cultivation in unfavorable edaphoclimatic environments (high temperature, low fertility, and areas with sandy soil) on a site previously covered with degraded pasture (post-pasture area) yields good results with N fertilization at 50 kg ha⁻¹ combined with B. japonicum inoculation increasing seed yield by 22%. Increased drought tolerance of soybean after N fertilization has also been reported by other authors. Purcell and King [79] observed decreased flower and/or sub-abortion, increased number of seeds per unit area, increased seed growth rate and reduced seed filling time under soil water deficit conditions as influenced by N fertilization, resulting in higher yields compared to plants dependent on BNF alone. According to Kulig and Klimek-Kopyra [55], in dry years, higher N application combined with early sowing can be an effective way to increase soybean productivity and BNF efficiency by extending the period of effective N uptake by plants.

On the other hand, other authors observed a negative effect of soybean fertilization with high single doses of mineral N on BWN and seed yield. A study by Bobrecka-Jamro et al. [83] showed that a single pre-sowing application of 60 kg N ha⁻¹ reduced soybean seed yield by up to 15% compared to a split application: 30 kg N ha⁻¹ pre-sowing and 30 kg N ha⁻¹ at the BBCH (Biologische Bundesanstalt, Bundessortenamt und Chemical Industry) 59. Other authors report that negative effects of mineral N fertilization of soybean were observed when N was applied to the soil surface or in the uppermost layers of the soil. Alternatives may include deep placement of N, with slow release below the nodulation zone, or late application of N at the generative development stage. These may be promising alternatives for achieving a yield response to N fertilization in high-yielding environments [54]. An important factor that allows to overcome the negative effects of drought by using macronutrients is the appropriate application time. According to many authors, the most beneficial way to maintain a high level of photosynthetic capacity of soybean, which contributes to increased yield and nitrogen use efficiency, is to apply nitrogen fertilization before sowing (as basal fertilizer at a dose of 1.8 g N/m²) in combination with top dressing in the R3 phase (4.2 g N/m²) [86]. This is also confirmed by Bobrecka-Jamro et al. [83], in which the application of nitrogen fertilizers before sowing (dose of 30 kg N ha⁻¹) and in the BBCH 59 phase, i.e., in the inflorescence development phase immediately before the flowering of the plants (dose of 30 kg N ha⁻¹), gave the best results in increasing the yield of soybean seeds.

3.1.2. Potassium

One of the main nutrients that is considered essential for plant growth and development is potassium (K). Plants require K at a rate of approximately 2–5% of plant dry weight for proper growth and development and productivity [87]. Although K is not part of any cellular organelles or structural parts of the plant, it is involved in a number of physiological processes that promote plant growth and development and control plant water management. Potassium regulates physiological processes such as photosynthesis, affects turgor pressure and activates enzymes [88]. According to many authors, under soil water-deficit conditions, the adverse effects of drought can be minimized by increased K fertilization [89,90]. In a study by Abd El-Mageed et al. [91], application of K at rates of 120 and 150 kg ha⁻¹ under water stress conditions significantly increased soybean seed yield by 14 and 30%, respectively, compared to the rate of 90 kg ha⁻¹. Furthermore, increased K fertilization improved plant morphological traits (shoot length, number and leaf area) and physiological indices (chlorophyll fluorescence and WUE index). Another study showed that K minimized the negative effects of drought related to leaf drop and pod abortion, which consequently contributed to increased yield under water deficit [92].

Potassium is involved in maintaining osmotic balance in plant cells and tissues. Under drought conditions, water availability is limited, so plants, in order to maintain an adequate level of turgor, have to carry out osmotic regulation, e.g., by accumulating various types of substances called osmolytes, which can be organic acids, carbohydrates, free amino acids or inorganic ions [93]. Of the inorganic ions, it is potassium (K⁺) that plays a significant role in osmotic regulation, helping to maintain cell turgor and stability, while also providing a mechanism for drought tolerance [92]. Under drought conditions, osmotic regulation is related to maintaining photosynthesis, stomatal conductance, leaf hydration status and plant growth rates at appropriate levels.

