Improving Chernozem Fertility and Barley Yield Through Combined Application of Phosphorus Fertilizer and Ash–Carbon Amendment

Abstract

Phosphorus deficiency and declining organic matter limit crop productivity in Northern Kazakhstan’s chernozem soils. This study evaluates whether the combined application of phosphorus fertilizer and an ash-carbon amendment from industrial by-products can improve soil fertility and barley yield. In a three-year field experiment (2018–2020), four P rates (1/10, 1/5, 1/2, and the full recommended dose, Prec) were tested with 100 kg ha⁻¹ of ash–carbon product (“Agrobionov”). Across growth stages, we measured cellulolytic microbial activity, water-stable soil aggregates (%WSA), and grain yield. Relative to the control, P + ash–carbon increased microbial activity by up to 57.6% and %WSA by up to 76%. The highest yield (1.32 t ha⁻¹) occurred with Agrobionov + ½ Prec, a 51.7% increase over the control. These results indicate that pairing moderate P rates with an ash–carbon amendment enhances soil biological and physical properties and improves yield in P-deficient chernozems, supporting the sustainable use of industrial by-products as cost-effective soil amendments. Future work should assess long-term effects on C sequestration, nutrient cycling, and economic feasibility.

1. Introduction

Soil dehumification is a global environmental concern [1,2,3,4]. This process is particularly severe in the north regions of the Republic of Kazakhstan (RoK), where the development of virgin lands since 1953 has led to a 20–25% decline in humus content in chernozem soils a trend that continues today [5,6,7]. Dehumidification is primarily caused by the low availability of essential macronutrients in the soil, especially, available phosphorus. For example, 47.2% of arable land in Northern Kazakhstan is classified as low in available phosphorus [8,9]. Despite Kazakhstan’s strong agricultural potential and its export of grain to 44 countries, average grain yields remain low, ranging from 1.0 to 1.2 t ha⁻¹, largely due to limited access to mineral fertilizers [10,11]. On average, only 5–6 kg ha⁻¹ of fertilizers are applied annually, far below application rates in the USA (140 kg ha⁻¹), EU (130 kg ha⁻¹), Latin America (90 kg ha⁻¹), and Russia (49 kg ha⁻¹). [6,7,12,13]. Of all fertilizers applied in Kazakhstan, phosphorus fertilizers account for only 3.5–4.2 kg ha⁻¹ of P₂O₅. This means that in the soils of Northern Kazakhstan, the lack of phosphorus—essential for root system development and nutrient transport—further limits the growth of agricultural crops [7,14].

With rising phosphorus fertilizer costs [13,15], an alternative solution is to utilize industrial by-products such as ash and slag waste (ASW), which are already repurposed for soil applications in countries like England, Germany, the USA (70% usage), Poland, and China (80%). Kazakhstan has accumulated more than 500 million tons of ASW, 370.5 million tons of which are located in the northern region, with an additional 19 million tons generated annually [11]. Although ASW poses environmental risks through dust and leachate emissions [16,17], its application as a soil amendment offers clear advantages, including improved soil structure, increased nutrient availability and microbial activity, and reduced fertilizer costs [18,19,20,21]. Recent research further suggests that combining carbon-rich organic materials with inorganic fertilizers can yield synergistic benefits for soil fertility, nutrient use efficiency, and crop performance. Materials such as fly ash, compost, biochar, and others enhance soil pH, cation exchange capacity (CEC), and moisture retention, which are especially beneficial in degraded or nutrient-depleted soils [22,23,24,25,26]. When applied with mineral fertilizers, these carbon-rich amendments improve nutrient retention, benefit seedlings’ emergence, reduce leaching losses, and stimulate beneficial microbial activity, especially under phosphorus and nitrogen fertilization regimes [27,28,29,30].

