The Role of Organic and Mineral Fertilization in Maintaining Fertility and Productivity of Cryolithozone Soils

Abstract

Considerable attention of the world community is now focused on the implementation of measures in the interests of achieving the global food security for future generations. Particular aspects of this problem include efforts aimed at increasing crop yields through the use of fertilizers. The purpose of this review is to analyze the experience of using organic and mineral fertilizers to increase crop yields in the cryolithozone of the European part of Russia for food security purposes. The fertility restoration methods of various Retisols considered in the review are of primary importance for the agriculture in the forest zone where 91% of arable lands are represented by this soil type. As these soils are low-fertility, poorly humified and acidic, they largely require the use of chemization and biologization means which are therefore an important factor in ensuring food security under the conditions of modern challenges.

1. Introduction

Soil is a strategic resource and a vital part of the environment, being the main source of agricultural products. The human race receives about 95% of food products in the form of crops from cultivated land [1]. At the present stage, there is a quickly developing relative decrease in the productive land resources on the planet and their replacement by depleted territories (deserts, badlands, ravines, built-up and flooded lands, etc.). These trends, combined with modern crises (migration of the population, disruption of fertilizer supplies, and high susceptibility of agriculture to climate change) increase food security risks [2]. This problem is of global importance since the prospects for sustainable development of all countries on the world that are involved in the processes of food production and consumption depend on its solution [3].

The relative decrease in soil area is not accompanied by a proportional or faster increase in the average global yield of agricultural crops grown for food, raw materials and livestock feed. The problem is complicated by the uneven distribution of soil resources suitable for use in agriculture throughout the continents and a steady increase in population [4]. In this regard, the use of cryolitic soils for agricultural purposes is particularly relevant for the solution of these issues. Various Retisols prevailing in the soil cover of the cryolitic zone (Figure 1) are low-fertility and in the case of insufficient application of fertilizers and ameliorants are subject to degradation; humus and nutrients reserves decrease, and physical-chemical and other soil properties deteriorate [5].

Retisols cover an estimated 320 million ha in Europe, North Asia and Central Asia, with minor occurrences in North America. Retisols are concentrated in two regions, each having a particular set of climate conditions: (1) the continental regions that had permafrost in the Pleistocene of northeastern Europe, northwestern Asia and southern Canada, which constitute by far the largest areas of Retisols; (2) the loess, cover-sand areas and old alluvial areas in moist temperate regions, such as France, central Belgium, the southeast of the Netherlands and the west of Germany [6].

The main task of land users is to preserve and increase soil fertility and then increase the productivity of agrocenoses and protect agroecosystems from pollution. The optimal physiological development of plants is ensured by adequate nutrition, primarily mineral nutrition. In the majority of soils, mineral salts are present in sufficient quantities. If not, their need for minerals can be fulfilled with mineral fertilizers.

The aim of this research work is to trace the experience of using organic and mineral fertilizers to increase crop yields in the context of the food security governance using the example of the cryolithozone of the European part of Russia.

2. Organic Fertilizers and Humates

In recent years, organic fertilizers have gained great importance in many countries. This is because the intensification of arable farming logically causes a soil fertility decrease as the content of soil organic matter (SOM) becomes low, and this is a serious problem [7].

In terms of the condition of long-term regular application of organic fertilizers, soil humus and nitrogen reserves increase. Organic fertilizers improve the soil chemical properties, reducing acidity, increasing the value of total absorbed bases, reducing the content of mobile aluminum to a harmless amount, and increasing phosphorus and potassium reserves [8].

The application of manure, peat and peat–manure composts at 30–40 t/ha in the grassland crop rotation increases the SOM reserves, soil moisture capacity and soil field moisture. The most noticeable improvement in the physical soil properties is achieved with the long-term regular use of organic fertilizers [9].

Intensive exploitation of the resources of the North and the need to provide the population with local high-quality food products obviously requires Eutric Albic Retisols to be agriculturally developed. In the conditions of the middle taiga zone, the authors have studied the circulation of elements in agrocenoses and their productivity. After liming and application of optimal doses of organic fertilizers, the productivity of field crops on poorly cultivated Eutric Albic Retisols reached 6.6–14.1 t/ha [10]. In the virgin phytocenosis of the spruce-green moss forest, the annual increment is 6.4 t/ha, which corresponds to the natural productivity of poorly cultivated Eutric Albic Retisol and is significantly lower than its productivity after liming and fertilization [11].

Long-term regular application of organic fertilizers not only preserves the total humus reserves but also increases their content in the soil [12]. Under the influence of fertilizers, especially organic fertilizers in combination with mineral fertilizers, the content of humic acids (HAs) in the composition of SOM increases and the ratio of humic to fulvic acids (FAs) increases whereas the content of highly mobile, easily washed-out organic matter fraction decreases. If fertilizers are not applied, then all agrochemical, chemical and physical-chemical soil properties deteriorate which, in turn, enhances the podzol formation process in the conditions of the flushing water regime [13].

From the study results on Eutric Albic Retisols (Loamic) in the middle taiga zone, the application of organic fertilizers in combination with liming increases the soil fertility, elevates the humus content (by 0.72%), increases the value of total exchangeable bases (by 6.4 mg-eq/100 g of soil), reduces the hydrolytic acidity (by 0.54 mg-eq/100 g of soil), reduces the bulk density (0.07 g/cm³) and significantly increases the productivity of agricultural crops (by 22–100%) [14].