Accumulation of K⁺ ions helps to protect plants from the effects of drought not only through osmotic regulation, but also through the post oxidation of reactive oxygen species (ROS) and the regulation of physiological processes related to gas exchange (higher photosynthetic intensity) or the stomatal opening [94]. Under water-deficient conditions, stomatal conductance is reduced, resulting in reduced intercellular CO₂ and accumulation of ROS in chloroplasts [95]. When K availability is low, free radical production can be increased, because the mechanism of stomatal opening and closing is disrupted, which limits photosynthesis. Therefore, under drought conditions, soybean has an increased requirement for K to maintain photosynthesis at an adequate level and protect protoplasts from oxidative damage [90]. Potassium also protects cell membranes from damage caused by drought stress. This reduces water loss from leaf tissues and makes cells more resistant to ROS [92]. In soybean, 19 metabolic pathways are upregulated in response to K⁺ availability, including the isoflavonoid biosynthesis pathway most strongly [96]. Under conditions of very low K⁺ availability, plants accumulate Ca²⁺, Mg²⁺, Fe²⁺, Cu²⁺ and B ions in the leaves, while the movement of NO³⁻, PO₄³⁻, Ca²⁺ and Mg²⁺ ions is limited [97]. Optimal K fertilization of soybean also improves the stability of photosynthetic mechanisms by preventing the destruction of photosystem II (PSII) due to excessively absorbed solar energy [94]. Potassium is also involved in the movement of assimilates from leaves to roots. Under K deficiency, the transport of assimilates is limited [90].

Adequate K fertilization influences the growth of structural plant elements such as height, leaf area and dry matter production. Plants fertilized with higher K doses are better able to cope with water shortages, which translates into greater drought tolerance, associated with more efficient water management. Potassium may also promote better plant recovery after stress, although the speed and extent of recovery after re-wetting depends mainly on the intensity and duration of the stress [98]. In a study by Soleimanzadeh et al. [99], in severely stressed plants, the photosynthetic rate the day after re-wetting was about 40–60% of the maximum value, with higher values characterizing plants fertilized with higher K doses, which may be related to the role of K in repairing oxidative damage to cells and minimizing yield losses.

3.1.3. Phosphorus

Phosphorus (P) is an essential plant nutrient, and its deficiency limits plant growth and biomass accumulation by reducing photosynthetic rates and stomatal conductance [100,101]. Soybean mainly take up P through the root system in the form of inorganic P, which is then transported to the shoots where it is used in metabolic processes [102]. Soybean roots can regulate P uptake through physiological metabolic responses to adapt to the stress of low soil supply of this macronutrient [102]. Soybean roots can regulate P uptake through physiological metabolic responses to adapt to the stress of low soil supply of this macronutrient [103,104]. Low P stress can promote the formation of new roots by soybean to increase the uptake area of P and other nutrients, and can also stimulate plant secretion of organic acids into the rhizosphere and the conversion of plant inaccessible forms of P into available forms in the soil to improve P utilization rates [105,106]. However, the most effective way to increase P availability is to fertilize with this nutrient. Optimal P supply to plants contributes to water use efficiency [107] and increases plant tolerance to drought [108]. Phosphorus fertilization mitigates the negative effects of drought on soybean yield while improving N metabolism. The increase in soybean seed yield due to P fertilization is mainly due to an increase in P and N accumulation, which is associated with an increase in root length after P application [101]. Drought and impaired P uptake by plant roots are the main factors contributing to P deficiency, a visible symptom of which is stunted leaf growth. When P levels are low, the intensity of photosynthesis decreases due to reduced production ATP and NADPH, which results a reduced ability to regenerate RuBP, which is crucial in the CO₂ assimilation process [109]. According to Rasnick [110], P translocation from roots to shoots is severely reduced even under mild drought stress. The application of P fertilizer can significantly improve plant growth under drought conditions [108]. The positive effect of P on plant growth during drought is attributed to an increase in stomatal conductance, photosynthesis, higher stability of cell membranes and improved water relations, resulting in greater drought tolerance [111].

Phosphorus is a component of nucleic acids, phospholipids, phosphoproteins, dinucleotides and ATP. It is the main element involved in energy processes in plants. The demand for P is crucial because of the role of ATP in regeneration processes [112]. It is essential for energy storage and transfer processes, regulation of certain enzymes and carbohydrate transport [113]. It is also necessary for the regulation of metabolic pathways in the cytoplasm and chloroplasts and in the synthesis of carbohydrates (sucrose and starch). Phosphorus in the form of ATP is involved in the biological process of BNF in legumes, including soybean [114]. Hungria et al. [115] showed that P deficiency in low fertility soils can inhibit the formation of root nodules in soybean. On the other hand, a study by Ogoke et al. [116] showed that increasing P fertilization levels increased N concentration in soybean seeds as a result of stimulation of BNF. He et al. [101] found that under P-deficient conditions, soybean reduced seed yield by 40% and P and N accumulation by 80% and 65%, respectively.