Building on these findings, the present study investigates the potential of Agrobionov, an ash–carbon soil amendment derived from local industrial by-products, to enhance phosphorus fertilization efficiency in degraded chernozem soils. The amendment is a powder comprising fly ash and technical carbon (~2:1). Fly ash supplies SiO₂, Al₂O₃, and Fe₂O₃, which can aid structure and nutrient retention; technical carbon (from tire pyrolysis) was included (≤6%) to improve aeration and support microbial activity [31].

Although prior work has been carried out to optimize Agrobionov application rates for enhancing nitrogen uptake in barley and to examine its effects on soil hydrophysical properties, oil flax yield, environmental safety, and microflora activity in chernozem soils [32,33,34], no studies have assessed its combined application with phosphorus fertilizer for barley production on Kazakhstan’s chernozem soils.

It has been proven that the application of ash and slag increases soil fertility and crop yields. These materials improve the soil’s physical properties, such as soil structure and water holding capacity, as well as agrochemical balance, and biological activity, thereby enhancing the yield of cereal crops. [27,30,35]. They also provide plants with essential macro- and micronutrients in available forms, enriching the soil’s nutrient profile, improving its physical and chemical properties, and supporting the adoption of environmentally sustainable agricultural practices. Research also shows that the addition of ash and slag can enhance the soil’s microbial composition and nutrient dynamics, leading to increased crop yields [21].

Cruz-Paredes et al. [16] demonstrated that incorporating ash and slag at a 5% concentration significantly increased microbial activity during a single growing season. Similarly, Mandpe et al. [21] found that the addition of fly ash improved microbial and enzymatic activity during composting processes. According to a review by Jambhulkar et al. [35], microelements derived from ash and slag can supplement nutrient-poor Indian soils, contributing to improved soil biology and plant growth. Ram and Masto [23] also emphasized the benefits of combining ash and slag with materials such as lime, gypsum, sludge, poultry manure, vermicompost, and biochar. These blended amendments not only enhance nutrient availability but also stimulate microbiological processes critical for long-term soil productivity.

Therefore, the objective of this study is to identify optimal phosphorus fertilizer rates in combination with the Agrobionov ash–carbon amendment to improve soil fertility and spring barley yields in Northern Kazakhstan. Specifically, this study evaluates the effects of these treatments on the agrophysical, agrochemical, and biological properties of chernozem soils, as well as crop performance. We hypothesized that there exists an optimal combination of Agrobionov and phosphorus fertilizer dose that produces a synergistic effect, resulting in greater improvements in soil quality and crop yield than either amendment applied alone.

2. Materials and Methods

The study was conducted from 2018 to 2020 in the experimental field at Ualikhanov Kokshetau University (KU) in Northern Kazakhstan. The region has a continental climate, characterized by cold winters and hot summers [36]. The average monthly temperature in July ranges from +19 °C to +21 °C (occasionally reaching up to +35 °C), while in January it ranges from −17 °C to −19 °C (with extreme lows down to −35 °C). The average annual precipitation is approximately 350 mm, with 150–200 mm falling during the growing season. The hydrothermal coefficient (HTC), according to Selyaninov, ranges from 0.8 to 1.0, which is the ratio of precipitation during the growing season to the sum of active temperatures. Overall, the agroclimatic conditions are favorable for cultivating spring barley. However, successful production requires water-saving technologies, minimal tillage, proper selection of crop varieties, and appropriate fertilizer management. The region is considered a zone of risk-prone agriculture and is particularly vulnerable to climate change [37].

Agrometeorological conditions during the study period varied significantly. According to the Zerenda hydrometeorological station, the 2018 growing season experienced 279 mm of rainfall, 105 mm above the long-term average of 174 mm. In contrast, 2019 and 2020 were marked by drought, receiving 112 mm and 114 mm of rainfall, 62 mm and 60 mm below the long-term average, respectively.