The experiments on Eutric Albic Stagnic Retisols (Loamic) evidenced that various types of compost positively affect both crop productivity and water, and physical and agrochemical soil characteristics [15]. The use of organic-mineral fertilizers based on lignin for Eutric Albic Retisol (Loamic) in the middle taiga zone raised the yield of potato and perennial grasses by 2–4 times compared to the control, and improved the content and composition of humus and mobile nutrients in the soil [16]. The combined use of lime and manure in a long-term experiment on Eutric Albic Retisol (Loamic, Ochric) enhanced the soil biological activity [17]. At the same time, the application of mineral fertilizers does not significantly affect soil microbiological activity [10].

On arable soils when field crops are harvested and so largely excluded from nature’s cycle, the source of organic matter is the aboveground and root residues of cultivated plants, as well as the organic fertilizers used [18]. The intensity of the processes of microbiological decomposition of plant residues in the soil depends on the environmental conditions, including the presence of mineral elements, humidity, temperature, degree of aeration, biological activity and physical-chemical soil properties, and above all, the composition of plant residues. The above intensity increases along with an increase in the composition of plant residues of substances being easily decomposed by microorganisms, especially water-soluble substances, and a decrease in the carbon to nitrogen ratio (C/N). Active decomposition proceeds in plant residues with a C/N ratio value of 20 or less. In experiments with perennial grasses, the lowest C/N ratio was found for Trifolium pratense aftergrass (14.3) and its roots (18.5). In aftergrass and root residues of the clover–timothy mixture it increased to 19.9 but still remained favorable for active decomposition. The ratio was high in plant residues of the cocksfoot (Dactylis glomerata L.) (32.8) and the vetch–oat mixture (34.6) [19].

The effectiveness of humic substances (HSs) for soils of different fertility, as well as under different hydrothermal conditions and under different agricultural crops, varies greatly [20,21,22]. Hence, the research on the effectiveness of HSs and fertilizers made on their basis for different soils, different bioclimatic conditions and different crops is relevant. HSs, as shown by numerous studies of scientists from different countries, can be used in a variety of fields of agriculture [23,24,25,26]. Additionally, although they are best-known as agents that stimulate the growth and development of plants, as well as adaptogens that neutralize the negative effects of the environment on agricultural crops, there are other areas of their application [27].

The experiments with sodium humate confirmed the increased oxygen consumption caused by the leaves of young sugar beet and corn plants [28]. The conclusion that is supported by the data of Popst and Schnitzer [29] is that the stimulation of root formation caused by FAs is associated with the function of carboxyl and hydroxyl groups since the acetylation of hydroxyls and the simultaneous etherification of carboxyls completely eliminate the physiological effect of FAs. These studies’ results are consistent with the results of experiments [30] according to which fractions of peat humic acids containing a high number of phenolic hydroxyls and quinoid groups are have high physiological activity. Other researchers have the same mechanism in mind [31,32,33] and note the stimulating effect of HSs on various agricultural crops.

In recent years, a confirmation of the stimulating effect of HAs on physiological processes of plants has been provided in many cultures. Researchers have described the effect of ammonium humate on biometric indicators of agricultural crops [34] and noted the positive effect of humic compounds on the germination, growth and development of seedlings during seed soaking [35]. HSs were identified to influence the biological activity of soils and the sowing qualities of seeds [36,37]. Sodium humate was found to stimulate grain germination by 30–50%, and cotton yield by 33–44% when added not only to distilled water or a nutrient solution, but also when added to soils poor in organic matter [38].

HAs affect not only the yield of crops but also the quality of agricultural products. There are data that indicate a sugar recovery increase of 0.38–0.5 t/ha after the application of sodium humate during the cultivation of sugar beet [39].

The effectiveness of humic fertilizers is different for different soils. Humic fertilizers are best for Eutric Albic Retisols. They are highly effective in poorly humified low-fertility soils, as well as in plowed-out soils and soils destructed by long-term irrigation since humic substances optimize soil properties [40,41,42,43].

A large number of scientific works are devoted to the changes in the humic state of various Retisols under anthropogenic impacts [44]. HSs from Retisols have a high migration capacity. The products of HS interaction with minerals differ in accordance with the water solubility degree; some of them migrate along the soil profile and so cause its prominent morphological and chemical differentiation. HSs in Retisols are dominated by FAs, the content of HAs being relatively low [45]. Being not fertilized and agriculturally used for a long period of time, Eutric Albic Retisols, the organic matter of which is highly mobile, normally develop undesirable humus features accompanied with a decrease in the C_HAs/C_FAs ratio mainly due to an increase in FAs (including aggressive fractions) and to a lesser degree due to a decrease in HAs. The most serious changes in the humic composition were recorded during long-term cultivation of row crops [46,47,48,49].

Thus, throughout the past decades, the scientific community has accumulated a large database to identify the effect of humic substances on the growth and development of plants in various conditions. However, the use of humic preparations in the agricultural sector remains insufficient although they are especially relevant for agrocenoses.