Under conditions of soil water deficiency, there is a significant reduction in P uptake by plants [117]. In the case of prolonged drought, lasting about 10 days, the diffusive flow of P from the soil to the plant almost completely stops, resulting in a significant loss of productivity. The direct role of P in maintaining plant productivity under low water availability is related to the maintenance of stomatal conductance [33]. In a study by Firmano et al. follow by [21], it was shown that under soil water-deficit conditions, plants fertilized with P at 200 kg ha⁻¹ had higher net photosynthesis and stomatal conductance compared to non-fertilized plants.

3.2. Secondary Macronutrients

3.2.1. Calcium

Calcium (Ca) is a very important nutrient for agricultural plants, despite the lower demand for this element compared to nitrogen and potassium [118]. Ca is a very important due to its participation in the formation of cell walls and membranes [119,120]. It combines with anions and acts as a detoxifying agent by neutralizing organic acids in plants [119]. Ca is important in the process of cell formation, where it acts as an intracellular messenger [121]. This nutrient is very important in soybean and other legumes as an activator of some enzymes in protein synthesis [119]. Moreover, Ca is also essential for the formation of complexes with proteins, ion transport, and cation balance [122]. Ca, as an intracellular messenger, coordinates responses to numerous development signals and environmental challenges [121].

The impact of Ca on plant yields also results from its influence on shaping plant growth conditions by reducing soil acidity [119]. Soil acidity and, as a result, low nutrient availability may be amongst of the major constraints in soybean production in some regions. According to Bedassa et al. [123], different cultivars of soybean may differ greatly in its sensitivity on soil acidity. Soybean is also a plant sensitive to water stress [124]. If water deficiency occurs during the seed-filling period, it results in the acceleration of plant senescence by 7–10 days [125]. The authors showed that this unfavorable phenomenon may be reduced by the proper nutrition of plants with Ca, which limited the yellowing of leaves and clear delaying of leaf senescence. Ca fertilization contributed to an increase in the chlorophyll content in soybean leaves in the third week after application. According to these authors, Ca may improve the cell membrane stability of plants. Ca plays a significant role in the maintenance of cell structure. It activates ATPase, which transports nutrients lost due to cell membrane damage under Ca deficiency conditions and regenerates the damaged plant [112]. Ca also plays the role of calmodulin, which counteracts plant metabolic activity and promotes plant growth under drought conditions.

According to Sawicki et al. [126], Ca is an indicator for auxins, which can reduce leaf, flower and fruit drop. It is a counter-cation for different anions in the vacuole, and it responds to numerous developmental cues and environmental challenges [121]. According to Ogunremi and Babalola [127], high levels of water in the soil also have an adverse effect on the growth and development of plants, including soybean. This is related to the limited availability of oxygen for plants and occurs even on sandy soils. In this case, the application of Calcium peroxide in the dose of 4 kg ha⁻¹ contributed to an increase in the number of leaves and pods per plant and the yield of soybean seeds. However, in this case, it is difficult to separate the influence of calcium from the influence of oxygen contained in the fertilizer used.

The results of research concerning the influence of Ca on the growth, development and yield of soybean are ambiguous. In studies by Fioreze et al. [128], Ca foliar application, at a rate of 235.8 g ha⁻¹, at budding, opening flowers or full flowering stages did not affect yield components, and final yield of the soybean crop. Also, the content of calcium in soybean flowers was not affected by foliar application of this nutrient. It is possible that the lack of a positive effect of calcium application in this study was due to the small dose applied in foliar form. In a study by Ogunremi and Babalola [127], the dose was many times higher. Moreover, Ca was used at seeding, so its influence on plants was possible throughout the soybean growth.

3.2.2. Magnesium

Magnesium (Mg) is an important “secondary” macronutrient. Mg is a crucial element of chlorophyll and plays a very important role in photosynthesis [119]. Moreover, Mg is involved in many physiological processes, like regulation of the cation-anion balance and as an activator of some enzymes (e.g., ATPases, RuBP, carboxylase, RNA polymerase, and protein kinases) [88,129]. According to Yang et al. [130], Mg deficiency is a big problem in agriculture. Mg affects some metabolic pathways; hence, its deficiency may be a reason for retarded plant growth and development. Mg is a cofactor in some enzymatic reactions and is involved in the stabilization of ribosomes and the structure of nucleic acids [119].

