PRODUCTION OF SOIL CONDITIONERS BASED ON OXIDIZED COAL, BENTONITE, PHOSPHATE ROCK, UREA, AMMONIUM SULFATE, AND POTASSIUM CHLORIDE

Abstract.This study presents the preparation of soil conditioners based on oxidized coal treated with hydrogen peroxide, bentonite, phosphate rock, urea, ammonium sulfate, and potassium chloride over a wide range of ratios. Optimal component ratios and activation conditions were determined to obtain soil conditioners with a balanced nutrient composition in a form readily available to plants.

Keywords: soil conditioner, brown coal, humic acids, phosphate rock, bentonite, urea, ammonium sulfate, potassium chloride.

At present, both natural non-metallic minerals and their activated analogues are widely applied in agricultural production. The use of these substances in crop cultivation as soil conditioners is explained by their unique properties: the presence of biogenic macro- and microelements, high cation-exchange capacity, pronounced sorption and catalytic activity. These properties allow such materials to substantially improve the agrochemical, agrophysical, and microbiological characteristics of soils, thereby increasing crop yields and improving product quality. Importantly, these materials act in a prolonged and environmentally safe manner, unlike conventional mineral fertilizers that are easily leached from the soil profile, often causing secondary salinization and contamination of groundwater.

An analysis of numerous national and international studies has shown that soil degradation processes are currently accelerating worldwide at an unprecedented rate. Among them, chemical degradation of soils has become the most widespread, caused by the intensification of agriculture and growing anthropogenic pressures. Soil contamination and depletion result both from global climatic changes and local factors, such as the application of excessive doses of mineral fertilizers and pesticides, irrigation with wastewater and sewage sludge, and the use of industrial and domestic by-products as unconventional fertilizers. All of these practices lead to the deterioration of agronomic properties, loss of soil fertility, and disruption of the natural biogeochemical balance [1,2]. In addition, degradation is often accompanied by a decrease in organic matter, compaction of the soil profile, and loss of natural water-holding capacity.

In this study, taking into account the specific beneficial properties of humic acids derived from brown coal, bentonite, phosphate rock, and mineral fertilizers, as well as the accumulated positive experience of their application in different regions, the task was set to develop soil conditioners based on these components and to investigate their chemical and physicochemical properties. Humic acids of brown coal are known as highly active natural polymers capable of stimulating plant growth, increasing their resistance to stress factors, and improving soil structure. Bentonite, due to its layered structure and high sorption capacity, is able to retain both water and nutrients, thereby creating optimal conditions for root development. Phosphate rock, as a source of slowly soluble phosphorus, provides a long-term supply of this essential nutrient, while nitrogen-containing fertilizers such as ammonium sulfate and urea ensure adequate nitrogen nutrition of plants.

In the experiments, representative samples of BR-2 grade brown coal dust from the Angren deposit (composition shown in Table 1) and bentonite from the Azkamar deposit (composition shown in Table 2) were used. Additional components included ammonium sulfate (wt.%: moisture – 0.21; total nitrogen – 21.1), potassium chloride (wt.%: moisture – 0.9; K₂O – 60.7), and urea (wt.%: moisture – 0.3; total nitrogen – 46.2). To increase the yield of humic acids and enrich them with functional groups, the brown coal was oxidized with hydrogen peroxide in the presence of oxalic acid. The oxidation process was carried out at a hydrogen peroxide concentration of 10%, oxalic acid concentration of 60%, and a weight ratio of coal (organic part) : H₂O₂ : H₂C₂O₄ = 1 : 0.1 : 0.01. As a result, the macromolecular structures were partially decomposed, and new oxygen-containing functional groups (carboxyl, hydroxyl, phenolic) were formed, which significantly increased the reactivity of humic acids. The chemical composition of the oxidized coal is presented in Table 1 [3].

Thus, this approach makes it possible not only to develop a complex soil conditioner with high agronomic efficiency but also to ensure the effective utilization of local natural resources. The proposed compositions exhibit a combined effect: they simultaneously improve soil structure and water regime, supply the soil with macro- and microelements, and stimulate the biological activity of the soil microbiome.

To prepare soil conditioners with balanced nutrient content according to plant requirements, oxidized coal was mixed with ground bentonite, phosphate rock, urea, ammonium sulfate, and potassium chloride. Mixtures were prepared in the ratios: oxidized coal (organic matter) : bentonite : phosphate rock : (NH₂)₂CO : KCl : (NH₄)₂SO₄ = 100 : (25–200) : (2–78) : (1–38) : (1–11) : (1–5). The mixtures were ground in a porcelain mortar to particle sizes below 0.16 mm and dried at 75–80 °C for 60 minutes.

Table 1. 

Composition of the Initial and Oxidized Coal, %

Table 2.

Chemical composition of bentonite from the Azkamar deposit, Navoi region

The obtained soil conditioner at the component ratio of oxidized coal (organic part) : bentonite : phosphate rock : (NH₂)₂CO : KCl : (NH₄)₂SO₄ = 100 : 25 : 2 : 1 : 1 : 1 contained the following composition: total organic matter – 59.88%, humic acids – 35.08%, nitrogen – 0.40%, P₂O₅ – 0.22%, K₂O – 0.77%, CaO – 1.28%, MgO – 0.21%, SiO₂ – 7.36%, Mn – 4261.20 g/t, Zn – 207.84 g/t, Mo – 1.14 g/t, Co – 5.58 g/t, Ni – 6.73 g/t. It was shown that, depending on the ratio of the initial components, the nutrient content can be varied within a wide range.Importantly, these soil conditioners can be applied together with plant seeds during sowing, ensuring a continuous supply of nutrients throughout the entire vegetation period. For a single plant, an application of only 10–20 g of the conditioner is sufficient, depending on soil conditions.The soil conditioners were tested under laboratory conditions to determine their effect on soil water-holding capacity. Depending on the form of retained water, maximum adsorption capacity, field capacity, capillary capacity, and total moisture capacity were determined. The conditioners prepared from oxidized coal, bentonite, phosphate rock, urea, KCl, and (NH₄)₂SO₄, at an application rate of 7.5 tons per hectare, demonstrated excellent water-holding properties across the entire range from total to minimum field capacity.For untreated soil, water retention was 30.12% at total moisture capacity, 27.94% at capillary capacity, and 13.97% at minimum capacity. In contrast, with the conditioner, water retention increased significantly: 43.47% at total moisture capacity, 38.84% at capillary capacity, and 20.84% at minimum capacity.Thus, the results clearly demonstrate the feasibility of producing soil conditioners that not only enrich soil with essential nutrients but also enhance its water-holding ability and improve soil structure.