The experimental plot soil was an ordinary chernozem—shallow, medium humus, heavy loam, classified by World Reference Base for Soil Resources (WRB) [38]) as a haplic chernozem (aric, loamic). Under the USDA Soil Taxonomy, it is classified as a Mollisol in the Ustolls suborder and, at the subgroup level, as a Typic Haplustoll. Organic matter was 4.2%, pH (water) 7.5, calcium occupied 90% of the exchange complex, and the cation-exchange capacity was 27 mol kg⁻¹, indicating good suitability for cereal crops. However, due to extensive arable use, the soil supply of easily hydrolyzable nitrogen and available phosphorus was very low (5.6 and 4.8 mg kg⁻¹, respectively), whereas exchangeable potassium was high (550 mg kg⁻¹).

Field experiments were conducted to assess the impact of different phosphorus fertilizer doses in combination with the Agrobionov (AGB) ash–carbon amendment on soil fertility and barley yield. Designed for enhancing soil fertility, improving water retention, and stabilizing degraded soils, the AGB amendment represents a sustainable solution that repurposes industrial by-products into a valuable agricultural resource. The chemical composition of AGB is 30% carbon, 44. % SiO₂, 18.5% Al₂O₃, 4.45% Fe₂O₃, along with other trace elements. The X-ray fluorescence analysis revealed insignificant levels of trace metals (Appendix A.1), much lower than the background levels in non-contaminated soils in Kazakhstan [39]. The ash–carbon product we used is marketed under the trade name Agrobionov.

The experiment included six treatments. There were a control without fertilizer or amendment, an AGB-only treatment with 100 kg ha⁻¹, and four treatments combining 100 kg ha⁻¹ of AGB with different phosphorus fertilizer rates: AGB+1/10Prec, AGB+1/5Prec, AGB+1/2Prec, and also Prec, which was the full calculated recommended dose without AGB. The full recommended phosphorus fertilizer dose was calculated based on the expected yield of spring barley (2.5 t ha⁻¹), taking into account phosphorus removal by the crop and the soil’s available phosphorus content, 4.8 mg kg⁻¹. The field experiment was conducted over three years, each year on new plots following spring wheat as the preceding crop. Each plot measured 125 m², with a registration area of 85 m², and the experiment was repeated four times on chernozem soil characterized by low nitrogen and phosphorus levels but high potassium availability. Soil samples were collected from 0–20 cm and 20–40 cm depths in spring, summer, and autumn to evaluate changes in soil structure, nutrient content, microbial activity, and crop performance. The study analyzed soil aggregate composition, water-stable soil aggregates (%WSA), bacterial activity, and barley yield. Potential heavy-metal contamination from the AGB amendment was assessed by testing the amendment material, soil, and grain.

Water-stable aggregates (%WSA) were determined by wet sieving under bench-top conditions [40] (Kemper and Rosenau, 1986) which evaluates the resistance of aggregates to dispersion under water through controlled immersion and mechanical agitation. Microbiological activity was analyzed by an accredited laboratory, to which soil samples were submitted for certified testing and reporting. Cellulolytic bacterial activity was measured using the method of E.N. Mishustin (1979) [41], wherein cellulose strips were buried at a depth of 0–20 cm and retrieved at three key phenological stages—May (after sowing), July (barley heading), and August (prior to harvest)—with three replicates per sampling period to assess microbial decomposition rates. To determine heavy-metal content in spring barley grain, 1 kg composite samples were collected from the overall lot, taking into account the yield of each treatment, in accordance with Interstate Standard GOST 13586.3-2019 [42]. Heavy metals in the grain were determined by stripping voltammetry in accordance with GOST 50686-94 and GOST 50683-94. Heavy metals in the Agrobionov amendment were determined using a portable X-ray fluorescence spectrometer [43].

Pairwise correlation was calculated using EXCEL, while ANOVA + Tukey HSD analysis of yield data was conducted in Statistika software, version 10 [44] applying the Fisher method [45] and R studio [46] with a significance level of α ≤ 0.05. We report LSD₀₅ (least significant difference at α = 0.05) from one-way ANOVA and Pearson’s r between P rate and response within each month.