The problem with Mg availability for plants may occur in conditions of high K concentrations in the soil solution, which may limit Mg uptake [129]. According to Santos et al. [131], Mg application to soil improves soybean drought tolerance. The favorable impact of Mg on soybean tolerance to water stress is associated with improved relative water content and magnesium accumulation, which leads to higher contents of photosynthetic pigments. An important task of Mg in the plant is also to support the movement of sugars [119]. In studies by Rodrigues et al. [132], a positive response of soybeans to foliar application of Mg was confirmed (at the dose of 500 g ha⁻¹ of MgCl₂). As a result, there was an increase in net CO₂ assimilation and an increase in total tissue sugar concentration, resulting in higher seed yield. Moreover, the application of Mg to the leaves also reduced oxidative stress by improving the use of energy accumulated in photosynthesis and by increasing the antioxidant enzyme activity. According to these authors, Mg foliar spraying results in obtaining more metabolically active plants and, as a result, with higher yield potential. Studies by Santos et al. [131] indicated that Mg supplementation can counteract growth limitation due to drought and can be an effective means of enabling soybean cultivation in regions characterized by significant rainfall deficits. Rodrigues et al. [132] stated that soybean treated with Mg reduced the substomatal CO₂ concentration (by 11%), increased net photosynthetic rate and reduced leaf transpiration and water-use efficiency. Mg fertilization has a beneficial effect on the growth of the plant root system (increases root length and surface area), which helps to increase water and nutrient uptake from the soil. It also participates in sucrose transport from leaves to roots, improves CHO translocation by increasing phloem export and reduces ROS generation and photo-oxidative damage to chloroplast under drought conditions [33].

The use of Mg is an important practice shaping the soybean growth conditions; however, the positive impact of this nutrient on physiological parameters does not always result in a significant increase in yield and its components.

3.2.3. Sulfur

Sulfur (S) plays an important role in plant growth and development processes, as it is involved in the synthesis of certain proteins and enzymes. It is a component of amino acids (cysteine, methionine), vitamins (biotin, thiamine), coenzymes, and is also involved in the synthesis of fatty acids [133,134,135]. Deficiencies of S-containing amino acids may limit the nutritional value of food and feed [136]. The direct source of S for plants is sulfate (SO₄²⁻) contained in the soil. More than 95% of soil S is organic bonded and divided into sulfate ester S and carbon-bonded S [137]. However, S in sulfate form is subject to leaching from the soil throughout the growing season, and the seasonal supply of this macronutrient to the soil is linked to organic matter mineralization and atmospheric deposition [138].

Currently, an increasing number of soils worldwide are deficient in S. The main soybean-producing regions of the world have also been reported to have low soil S availability [137,139,140,141]. The biggest problem is observed in America and Asia, especially in China, where 30% of the land suffers from S deficiency [142]. Therefore, the desirability of fertilizing soybean with this macronutrient is indicated. A study by de Borja Reis et al. [143] conducted at 18 locations in eight US states showed that the application of mineral S (sulfate/elemental S) fertilization was followed by an increase of 0.3% in soybean protein and about 1% in amino acids S (cysteine and methionine). Under drought conditions (18–29% reduction in potential transpiration), no changes in yield or composition of soybean seeds were observed under S fertilization.

The effect of S supply on BNF has received less attention, as deficits of this nutrient have rarely been diagnosed in agricultural soils. However, due to the depletion of some soils in S, this issue is gaining importance. A study conducted under rainfed conditions showed that S application along with inoculation resulted in a significant increase in overall plant growth and BNF [144]. Deficiency in S can reduce BNF by legumes, negatively affecting the development and function of nodules (Figure 3) [145,146]. According to Hu et al. [147], sulfur deficiency severely limits soybean growth, inhibiting the rhizobia nitrogenase and soybean protein synthesis. Similar to P and K, S deficiency reduces the weight and number and activity of nodules on roots of soybean, as well as other legume species (Pisum sativum L. and Medicago sativa L.) [148]. Under S-deficient conditions, the activity of nitrogenase and other important enzymes, such as PEP-carboxylase, malate dehydrogenase or glutamate synthase [149]. Hu et al. [147] showed a negative effect of S deficiency on soybean growth BNF, yield and root morphology-related parameters, and root nodule growth with hydroponic experiments. These authors emphasize that low S concentration and rhizobia inoculation enhance soybean growth, nitrogen fixation, and yield but reduce soybean root efficacy, increasing reliance on root nodules. In other studies, S fertilization did not affect the number of nodules per plant, but increased the percentage of average nodule at the R4 growth stage of soybean [150]. When fertilizing soybean with S, the timing of the treatment is also important. A late supply of S, i.e., after the R5 growth stage, has been shown to be critical as S uptake during seed fill increase with yield, while remobilization from vegetative tissue is relatively less than that of nitrogen [52]. The amount of S uptake by soybean plants at the end of the growing season (after the R5 growth stage) is more yield-dependent, i.e., the uptake of this macronutrient increases with higher seed yield [138]. Almeida et al. [151] indicated that changes in plant N status are positively correlated to yield response to S.

