1. Introduction
2. Materials and Methods
2.1. Soil Sampling and Analysis
2.2. Statistical Analysis
3. Results and Discussion
3.1. Soil Chemical Properties
3.2. Comparison of SOM by Analytical Method
4. Conclusions
1. Introduction
Soil organic matter (SOM) is a central component of soil health, contributing to nutrient availability, pollutant immobilization, and improved soil physical properties such as aggregate stability and porosity [1, 2]. SOM also supports microbial diversity and activity in the soil ecosystem [3, 4]. Quantifying SOM is often achieved by converting soil organic carbon (SOC) values using correction factors, which range from 1.4 to 2.5 depending on soil type and depth [5]. In Korea, a conversion factor of 1.724 is commonly used, based on the assumption that SOC comprises 58% of SOM.
SOC can be determined through wet or dry oxidation methods. The Tyurin method, a wet oxidation technique using potassium dichromate and sulfuric acid, is prevalent in Korea due to its simplicity and high recovery rates. However, it poses environmental risks and may yield underestimates due to incomplete oxidation or interference from Cl⁻, Fe²⁺, and MnO₂ [5, 6, 7]. In contrast, dry combustion employs high-temperature oxidation and automated elemental analyzers, offering greater accuracy but at higher cost and operational complexity [8].
Previous studies have shown that the correlation between methods depends on soil properties. For example, Seo et al. [9] reported minimal differences in soils with low inorganic carbon, while Kim et al. [10] highlighted the importance of pretreatment in calcareous soils. This study evaluates and compares SOM contents derived from the Tyurin and dry combustion methods across three contrasting soil types: organic farming, calcareous, and reclaimed tidal soils.
2. Materials and Methods
2.1. Soil Sampling and Analysis
A total of 58 surface soil samples (0–15 cm) were collected: 24 from organic upland farming fields, 25 from calcareous soils (9 paddy and 16 upland), and 9 from reclaimed tidal upland soils. Samples were collected using a soil auger, stored in polyethylene bags, air-dried for one week, and sieved to 2 mm before chemical analysis.
Soil organic matter (SOM) content was quantified using both the Tyurin method and dry combustion via a Vario MAX CN analyzer (Elementary, Germany). For the Tyurin method, samples were pretreated to remove inorganic carbon (1:4:2 soil:HCl:water, 4 h standing time). Additional soil properties were analyzed as listed in Table 1. Instrumental settings for dry combustion are provided in Table 2.
Table 1.
Chemical analysis methods and equipment used for determining soil chemical properties
Table 2.
Settings for the dry combustion method used to determine organic carbon content in soils
2.2. Statistical Analysis
Regression analyses were performed using SAS 9.4 (SAS Institute Inc., Cary, NC, USA) to compare SOM values from both methods. A 5% significance level was used.
3. Results and Discussion
3.1. Soil Chemical Properties
The chemical properties of the three soil types varied considerably, reflecting differences in land management and parent material. Organic farmland soils exhibited a neutral to slightly alkaline pH (mean 7.0), high available phosphorus (Av. P₂O₅ = 560 mg kg⁻¹), and elevated CEC (17 cmolc kg⁻¹), likely due to continuous application of organic inputs. Organic amendments contribute to soil buffering capacity and cation availability by releasing base cations during decomposition [1, 2]. The high Av. P₂O₅ concentration suggests long-term accumulation from repeated applications of livestock manure compost [10].
Calcareous soils were characterized by a higher pH (mean 8.0), consistent with the presence of calcium carbonate (CaCO₃), which contributes to alkalinity and increases soil calcium and CEC [11, 12]. Av. P₂O₅ levels (315 mg kg⁻¹) were within the optimal range for upland crops, while CEC averaged 12 cmolc kg⁻¹. Despite moderate K⁺ and Mg²⁺ concentrations, the overall fertility status was sufficient for crop production.
Reclaimed tidal soils exhibited slightly alkaline conditions (pH 7.7), low Av. P₂O₅ (27 mg kg⁻¹), and low Ca²⁺ (1.2 cmolc kg⁻¹), but relatively high Mg²⁺ (2.7 cmolc kg⁻¹). These patterns are consistent with saline soil profiles, where marine-derived salts accumulate during reclamation processes [13, 14]. The low P availability and moderate CEC (8.1 cmolc kg⁻¹) suggest limited nutrient retention capacity. However, ongoing farming and soil remediation efforts can gradually improve these properties [15].
These trends and numerical values are summarized in Table 3, which provides a statistical overview of the chemical characteristics across the three soil types.
Table 3.
Statistics of the chemical characteristics across the three soil types
3.2. Comparison of SOM by Analytical Method
The SOM content determined by the Tyurin and dry combustion methods showed method- and soil-dependent variations (Table 4). In organic farming soils, both methods yielded comparable average SOM contents of 36 g kg⁻¹, though the dry combustion method slightly exceeded the Tyurin values across the range.
Table 4.
Statistical summary of soil organic matter (SOM) contents measured by the Tyurin and dry combustion methods
Regression analysis (Fig. 1a) showed a strong linear relationship (Y = 0.9092X, R² = 0.88, p < 0.001). The consistency between methods suggests that in soils with low carbonate and chloride interference, the Tyurin method remains a viable alternative to dry combustion. However, incomplete oxidation of stable organic compounds in the Tyurin method may still contribute to slight underestimations [5].
In calcareous soils, the dry combustion method reported significantly higher SOM values (mean 49 g kg⁻¹) than the Tyurin method (mean 26 g kg⁻¹), with some values nearly doubling. The regression (Fig. 1b) yielded a moderate correlation (Y = 0.4585X, R² = 0.79), though not statistically significant. This discrepancy is likely due to the presence of unremoved carbonates, which decompose at high temperatures during combustion, releasing CO₂ and falsely inflating SOM estimates [7]. Additionally, acid pretreatment in the Tyurin method may not completely eliminate carbonate interference. Kim et al. [10] emphasized the importance of rigorous carbonate removal in calcareous soils to improve inter-method consistency.
In reclaimed tidal soils, the Tyurin method produced higher SOM values (mean 4.1 g kg⁻¹) than dry combustion (mean 3.0 g kg⁻¹). Regression analysis (Fig. 1c) showed a positive relationship (Y = 1.1871X, R² = 0.78), indicating that the Tyurin method may overestimate SOM in these soils. The likely cause is oxidation of chloride (Cl⁻) during the wet oxidation process, which can artificially elevate SOC estimates [16, 17]. Additionally, high salinity and uncertain pretreatment effectiveness may result in analytical variability [18]. Although acid treatment is commonly applied to remove inorganic carbon, it can also lead to organic matter losses, particularly of recalcitrant fractions. Park et al. [19] noted that in saline soils, the precision of dry combustion results depends heavily on the effectiveness of pretreatment protocols.
These findings highlight that while the Tyurin method is suitable for low-interference soils, it requires modification and caution when applied to carbonate- or salt-affected soils. Conversely, although dry combustion is more robust across varying soil types, it is not immune to overestimation risks and requires proper pretreatment to ensure accuracy.
4. Conclusions
SOM quantification varied by method and soil type. The Tyurin method produced reliable results in organic soils but was less accurate in calcareous and saline soils. Dry combustion, although more robust, also requires pretreatment to avoid overestimation. Tailoring analytical methods to soil properties is essential for accurate SOM measurement.