3. Results

3.1. Cellulose-Decomposing Microbial Activity

Over the three-year study, the activity of cellulose-decomposing bacteria in the control plots averaged 21.77%, while in fertilized variants, it increased significantly to 32.30–34.30%, depending on the phosphorus fertilizer dose. The highest microbial activity was observed in the AGB+1/2Prec (34.30%) and AGB+1/5Prec (33.62%) treatments, representing a 57.6% and 54.4% increase over the control, respectively. This enhancement in cellulose-decomposing bacterial activity suggests an improvement in soil microbial function due to the combined effect of phosphorus fertilization and the Agrobionov amendment. A moderate positive correlation (r = 0.50) was established between phosphorus fertilizer doses and bacterial activity, indicating that higher phosphorus availability supported microbial decomposition processes in the soil (Figure 1).

Figure 1 indicates that microbial activity varied across different months, with measurable differences in response to phosphorus application. The least significant difference (LSD₀₅) and correlation coefficient (r) values suggest variations in microbial response depending on the time of measurement. In May, microbial activity showed a stronger correlation with phosphorus application (r = 0.54) and an LSD₀₅ value of 1.50, indicating significant differences between treatments. By July, the correlation weakened (r = 0.09), while the LSD₀₅ remained at 1.50, suggesting that microbial activity had stabilized. In September, the correlation was minimal (r = 0.02), with an LSD₀₅ value of 0.70, indicating fewer statistically significant changes among treatments. On average, across all months, the LSD₀₅ was 1.40, with a moderate overall correlation of r = 0.45, reinforcing that phosphorus application in combination with the Agrobionov amendment influenced microbial activity, particularly in the early stages of barley growth.

3.2. Water-Stable Soil Aggregates (%WSA)

The soil aggregate stability (%WSA) showed no significant overall changes; however, a notable improvement in water-stable aggregates was observed in the top 20 cm layer of treated plots, particularly in the AGB+1/5Prec, AGB+1/2Prec, and Prec variants. The water resistance of soil aggregates increased from 29% in the control to 34.0–51.3% in treated plots, indicating that the combination of ASW-based materials and phosphorus fertilizer contributed to enhancing soil physical properties. The highest water stability was recorded in AGB+1/5Prec (51.3%), AGB+1/2Prec (46.0%), and Prec (46.3%) (Figure 2), suggesting that these amendments reinforced soil structure and aggregation. A weak but positive correlation (r = 0.46) was found between the water stability of soil aggregates and phosphorus fertilizer doses.

In the 20–40 cm soil layer, %WSA also increased in fertilized plots, ranging from 29% to 44%, compared to 15% in the control. The most significant improvements were observed with the AGB+1/5Prec (44.0%) and Prec (36.7%) treatments. These results suggest that the application of Agrobionov amendment and phosphorus fertilizers contributed to greater soil stability and resilience, particularly in the topsoil, where organic matter and microbial activity play a crucial role in soil aggregation and structure formation.

3.3. Barley Crop Yields

Spring barley yields in treated plots significantly exceeded those in the control, ranging from 1.11 to 1.32 t ha⁻¹. The highest yield (1.32 t ha⁻¹) was observed in AGB+1/2Prec plots, representing a 51.7% increase compared to the control (

Table 1. Effect of doses of phosphorus fertilizer in combination with the Agrobionov (AGB) amendment on the yield of spring barley.

Option	Productivity by Year, t ha−1				Increase Over Control
	2018	2019	2020	Mean	t ha−1	%
Control	0.77	0.75	1.08	0.87	-	-
AGB * only	0.82	0.87	1.20	0.96	0.09	10.3
AGB+1/10Prec **	1.16	0.91	1.37	1.15	0.28	32.1
AGB+1/5Prec	1.00	0.94	1.39	1.11	0.24	27.6
AGB+1/2Prec	1.44	1.06	1.46	1.32	0.45	51.7
Prec	1.00	0.92	1.40	1.10	0.23	26.4
LSD05	0.07	0.10	0.10	0.09

* Agrobionov amendment; ** the full calculated recommended dose without AGB.

). This substantial yield improvement suggests that the combination of Agrobionov amendment and phosphorus fertilization played a critical role in enhancing soil fertility and plant productivity.