3. Mineral Fertilizers

3.1. Nitrogen Fertilizers

The use of nitrogen fertilizers for agricultural crops in naturally low-fertility soils, including Eutric Albic Retisols, is among the main conditions necessary for increasing the productivity and sustainability of agriculture in the non-Chernozem zone. A high long-lasting effect of this strong optimization means of the plant production process can be attained only under the condition of a balanced level of plant root nutrition via fertilizers and under the creation of favorable conditions for the effective utilization of applied nitrogen by crops. Applying nitrogen to soil in an amount exceeding the need of crops, especially against recommendations on the regular use of nitrogen fertilizers in crop rotation, can lead to a number of undesirable environmental consequences, among which the following should be highlighted: increased migration of nitrates along the soil profile creating a risk of contamination of natural waters, excessive mineralization of humus leading to its unproductive losses, violation of the balance of natural nitrogen cycles in the soil and its acidification [50]. All of these make it necessary to create a nitrogen balance-ensuring system using the soil and plant diagnostic methods to determine the need of crops in fertilizers.

The phenomenon of an increasing plant nitrogen nutrition level partly improves their resistance to moisture deficiency occurs because nitrogen plays an important role in water regime regulation. In drought conditions, nitrogen is often a more limiting factor than moisture is, and crops grown in soils with a low availability of this nutrient suffer more from drought than those in soils with a high level of assimilable nitrogen do. As shown by many researchers, in the case of insufficient moisture, nitrogen consumption by plants is limited due to a decrease in the amount of its available compounds, primarily nitrates, in the soil since the nitrification process proceeds poorly under unfavorable moisture conditions [51]. Therefore, application of nitrogen fertilizers, optimizing the nitrogen nutrition of plants, has a positive effect on the water regime in case of moisture deficiency. This is because nitrogen-fertilized plants form a root system with a high absorption capacity which allows a more efficient use of the subsurface moisture reserve. Noting that the use of soil moisture is closely related to the plant nitrogen nutrition level, many authors [52] emphasize that nitrogen fertilizers, stimulating the active growth of aboveground biomass, can contribute to the depletion of soil moisture reserves even before the period of their maximum consumption. In this regard, taking a decision on the optimal application dose of nitrogen, it is recommended to balance it with the available soil moisture reserves. There is an opinion [53] that in order to avoid early consumption of moisture and to increase the productivity of its use by crops, the application of nitrogen fertilizer in split portions should be practiced.

The stationary field experiments on poorly and medium-cultivated Eutric Albic Retisols have allowed the dependence of the effectiveness of the nitrogen fertilizer on the crop’s phosphorus and potassium supply [54]. The crop rotation includes six fields sown with grain crops rotating by 66% and potato and clover rotating by 17% each. Along with the enrichment of phosphorus-potassium background, the yield increments from nitrogen fertilizer increased. The yield increases in the second rotation were lower than those in the first field rotation which seems to be associated with the improvement of the soil nitrogen regime under the influence of regular liming and cultivation of clover in the crop rotation [55].

3.2. Phosphorus Fertilizers

Phosphorus is among the most important elements of plant nutrition, the availability of which is considered to be one of the main indicators of soil cultivation. Therefore, the creation of an optimal phosphate level in the soil, ensuring the formation of high and stable yields of agricultural crops, is one of the priorities of modern agriculture [56]. The role of phosphorus in plant nutrition is determined primarily by the participation of its compounds in the conversion of solar energy into chemical energy of macro-energy bonds during photosynthesis, as well as in the synthesis of carbohydrates, proteins, fats and nucleic acids. Under normal conditions, the phosphate ion of the soil solution is rapidly absorbed by the root. It can be assumed that in the root most of the inorganic phosphates are already included in organic compounds [57].

Phosphates are present in the soil in the form of mineral and organic compounds. Among the minerals containing phosphorus, iron and aluminum phosphates predominate in Eutric Albic Retisols, including strengite, variscite, crandallite, augelite, etc. [58].

Phosphates can also be absorbed by soil colloids. The sorption bond of phosphates with the soil absorbing complex increases with a decrease in pH; therefore, the availability of absorbed phosphates to plants decreases with the acidification of Eutric Albic Retisol [59].

The severity of the phosphorus problem in agriculture in the non-Chernozem zone of Russia is aggravated by the fact that this element is mostly concentrated in agricultural end products to be unilaterally excluded with harvests from nature’s cycle [60]. To increase the yield of agricultural crops in conditions of high phosphorus deficiency, it is particularly interesting to determine the optimal level of phosphorus availability for different soil types. The establishment of optimal phosphate levels for the main field crops in the six-field crop rotation in various zones of the country, as well as the estimation of the cost of fertilizers to achieve them, will allow access to the needs of agriculture by phosphorus fertilizers. The availability of phosphorus in the soil is the main indicator of its fertility, which determines (up to a certain limit) the yield level of all agricultural crops [61].

