On Gleyic Chernozems soil, initially rather highly fertile, the average crop yield during the experiments in the control variants (without fertilizers) of fodder and grain-grass crop rotations was 3.39-3.86 ton of fodder units per hectare (t f.u. ha
-1), whereas when making the calculated doses of NP fertilizers -5.20-5.24 t f.u. ha-
1 (Table 1).
The application of only NP fertilizers on Luvic Greyzemic Phaeozem soil for vegetables and potatoes was ineffective (the average annual yield in control and NP, respectively, 6,5 and 7,6 t f.u. ha
-1), apparently due to the limited potassium nutrition of plants. The balanced application of mineral fertilizers in the NPK variant contributed to a significant increase in the yield of vegetable crops and potatoes (average annual - 11,4 t f.u. ha
-1).
Long-term agricultural use of the soil, both intensive and extensive, inevitably affects, to one degree or another, the physicochemical properties of the soil, which can affect the transformation processes of the fund of macro- and microelements contained in it, including magnesium (
Zav’yalova, 2015;
Mohamad et al., 2016; Dornbush and Von Haden, 2017;
Dubovik and Dubovik, 2019;
Yang et al., 2019). The different intensity of the use of mineral fertilizers in the variants of the experiments carried out, the specificity of individual crops and the technologies of their cultivation led to changes in the effective fertility of the studied soils (Table 2).
The humus content (determined by Tyurin’s method) in the Gleyic Chernozems soil changed insignificantly as a result of long-term cultivation. This is associated with a significant proportion of perennial grasses in crop rotations and, accordingly, a large amount of root and crop residues entering the soil (
Weißhuhn et al., 2017;
Boiko, 2019; Levin et al., 2021). The pH value of the soil under study increased slightly during the experiment - by 0.2-0.3 units. To the greatest extent, changes in soil properties affected the fund of ash elements - phosphorus and potassium: the content of mobile phosphorus (according to Chirikov) in variants with its introduction increased 2 times in comparison with the original soil and the content of exchangeable potassium (according to Maslova) with a permanent deficit of its balance similarly decreased (Table 2).
In the variants of the experiment on Luvic Greyzemic Phaeozem soil, the humus content sharply decreased in comparison with virgin soil (Table 2), which is probably due to the permanent mineralization of soil organic matter with a small input of plant material. In the NPK variant, the humus content was higher than in the control and NP. This associated with a much higher yield with optimized crop nutrition and a corresponding input of plant residues into the soil
(Voronkova et al., 2019; Matvienko et al., 2023).
The reaction of the soil environment of the control variant during the experiment changed little, relative to virgin lands and in the variants, with the introduction of nitrogen fertilizers, it significantly decreased. The additional use of potash fertilizers did not affect the acidity of the soil in comparison with the NP option. The level of mobile phosphorus in the soil of the control variant decreased significantly in comparison with virgin soil and as a result of systematic long-term use of phosphorus fertilizers, it increased significantly; a similar situation was observed with the potassium state of the soil with or without potassium (Table 2). It should be noted that all changes in the indicated soil agrochemical properties in all variants of the experiments occurred mainly in the 0-20 cm soils layer. The absorption capacity of the studied soils in the upper layer of various variants of long-term experiments slightly differed from the virgin (initial) values.
The level of easily exchangeable magnesium gives an idea of the degree of depletion of the soil, its ability to desorb the ions of the element into the soil solution. These cations make up the most mobile fraction of exchangeable magnesium, which is dislocated on the outer (planar) faces of crystallites and in the organic matter
(Xie et al., 2020; Buernor et al., 2024). The content of easily exchangeable magnesium in the upper root layer of Gleyic Chernozems soil during its long-term extensive (without fertilization) agricultural use practically did not change (Table 3). This was facilitated by the predominant cultivation of grasses, a stable level of organic matter in the soil, which is the soiled carrier of this fraction of magnesium, as well as an increased, in general, the magnesium fund of the studied soil (
de Mello Prado, 2021;
Chebotarev and Brovarova, 2023). On the contrary, in the control variant of the experiment on Luvic Greyzemic Phaeozem soil, the content of easily exchangeable magnesium significantly decreased in comparison with virgin soil. The level of humus in this soil during its agricultural use has significantly decreased (Table 2) and the cultivation of row crops contributed to a certain activation of the leaching processes; it is known that magnesium ions have an increased migratory ability
(Senbayram et al., 2015; Chernousenko et al., 2023) In addition, the removal of the element with the alienated main and secondary plant products could also have a certain effect (
Yakimenko and Naumova, 2021).