Fertilization of soybean with S in soils deficient in S has a beneficial effect on soybean seed yield and quality. A number of studies have identified a dose of 20 kg S ha⁻¹ as optimal for soybean cultivation [152,153,154,155]. Chauhan et al. [154] showed a 91% increase in nodule number at 20 kg S ha⁻¹, while increasing the application rate to 40 kg S ha⁻¹ resulted in a decrease in nodule number, seed yield and seed oil content. In other studies, in soils poor in this element, soybean fertilization with a dose of 30 kg S ha⁻¹ had a positive effect on nitrogenase activity and BNF, as well as on biomass production and seed quality (increase in protein and fat content) [133,155]. In contrast, the results of Singh et al. [156] showed that seed and straw yield of soybean increased when S was applied at a rate of 40 kg ha⁻¹. The response per 1 kg S apply was 14.5 kg of soybean grain. In soils with low water availability, a dose of 80 kg S ha⁻¹ was considered a viable alternative to mitigate the detrimental effects of water shortage, but the response of soybean to the S dose was dependent on the genotype [157].

Sulfur has been shown not only to improve plant productivity under favorable conditions, but also to provide protection against various abiotic stresses such as salinity, drought, excessive temperature, high light levels and toxic metals or non-metals [135,158]. Da Silva et al. [159] report that S supplementation mitigates the damaging effects of drought on soybean.

3.3. Synergistic or Antagonistic Effects of Macronutrients on Soybean

In agricultural practice, most nutrients are usually supplied to plants in combination with others. An approach to understanding the balance between macronutrients in soybean plants can be achieved by analyzing stoichiometric proportions, e.g., N:P ratio [143,160,161,162,163], N:K ratio [160,162], N:S ratio [143,164,165,166,167], and P:S ratio [143]. It has been shown that the demand for nutrients per unit of soybean yield decreases with increasing seed yield [143].

There are few reports in the literature on the synergistic effects of various macronutrients on the growth, yield and chemical composition of soybeans under conditions of limited water availability. It has been stated that nitrogen should be in balance with other macronutrients. Setubal et al. [168] showed that limited water availability and lack of mineral N fertilization reduced the accumulation of nutrients such as N, P, K, C, Mg, S, Cu, Fe, Mn, Zn and B by soybean. Water availability as well as N supplementation or its absence had no effect on N accumulation in soybean with indeterminate growth, and the maximum N accumulation occurred in the R8 phase. The authors also showed that the uptake of P and Ca by plants depends more strongly on the water regime than on N fertilization. In conditions of water deficiency, N supplementation influenced the greater accumulation of K, Mg and S. In the absence of N fertilization, soybean plants subjected to water stress showed a 70% reduction in S accumulation compared to plants fertilized with N (Figure 4). A synergistic relationship between N and S is known [168], which explains the lower accumulation of S by soybean plants in the absence of N fertilization. Synergistic interactions occur in the uptake of N and P as well as N and K. They are important for plant yield, as well as helping to explain their combined effects on root growth and development, and the importance of combined application of N and K fertilization during the growing season [169]. Synergy between P and K uptake and antagonism between K and Mg uptake were also detected [169,170].

4. Conclusions

In field conditions, plants are exposed to many stresses; therefore, improving plant growth under conditions of water deficit in the soil is a priority task in the context of climate change and pressure from the growing human population. Current knowledge of biology, physiology and functional genomics is used in breeding programs to create new plant varieties, but practical use of the potential of these varieties is possible thanks to the use of appropriate, sustainable agricultural practices. The optimal nutrition of plants with macroelements has a beneficial effect on the growth and development of plants by maintaining internal homeostasis, improving plant health and competitiveness, and consequently using the yield potential. The proper nutrition of plants in drought conditions can be an effective way to mitigate the effects of stress by reducing ROS toxicity, maintaining water balance and effective gas exchange adapted to environmental conditions. Balanced mineral fertilization with macroelements is therefore a feasible technique for reducing environmental stress and increasing crop productivity.