According to the world’s chemicalization experience, to obtain high and stable yields, it is necessary to create an optimal level of available phosphate content in the soil via a single application of high fertilizer doses or the annual use of phosphorus in quantities exceeding the removal of nutrients. In the future, the yield level and its quality are regulated via the after-effect of phosphorus fertilizers against the application of other nutrients, as well as progressive agrotechnical techniques [62]. The economic feasibility of regular application of phosphorus fertilizers in a number of years has been shown by numerous studies. The correct scientifically justified inclusion of phosphorus fertilizers into crop rotations with concentrations under those of the most responsive crop is particularly important in this process [63]. Under the condition of equal P amounts applied to soil within fertilizers in the form of phosphorous flour and superphosphate, the soil after the application of phosphorous flour contains more acid-soluble phosphorus than it does that after the application of superphosphate. Even in a case when phosphorus is applied within superphosphate in higher quantities than it is within phosphorous flour, the content of acid-soluble phosphorus is higher in the soil treated with phosphorous flour compared to that after superphosphate treatment. This is probably explained by the fact that phosphorus within phosphorous flour, in contrast to phosphorus within superphosphate, is better-retained in the composition of phosphate forms available to plants after their long-term interaction with soil [64]. Thus, the P of water-soluble fertilizer superphosphates undergoes chemical sorption in the soil and largely transforms into compounds that are not as available to plants as the P of phosphorite is. This property of phosphorite determines its advantage over superphosphate when applying these as the main phosphorus fertilizers.

When studying the reserve application of phosphorous flour to medium-loamy Eutric Albic Retisol, phosphorous flour introduced at a dose of 140 kg P per hectare resulted in a greater increase in crop productivity for 7 years than its annual use did. The high efficiency of phosphorous flour for soils with an acidic medium reaction is beyond doubt. However, convincing materials have been accumulated, evidencing the positive effect of phosphorous flour in unusual conditions for its use, i.e., for soils with a slightly acidic, neutral and even slightly alkaline medium reaction. An example is the results of field experiments conducted in the Volgograd region, Tatarstan, Western and Eastern Siberia, and the Far East, where phosphorous flour was prepared from local phosphorous raw materials. Now, local agricultural ores are the most reliable source of improving the nutrient regime of soils and increasing the yield of agricultural crops [65]. Phosphorous flour from local deposits is the main source of phosphorus nutrition for agricultural crops and a means of reproducing soil fertility in the Russian Federation.

In the zone of mandatory soil liming (Retisols, Phaeozems, and leached and podzolized Chernozems), the solution of phosphorus problems in agriculture is associated with the use of ground (milled) phosphorites [15]. One of the most important factors of the effectiveness of phosphorites is soil acidity [66]. There are two opposing opinions about the possibility of using ground phosphorite together with liming. At the beginning of the twentieth century, ground phosphorite was assigned only a narrow field of application which included acidic soils and the ploughing of wastelands enriched with humus. In modern agriculture, with numerous repetitions of liming cycles, there are soils with different medium reaction levels to which phosphorus fertilizer should be applied.

The effectiveness between ground phosphorite and superphosphate did not significantly differ for heavy-loamy Eutric Albic Retisol with an acidic medium reaction after liming to a slightly acidic-neutral level (pH 5.2–6.4) [67]. Regular liming did not reduce the effectiveness of ground phosphorite. Phosphorite is significantly superior to superphosphate in terms of energy expense recoupment, and does not differ from superphosphate in terms of its effect on the soil phosphorus regime. Based on a series of studies, phosphorites improve soil fertility via enriching the soil with available forms of macro- and microelements. At the same time, phosphorous flour, ground raw phosphorites, can be used to increase both the soil phosphate level and the content of other nutrients. In economic terms, this will be much cheaper than using industrial mineral fertilizers will be. In addition, local raw materials are more accessible and can be used directly in the deposit area of agricultural ores after mechanical crushing.

Up to 60% of organic phosphates in Eutric Albic Retisols are in the form of inositol phosphates accumulating as iron, aluminum, calcium and magnesium phytates [68]. Along with this, some organophosphates are included in the composition of HSs of a specific nature, mainly FAs. The availability of soil organic phosphates to plants is determined by the conditions of their mineralization and is closely related to soil biological activity, particularly the activity of phosphorylase [69]. There exists a close relationship between the content of mobile phosphates in the soil, plant productivity and responsiveness to phosphorus fertilizers. The optimal level of mobile phosphates extracted with a 0.2 M HCl solution in the soil for cereals and potato lies within 10–15 mg/100 g of soil. Similar results were obtained during the long-term stationary field experiments on heavy-loamy Eutric Albic Retisols [70].

The use of phosphate fertilizers is the main way to optimize the phosphate level of Eutric Albic Retisols. The role of fertilizers in this regard account for up to 70% optimization, with liming accounting for 15–20% [15]. The enrichment of soils with mobile phosphates was achieved through the regular use of manure and phosphorus fertilizers [71]. For such soils, the cultivation of agricultural crops is based mainly on the use of nitrogen and potassium fertilizers, and the application of phosphorus only compensates for its removal by crops and stabilizes the achieved soil phosphate level. The phenomenally long duration of the effects of phosphorus fertilizers is evidenced by the data of the Rothamsted Experimental Station [72]. The ability of plants to affect difficult-to-dissolve soil phosphates and transform them into plant-available forms is also associated with the carbon dioxide released by the roots. The root zone always has a comparatively lower pH value and so the soil phosphates adjacent to it are dissolved by acidic root secretions and absorbed by plants. This mechanism of phosphorus assimilation by plants is of considerable importance in cases when phosphate ions in the soil solution are present in an insoluble form [73].