In 2018, on the treated plots, barley yield increased significantly depending on the phosphorus fertilizer rate by 0.23–0.67 t ha⁻¹ (29.9–87.0%) compared with the control (control yield 0.77 t ha⁻¹). The least significant difference at the 5% level (LSD₀₅) was 0.07 t ha⁻¹. The highest yield, 1.44 t ha⁻¹, was obtained with the AGB+1/2 of the calculated dose of available phosphorus. The exception was the AGB 100 t ha⁻¹ treatment, where the yield increase was not significant (yield 0.82 t ha⁻¹). In 2019, all treated variants produced a significant yield gain of 0.12–0.31 t ha⁻¹ (16.0–41.3%) (LSD₀₅ = 0.10 t ha⁻¹). In 2020, the yield increase was 0.12–0.38 t ha⁻¹ (11.1–35.2%). On average, in 2018–2020, barley yield from the treated variants was 0.96–1.32 t ha⁻¹, exceeding the control by 0.45–0.90 t ha⁻¹ (10.3–51.7%) (LSD₀₅ = 0.09 t ha⁻¹). The maximum average yield was achieved with AGB+1/2 of the calculated dose of available phosphorus—1.32 t ha⁻¹, which was 0.45 t ha⁻¹ (51.70%) above the control (Figure 3).

These results align with previous studies demonstrating that ash and slag waste (ASW) applications contribute to soil fertility improvements and yield increases by enhancing soil physical, chemical, and biological properties [16,23,24,47]. The integration of ASW with phosphorus fertilization has been shown to stimulate beneficial soil microflora, increase microbial activity, and improve nutrient availability, leading to better root development and plant growth. These findings reinforce the potential of ASW-based amendments as a sustainable agricultural practice, supporting long-term soil health, nutrient efficiency, and improved crop productivity.

Application of the ash–carbon product in combination with a phosphorus fertilizer increased the copper content to 0.14–2.01 mg kg⁻¹ (vs. 0.10 mg kg⁻¹ in the control), which is far below the maximum permissible concentration (MPC) of 33 mg kg⁻¹. Zinc did not exceed the control concentration (1.6 mg kg⁻¹) and ranged from 1.0 to 2.3 mg kg⁻¹. Lead in spring barley grain increased to 0.30–0.43 mg kg⁻¹ in the fertilized treatments (vs. 0.25 mg kg⁻¹ in the control) but remained well below the MPC of 32 mg kg⁻¹ [48]. From an environmental safety standpoint, the most favorable options were treatments with ½ of the calculated P-fertilizer rate combined with the Agrobionov product at 100 kg ha⁻¹, since granular double superphosphate contains lead compounds. In contrast, no detectable Pb was observed in the Agrobionov amendment. Cadmium, arsenic, and mercury levels in the barley grain were below detection limits.

4. Discussion

The application of ash and slag has been shown to enhance soil fertility and crop productivity. These materials improve stability of soil aggregates, water holding capacity, agrochemical balance, and biological activity in the soil, thereby increasing grain crop yields [23,49]. They also contribute essential macro- and micronutrients in plant-available forms, enriching the soil’s nutrient profile, enhancing physical and chemical characteristics, and promoting environmentally sustainable agricultural practices. Studies further indicate that the addition of ash and slag can improve soil microbial composition and nutrient dynamics, resulting in increased crop yields [21].

The combined application of phosphorus fertilizer and the Agrobionov (AGB) amendment significantly improved soil physical and biological properties, as evidenced by increased water-stable aggregate formation, microbial activity, and spring barley yield. These improvements were consistent across both topsoil (0–20 cm) and subsoil (20–40 cm) layers, indicating a broad impact on soil structure and function. Similar synergistic effects have been reported when organic or industrial by-product amendments have been combined with mineral fertilizers to enhance soil fertility and crop performance [23,27,49,50].