Long-term application of mineral fertilizers in the variants of experiments on the studied soils caused a significant decrease in the content of easily exchangeable magnesium in the arable and subsoil layers (0-40 cm), both relative to the control and virgin lands (Table 3). With comparable crop yields in fertilized and non-fertilized experiments (for example, control and NP on Luvic Greyzemic Phaeozem soil, Table 1), it can be assumed that the removal of magnesium by plants was not decisive in the depletion of its soil reserves; the main influence was probably provided by the leaching processes (
Gransee and Führs, 2013;
Lu et al., 2024). It is known that monovalent cations (NH
4, K, H) - are strong antagonists of Mg, suppressing the processes of its fixation in the soil and absorption by plants
(Xie et al., 2020). In our experiments, the long-term application of ammonium nitrate caused the loss of the most mobile part of the magnesium fund of the upper soil layer (0-40 cm). Additional application of potassium chloride (Luvic Greyzemic Phaeozem soil) and a concomitant significant increase in the yield of cultivated crops did not fundamentally affect this process, nevertheless, the tendency for a further decrease was obvious. Earlier, the scientific press reported on the intensification of the processes of magnesium leaching from the soil with the intensive use of mineral fertilizers (
Shil’nikov et al., 2008;
Bogdevich and Lomonos, 2009;
Tikhomirova, 2011;
Marschner, 2012;
Senbayram et al., 2015 ).
Long-term extensive agricultural use of Gleyic Chernozems soil did not affect the soil level of exchangeable Mg (Table 3). Probably, the predominant cultivation of grasses weakened the processes of infiltration and leaching and the mineralization in the upper soil layers of a large number of plant residues (
Boiko, 2019) contributed to the compensation of the removal of the element with the harvest. However, judging by the slight decrease in the content of the easily exchangeable fraction of magnesium, it is obvious that, despite the external invariability of the pool of intracellularly absorbed cations, the soil’s ability to desorb magnesium ions into the soil solution to a certain extent decreased. Some equilibrium between its forms, which existed in virgin soil, turned out to be disturbed.
In the control variant of the experiment on Luvic Greyzemic Phaeozem soil, the level of exchangeable magnesium slightly decreased in comparison with virgin soil (Table 3). This could be due to the long-term cultivation of vegetables crops and potatoes with a small supply of post-harvest plant residues to the soil, as well as infiltration processes.
However, the main factor influencing the change in the conditions of exchangeable magnesium in the soils of the experiments was the use of mineral fertilizers.In Gleyic Chernozems and Luvic Greyzemic Phaeozem the level of exchangeable magnesium in NP variants decreased in comparison with virgin lands and control variants not only in the arable layer but also in the subsoil layer (Table 3).
Probably, the long-term use of ammonium nitrate, as noted above, contributed to the displacement from the soil complex not only of extramicellar easily exchanged magnesium cations but also of its more strongly intramicellar absorbed ions. With the additional application of potash fertilizers and an increase in crop yields, there was a tendency for a further decrease in the content of exchangeable magnesium in the arable and sub-arable soil horizons.
The scale of losses of soil magnesium in intensive agrocenoses, as well as the nearest resources for replenishing the level of its exchangeable form, can be estimated by the content of non-exchangeable magnesium in the soil. It is believed that non-exchangeable cations are genetically related to three-layer clay minerals (
Maguire and Cowan, 2002;
Härdter et al., 2005;
Gransee and Führs, 2013). The quantity and quality of these minerals in a particular soil determine both the absolute content of the exchangeable and non-exchangeable forms of soil magnesium and the ratio between them. The content of non-exchangeable magnesium in the soils of the variants of our experiments, possibly due to the relatively high absolute values, did not vary as clearly as in the case of the more mobile forms of this cation (Table 3). Nevertheless, it can be assumed that with long-term extensive agricultural use of soils, the content of non-exchangeable magnesium in them changes little. At the same time, with the long-term use of mineral fertilizers and increased crop yields, there is an obvious tendency to a decrease in the non-exchangeable form of magnesium in the soil. This reflects a weakening of the potential capacity of the soil due to a decrease in the level of mobile forms of this element.