The phosphorus deficiency dramatically reduces plant productivity. At the same time, phosphorus, unlike nitrogen, has no natural sources in soil. Phosphorus intake by plants and the restoration of its reserves in soils is possible only through the application of phosphorus-containing fertilizers. Numerous studies indicate that the regular use of fertilizers for Eutric Albic Retisols increases the total phosphorus content in the arable soil horizon, enlarges the reserves of its available compounds and improves their uptake by plants. Moreover, the phosphorus of fertilizers transforms in the soil into chemical compounds characteristic of this soil formation type. Eutric Albic Retisols with low pH values contains mainly aluminum and iron phosphates. Enrichment of the arable layer of Eutric Albic Retisols normally includes inorganic phosphorus compounds. The content of organic phosphates increases insignificantly [49]. Among the mineral phosphorus compounds, soil is dominated by aluminum, and not by iron and calcium phosphates. The stationary field experiments on medium-loamy Eutric Albic Retisols of weak and medium cultivation levels allowed a close interaction between phosphorus, nitrogen and potassium fertilizers when applied in addition to regular liming [70]. Long-term application of phosphorus fertilizers increases the utilization rate of phosphorus. These functions can be successfully performed by phosphorous flour. According to many researchers, it has the same effect as superphosphate and in the conditions of the North is even more efficient.

3.3. Potassium Fertilizers

Potassium performs numerous functions in plant life. By optimizing the potassium regime in the agroecosystem, improving crop cultivation technologies, it is possible to significantly affect the productivity of agrocenosis, especially in extreme conditions [74]. Potassium in soils is the main source of plant potassium nutrition. Its gross content far exceeds that of the reserves of nitrogen and phosphorus in the soil and is determined mainly by the parent rock and soil texture (granulometric composition). Sandy Eutric Albic Retisols contain 0.5–1.2% K, and loamy Eutric Albic Retisols contain 1.5–2.1% K. Potassium is found in the soil mainly in the form of primary and secondary clay materials, such as feldspar, mica, illite, vermiculite, chlorite, and montmorillonite [75]. In the practice of agricultural soil use, there is an active involvement of a non-exchangeable form of potassium in plant nutrition which largely determines the peculiar buffering of soils with respect to potassium. According to the studies on the interaction of potassium fertilizers with soil, up to 70–90% of the introduced potassium was fixed in a non-exchangeable form [76].

The ability of potassium to exchange absorption increases along with an increasing content of a highly disperse fine-soil fraction, mainly that of silty particles. Exchangeable potassium is absorbed on the surface of negatively charged colloidal particles. Potassium is not as firmly retained in the exchange positions of organic matter as it is in those of clay minerals [77]. According to the studies conducted in sandy-loam and heavy-loam Eutric Albic Retisols, the use of potassium fertilizers noticeably increases the content of exchangeable potassium in the arable layer only against the background of a triple dose of potassium (220 kg/ha) applied in addition to a nitrogen–phosphorus fertilizer [78].

After the application of a triple dose of potassium fertilizer. the potential greatly reduced which, according to the existing gradations, corresponds to the optimal provision of the soil with potassium. The content of easily exchangeable potassium, determined via the curves of the potential soil buffering capacity in relation to potassium, against the background of a triple dose of potassium fertilizer significantly increased only in sandy loam and not in heavy loam. The latter, as the authors suggest, is related to the ability of heavy loam to firmly retain the introduced potassium due to the high content of fine fractions in its texture [79].

Along with the regular use of potassium fertilizers for Eutric Albic Retisols of various textures at doses of 50–75 kg/ha, only sandy loam was identified for a noticeable migration of potassium down to a depth of 100 cm. In heavy loam, potassium moved no deeper than 40 cm, and in medium loam, it moved 60 cm down the soil profile [80]. The mobility degree of exchangeable potassium decreased along with the soil texture change to heavy variants [81]. The prominent absorption of potassium by clay minerals in soils of a heavy physical texture causes its poor downward movement along the soil profile; the exception for this is light soil from which potassium can be washed out. Consequently, it is possible to apply potassium fertilizers for most soil types, including Retisols, beginning in autumn, without fear of large potassium losses with leaching. However, based on the results of the long-term stationary experiment, potassium moves into the underlying horizons in amounts of 12–19 kg/ha per year [82]. The studies conducted in the same field experiment revealed the accumulation of mobile potassium in the meter-thick soil layer under crops after 62 years of fertilizer application. There is also information about more significant amounts of potassium leaching from the soil. From the arable layer of cultivated Eutric Albic Retisol bare fallow, up to 128.6 kg/ha K per year migrated to the underlying horizons. Losses due to leaching under annual and perennial grasses were significantly lower and amounted to 22–30 kg/ha annually [83].