Phosphorus fertilization contributed to soil structural improvement by stimulating root growth and enhancing the secretion of polysaccharides, which are natural biopolymers that bind soil particles and promote aggregate formation [51,52,53]. In tandem, components of the Agrobionov amendment—particularly, fly ash—supplied silica and alumina, which reinforced soil cohesion and mechanical strength [54]. The presence of technical carbon and alkaline binding agents is likely to have further enhanced aggregate stability by promoting cementation processes. Fly ash, rich in SiO₂, Al₂O₃, and Fe₂O₃, has been shown to improve soil aggregation and water-holding capacity when applied at appropriate rates [55].

Enhanced microbial respiration and enzymatic activity stimulated by the mineral and carbonaceous components of Agrobionov can play a key role in nutrient cycling and long-term soil function [47]. These biological responses are particularly pronounced in phosphorus-deficient soils, where fertilization alleviates nutrient stress and encourages root exudation, microbial proliferation, and extracellular polysaccharide production [56]. A moderate positive correlation (r = 0.46) between phosphorus application rate and aggregate stability supports the involvement of these biological mechanisms in soil structure development [57].

The observed increase in water-stable aggregates (2.3–6.5%) reflects the synergistic effects of Agrobionov’s mineral and carbon inputs, which improved the physical structure of the soil while fostering a favorable environment for microbial activity. Root-induced changes in the rhizosphere, combined with microbial enzyme production, probably created a positive feedback loop that sustained aggregate formation and enhanced nutrient retention.

Spring barley productivity was particularly responsive to moderate phosphorus inputs in combination with AGB. The AGB+1/2Prec treatment produced the highest yield (1.32 t ha⁻¹), representing a 51.7% increase over the control. This result demonstrates a notable improvement in nutrient use efficiency, as similar or superior outcomes were achieved with lower fertilizer input compared to the highest phosphorus-only treatment (P68). These findings highlight the potential for nutrient savings and reduced environmental loading when AGB is integrated into fertilization regimes.

Beyond their agronomic benefits, Agrobionov amendments offer a sustainable approach to soil fertility management by repurposing industrial by-products such as fly ash and technical carbon derived from waste tires. Tire pyrolysis char (TPC), which shares similarities with biochar, has gained interest as an agricultural amendment due to its high porosity and carbon content, and its potential to improve soil aeration and water retention and stimulate microbial activity [51,52,53]. However, TPC and similar materials must be used cautiously due to potential contamination by trace metals and polyaromatic hydrocarbons [53]. Pre-treatment methods such as acid washing have been shown to reduce toxicity and improve safety for agricultural use [16]. In this study, chemical analyses confirmed that the AGB amendment contained only trace concentrations of metals, well below background levels [39], supporting its suitability as a safe and effective soil amendment.

Overall, the integration of Agrobionov with phosphorus fertilization significantly enhanced soil aggregate stability, microbial function, and barley productivity, particularly in the biologically active topsoil layer. These improvements are attributed to a combination of organic binding agents, root exudation, and microbial stimulation.

5. Conclusions

Microbial activity, as measured by flax cloth decomposition, increased by 54–58% in AGB+1/5Prec and AGB+1/2Prec treatments. Aggregate stability in the 0–20 cm soil rose from 29% (control) to 46–51% in the AGB+1/5Prec, AGB+1/2Prec, and Prec treatments. Spring barley yield increased from 0.87 t ha⁻¹ in the control to 1.11–1.32 t ha⁻¹ in treated plots, with the highest yield observed in AGB+1/2Prec (a 51.7% increase).

These results demonstrate the effectiveness of Agrobionov–phosphorus combinations in improving microbial function, soil structure, and crop yield. The amendment also reduced the need for high phosphorus doses while maintaining yield performance, highlighting its role in enhancing nutrient use efficiency.

Composed of fly ash and technical carbon, Agrobionov repurposes industrial by-products in line with circular economy principles. Given its low trace-metal content, the Agrobionov product aligns with circular-economy principles and appears safe at the tested rate; multi-year monitoring of C stocks and nutrient dynamics is warranted.