To characterize the potassium availability in Eutric Albic Retisols, the most frequently used value is the content of exchangeable soil potassium. Summarizing the field experiment results on grain crops demonstrates that the grain yield increases as the content of mobile potassium (according to Kirsanov) also increases by up to 14–20 mg/100 g of soil [84]. Based on the data of experiments with fertilizers, there exists a close dependence of potato productivity and its responsiveness to potassium fertilizer on the content of exchangeable potassium in the soil. An increase in the content of exchangeable soil potassium from 3.1 to 28.6 mg/100 g of soil was accompanied by an increase in the yield of potato tubers from 12.8 to 27.3 t/ha against the NP (nitrogen and phosphorus fertilizers) background, whereas the increase in yield after the application of potassium fertilizers decreased from 32 to 7% [85]. Similar data were obtained in studies on other crops of a field crop rotation. When growing flax, barley, lupine and winter rye in Eutric Albic Retisols, the yield of these crops increased with an increase in the content of exchangeable soil potassium to 15–20 mg/100 g. Any further increase in this indicator no longer led to an increase in crop rotation productivity [86].

4. Complex Fertilizers

The content of total nitrogen, as well as that of humus, is a quantitative indicator of potential soil fertility and a characteristic property of a particular soil type. Most researchers note that mineral fertilizers, when used for a long time without manure and other organic fertilizers in arable soils, stabilize the humus content not as efficiently as they do in natural conditions without anthropogenic impact [87,88,89].

Mineral fertilizers can participate in the formation of humus. In experiments with the use of N during the growing season, 20–25% of ammonia nitrogen and 6–7% of nitrate nitrogen from applied fertilizers were bound in organic matter by microorganisms. The portion of nitrogen from mineral fertilizers that passes into organic form can range from 5.4 to 23%, depending on the form [90]. Nitrogen was largely fixed in the soil after the application of ammonia forms in amounts of 10.5–23.0% and was fixed in comparatively lower amounts in the case of nitrate forms—5–17%. Ammonia nitrogen is a universal and more accessible form for the nutrition of microorganisms and so largely changes into an organic form. The ratio of nitrogen to carbon in the soil indicates the intensity of organic matter decomposition. In experiments without nitrogen fertilizers, the 0–20 cm upper soil layer was characterized by a fairly wide ratio of C/ N = 1/10, which indicates the relatively low biological activity of nitrogen in the soil. Nitrogen fertilizers partly increased the activity of biological processes. Here, the C/N ratio in the arable horizon was 1/8 [30,91].

The intensive use of Eutric Albic Retisols without extra application of organic matter reduces the concentration of humus from 1.9% in the initial soil to 1.34% in clean fellow. Simultaneously, HAs, being complex in terms of composition and properties, become destructed and “aggressive” FAs accumulate. Application of 7–8 t/ha of manure, (NPK60) for cereals in a typical crop rotation and the saturation of the crop rotation with legumes by 42% allow the maintenance of humus levels at 1.8–1.82%, contributing to the accumulation of HAs of the first and second fractions and expanding the C_Has/C_FAs ratio to 0.78–0.79. Based on studies on the fruit-bearing crop rotation in Eutric Albic Retisol, the combined application of organic (80 kg/ha for potato) and mineral fertilizers (NPK60) improved the soil agrochemical properties, increased the organic matter quality (C_HAs/C_FAs rose from 1.10 to 1.78) and productivity (this increased by 6% compared to the grain-grass crop rotation) [92].

The data on the effect of fertilizers on the humus state of Eutric Albic Retisols are given in works performed during 30–40-year-long stationary field experiments. For 40 years of growing crops using a crop rotation with clean fallow, potato, winter wheat and oats, the carbon content in the arable soil layer decreased from 1.06 to 0.70%; after the application of full mineral fertilizer (N60P60K60), it decreased to 0.77% and after the application nof manure (9 t/ha), it decreased to 0.93%. Consequently, in these conditions, the use of manure was insufficient to stabilize the humus content in the soil. The humus balance was achieved by using a lower dose of manure (3 t/ha) in a crop rotation with 2-year cultivation of perennial grasses along with pre-liming. Application of 9 t/ha of manure made the carbon content in the arable soil layer exceed the initial level (1.25%) [93]. The obtained results indicate that the ratio of HS decomposition to formation in the soil is largely determined by the crop rotation structure and the amount of plant residues entering it, which are the initial humus-formation material.

The use of liquid fractions of poultry and pig manure and semi-liquid cattle manure has the greatest impact on the humus regime of the soil and the productivity of the grain-grass chain of crop rotation. One of the important factors in stabilizing the humus state of Eutric Albic Retisols is liming, the role of which increases against the regular application of physiological acidic mineral fertilizers in the crop rotation. Liming shares 66.2% and fertilizers share only 15.6% of the total impact of the studied factors on the SOM [94]. This is due to the increased mobility of organic substances during soil acidification caused by the use of mineral fertilizers which makes soluble humus fractions, mainly FAs, move from the arable layer to the underlying soil horizons. During the 65-year cultivation period of agricultural crops in Eutric Albic Retisol, humus enrichment of its sub-arable horizons proceeded to a much greater extent than that in fallow did. For example, the 20–40 cm thick soil layer of fallow contained 0.46% of humus; in the case of a field crop rotation without fertilizers, it contained 1.18% of humus, with mineral fertilizers, it contained 1.40% of humus, and with a combination of mineral fertilizers and manure, it contained 1.38% of humus [95].

During a stationary field experiment, the effect of mineral fertilizers and lime on the fractional group composition of humus and the HA optical properties in the heavy-loamy Eutric Albic Retisols was studied. The maximum amount of humate-type HSs accumulated in the soil after repeated liming against the background of 2 NPK, and the C_HAs/C_FAs ratio = 0.96. The humus type was fulvate-humate. Liming contributed to the accumulation of calcium-bound HAs in the humus composition of Eutric Albic Retisol. Soil liming also favored the formation of more “mature” and optically dense HAs. The HA solutions of all three fractions extracted from calcareous soil had high extinction values at 465 nm, which indicated a high content of aromatic structures in their molecules [13,96].

Agricultural use of Eutric Albic Retisols has a significant impact on the content and composition of exchangeable cations in the soil-absorbing complex. Particularly, noticeable changes occur under the influence of long-term application of fertilizers and liming, which affect not only the arable layer but also the underlying soil horizons. During a stationary field experiment, data on the degree of impact of fertilizers and liming on the physical-chemical properties of silty-loam Eutric Albic Retisol were obtained [12,49]. Mineral fertilizers (superphosphate, ammonium nitrate, and potassium chloride) were annually applied in the crop rotation at the rate of 60 kg/ha of the active substance of each element. The acidity of the soil limed before the application of fertilizers decreased from pH 6.6 to 5.2 during the first 14 years, remaining within the limits favorable for most cultivated crops, and only after 17 years of application of mineral fertilizers it reached the initial level (pH 4.6–4.8). The increase in soil acidity continued in the subsequent period, and in 26 years the neutralizing effect of lime was practically zero. The soil acidity by this time with pre-liming was pH 4.3 and that without it was 4.1. In both cases, there was no difference in the soil acidity between the variants with and without mineral fertilizers, and the acidifying effect of different mineral fertilizers was the same. The studies on heavy-loamy, poorly cultivated, strongly acidic (pH_KCl 3.9–4.4) Eutric Albic Retisol with a high content of mobile aluminum (5.0 mg/100 g of soil) revealed a high positive effect of liming on its physical-chemical properties and the productivity of field rotation crops [97]. Against the background of the liming of acidic Eutric Albic Retisols, the effectiveness of mineral fertilizers in clover crops significantly increases (by 1.5 times). Each ton of lime increases crop yields in a seven-field crop rotation by 0.7 t/ha. Liming of acidic Eutric Albic Retisols increases the efficiency of fertilizers by 30–40% but it should precede the application of fertilizers [98]. When the acidity of the soil is neutralized and the content of mobile aluminum in it is reduced to a minimum value via liming, a significant improvement in the phosphorus nutrition of plants is observed [26].

Neutralization of soil acidity via liming has a high positive effect on the mineralization of soil nitrogen compounds and the effectiveness of nitrogen fertilizers. The soil pH step change from 4.3 to 6.3 increased the nitrifying ability of Eutric Albic Retisol by four times [99]. Nitrogen fertilizers, due to the manifestation of characteristic physiological acidity, strongly acidify the soil under condition of their long-term application in crop rotation; consequently, periodic liming of the soil is an indispensable condition for increasing their effectiveness [100]. Liming is characterized by a stable and very long-lasting effect on the agrochemical properties of soils. However, one-time liming of acidic soils cannot change the factors under the influence of which the soil becomes acidic; therefore, it must be done on a regular basis.

Eutric Albic Retisols are poorly provided with nitrogen, the total content of which in the arable layer of loams is 0.10–0.16%, and that in sandy loams and sands is 0.08–0.13% and 0.07–0.10%, respectively. Soil nitrogen usually consists of organic compounds that are part of humus (93–95%), plant and animal residues of microorganisms [101]. A significant part of organic soil nitrogen is broken down into amino acids during acid hydrolysis, which serves a basis for the development of a hypothesis about its protein origin. However, organic nitrogen is found in the soil not in the form of proteins, but in the form of condensation products of amino acids formed during the autolysis of plasma protein of microorganisms with carbohydrates and HAs. According to the analysis of the amino acid composition of hydrolysates from various soils, their ratio is very close to the ratio characteristic of bacterial cell protein, and this gives reason to assume the bacterial origin of soil organic nitrogen [102].

Soil organic nitrogen becomes available to plants only after its mineralization by microorganisms ammonifying and nitrifying bacteria. The intensity of this process depends both on the nature of the organic matter itself and on environmental conditions: humidity, temperature, aeration, soil acidity and other factors [22,30]. Under the conditions of modern agriculture, which are characterized by the use of high doses of nitrogen fertilizer, the leaching of mineral nitrogen compounds is so great that there is a danger of natural water contamination. Similar data were obtained via the soil-agrochemical survey of Eutric Albic Retisols. A meter-thick soil layer was found for a significant accumulation of mineral nitrogen at the tillering phase of cereals in spring (100–380 kg/ha) thanks to a significant excess of its intake with fertilizers over its removal in the structure of the nitrogen balance in agrocenoses [103].

An essential source of nitrogen balance regulation in agrocenoses and replenishment of the nitrogen fund for Eutric Albic Retisols is an increase in the portion of legumes in the structure of field crop rotations with their ability for symbiotic nitrogen fixation [104]. Numerous experimental data obtained by researchers indicate the high need of agricultural crops for nitrogen in Eutric Albic Retisols [15,44]. Nitrogen applied together with mineral fertilizers is not only enhanced by plants but is also exposed to soil microorganisms that immobilize it. The application of nitrogen fertilizers is accompanied by an increase in the use of soil nitrogen reserves by plants (this effect of increased soil nitrogen mobilization is called “extra” nitrogen [105], which is confirmed in the studies of many authors). It occurs both due to an increase in the absorption capacity of the plant root system and due to an increase in the activity of microorganisms. The efficiency of using nitrogen fertilizers for winter grain crops increases under the condition of the optimization of phosphorus and potassium nutrition of plants. Simultaneously, the cost per unit of the formed grain yield of winter rye decreases from 58 to 25 kg/ha, and that of winter wheat decreases from 50 to 26 kg/ha, i.e., by more than two times [106].

Based on the results of the field experiments on light- and medium-loamy Eutric Albic Retisols, the application of mineral fertilizers appeared to be highly efficient for winter wheat sown after its different predecessors were in the crop rotation. Nitrogen fertilizer was of decisive importance in the intensification of the production process for this crop [85]. When winter wheat was sown after perennial grasses (clover + timothy) were against the background of a low soil phosphorus content, its productivity after the application of phosphorus and potassium fertilizers increased in 3 years by an average of 0.54 t grain/ha. Nitrogen applied in addition to phosphorus and potassium (N90P90K120) ensured an increase in the yield of this crop by 1.7 t grain/ha. Re-sown winter wheat also reacted well to the application of nitrogen fertilizer. The complex mineral fertilizer applied in the same dose as that of the nitrogen application in split portions (30 kg/ha before sowing and 60 kg/ha during spring fertilizing) increased grain harvest by 1.5 t/ha. The yield increase after phosphorus fertilizer was 0.48 t grain/ha. As shown by the author, the optimal level of the spring fertilization of winter wheat seeds depended on the nature of the predecessor in the crop rotation. When sown in black fallow, it was enough to add 30–60 kg/ha of nitrogen, when sown after early potato, it added 60 kg/ha of nitrogen, and when sown after stubble predecessors, it added 90 kg N/ha [107].

The highest yield of the field pea–oat mixture with the additional sowing of annual ryegrass (484–523 c green mass/ha) was obtained via an application of nitrogen fertilizer in split portions (N80+N120). The yield increase caused by the nitrogen compared to the background P120K180 was 19.3–20.8 t/ha [108]. The efficiency of nitrogen fertilizer for potato crops was also quite high. When applied in a moderate dose (60 kg N/ha), the growth of the tuber yield was 3.9–4.1 t/ha; additional nitrogen fertilizing at a plant height of 15–20 cm was ineffective. The positive effect of nitrogen fertilizer (60–90 kg N/ha) on the productivity of barley crops was fixed only in conditions of moderate moisture deficiency, providing an increase in grain harvest by 0.5–0.7 t/ha against the harvest with a P90K120 application of 4.86–5.09 t/ha. In the case of excessive moisture (precipitation of 164 mm in June–July) and reserves of assimilable nitrogen in the 0–40 cm soil layer before sowing, equal to 156–170 kg N/ha, there was a lodging of barley crops and a decrease in yield by 3–6 kg/ha under conditions of the use of not only elevated (90–120 kg/ha) nitrogen doses, but also of moderate (60 kg/ha) nitrogen doses. The greatest effect of nitrogen fertilization of grasses (yield increases of 6.38 and 4.37 t/ha) was achieved when it was applied 5–10 days after spring revegetation [44]. The reason for such a sharp difference in the effectiveness of nitrogen fertilization of winter cereals and perennial grasses, carried out at the petiole phase, compared with later dates of its implementation, obviously lies in the loss of most of the spring nitrogen with the runoff of meltwater.

5. Conclusions

The most severe and widespread manifestations of protracted crises have a negative impact on food systems. Modern agriculture needs new conceptual approaches to resolve the issue of plant nutrition regulation, which will allow solutions to possible agrochemical, environmental and economic problems. For the well-being of agroecosystems, it is necessary to develop an optimal fertilization program that will ensure their sustainable use and the balanced supply of nutrients to both plants and soil. The main requirement of the agricultural technology of crops grown in Retisols is to ensure that the physical-chemical properties of these soils do not deteriorate but improve. This task can be fulfilled via cultivating these soils with the application of fertilizers.

The fertility restoration methods of various Retisols considered in the review are of primary importance for the agriculture of the forest zone where 91% of arable lands are represented by this soil type. The natural and organic fertilizers, as well as calcium neutralizing fertilizers, play a decisive role in relation to the analyzed soils, which is important in the context of managing the safety of food sourced from low-fertility, poorly humified and acidic Retisols.

In the conditions of the middle taiga subzone, no long-term scientific research has been carried out that is aimed at fertility restoration and the productivity increase of Retisols via cultivating agricultural crops and the complex application of organic and mineral fertilizers. In this regard, our research is highly relevant to increasing the productivity of agricultural soils of the cryolithozone, a factor ensuring food security under the conditions of modern challenges.