Document Type : Research Article
Authors
1
Soil Science and Engineering Department, Faculty of Agriculture, Shahrekord University, Shahrekord, Iran.
2
ShahreKord University
3
Soil Science and Engineering Department, Faculty of Agriculture and Natural Resources, Tehran University, Tehran, Iran.
10.22067/jsw.2026.100377.1578
Abstract
Introduction
The application of biochars produced from agricultural residues, including sugarcane bagasse and rice husks, as growing media in soilless cultures has gained increasing attention in recent years. However, the high electrical conductivity (EC) and alkaline pH of these biochars limit their application, particularly at high incorporation rates in growing media. The high EC of biochar is mainly associated with soluble salts, which may adversely affect plant growth when a considerable proportion of the growing medium is replaced with biochar. Previous studies have shown that water leaching can reduce biochar EC; however, its effect on reducing the pH of alkaline biochars is limited. The use of biochars with high pH may reduce nutrient availability to plants, thereby limiting plant growth. Leaching biochar with dilute acid solutions, such as sulfuric acid, has been suggested as a potential approach for reducing biochar pH. Therefore, this study aimed to evaluate the effects of distilled water and acidified water leaching on the chemical properties of sugarcane bagasse and rice husk biochar.
Materials and Methods
A factorial experiment was conducted using a completely randomized design (CRD) with two factors: biochar type at two levels (sugarcane bagasse and rice husk biochars) and leaching method at three levels (non-leached [raw biochar], leached with distilled water, and leached with acidified water), with three replicates. Sugarcane bagasse and rice husk were air-dried, ground, and sieved prior to pyrolysis. Biochars were produced in a furnace under oxygen-limited conditions at 500 °C for 2 h. The sugarcane bagasse and rice husk biochars were leached with distilled water until their EC decreased to below 0.5 dS m⁻¹. In contrast, leaching with acidified water was continued until their pH decreased to the range of 5.5–6.3, which is considered acceptable for use as a growing medium in soilless cultures. The chemical properties of sugarcane bagasse, rice husk, and their biochars were determined in three replicates before and after leaching. The evaluated properties included EC of a 1:10 sample-to-distilled-water extract, pH of a 1:10 sample-to-distilled-water suspension, carbon (C) and hydrogen (H) contents, C/N ratio, cation exchange capacity (CEC), and calcium carbonate equivalent (CCE). In addition, the total concentrations of nutrients, including N, P, K, Na, Ca, Mg, S, Fe, Mn, Zn, and Cu, in the raw feedstocks and their biochars were measured using standard laboratory methods. Analysis of variance was performed using SAS software version 9.4, and mean comparisons were conducted using the least significant difference (LSD) test at p < 0.05.
Results and Discussion
The results showed that, after 12 leaching cycles, the EC of sugarcane bagasse biochar decreased from 2.78 to 0.42 dS m⁻¹, while the pH decreased from 8.30 to 7.94. For rice husk biochar, after 8 leaching cycles, the EC decreased from 1.62 to 0.40 dS m⁻¹, while the pH decreased from 8.04 to 7.45. Leaching with distilled water decreased both EC and pH, whereas leaching with acidified water markedly decreased pH but increased EC. After 5 leaching cycles with 0.05 N sulfuric acid, the pH of sugarcane bagasse biochar decreased from 8.30 to 5.61, while its EC increased from 2.78 to 4.30 dS m⁻¹. For rice husk biochar, after 5 leaching cycles with 0.02 N sulfuric acid, the pH decreased from 8.04 to 5.54, while its EC increased from 1.62 to 2.32 dS m⁻¹. Leaching the biochars with distilled water significantly decreased CCE and the concentrations of C, H, N, K, Ca, Mg, S, and Na, whereas the C/N ratio and CEC significantly increased compared with the raw biochars. In contrast, leaching the biochars with acidified water significantly decreased CCE and the concentrations of C, N, and Na, whereas the C/N ratio, CEC, and the concentrations of Ca, S, Fe, Mn, and Cu significantly increased relative to the raw biochars. The observed decreases in the concentrations of some elements may be attributed to the leaching of soluble and readily exchangeable forms from the biochar matrix. The decrease in CCE following leaching with acidified water was likely due to the dissolution of carbonate-containing compounds in the acidic solution, whereas the decrease observed after leaching with distilled water suggests that a fraction of the carbonates was water-soluble. The increase in the C/N ratio was more pronounced following leaching with acidified water than with distilled water, likely due to the greater reduction in N concentration, particularly after acidified-water leaching. The increase in CEC was likely associated with the formation of oxygen-containing functional groups, such as carboxyl and hydroxyl groups, which increased the negative surface charge and enhanced cation retention. The reduction in Na concentration following leaching with both distilled and acidified water is beneficial for reducing the potential adverse effects of excessive Na on plant growth and growing media quality. The increased concentrations of S, Ca, S, Fe, Mn, and Cu following acidified-water leaching may improve the nutrient supply to plants, with the increases in Fe and Mn likely associated with enhanced dissolution of their oxides and hydroxides under acidic conditions.
Conclusion
The results demonstrated that leaching substantially modified the chemical properties of sugarcane bagasse and rice husk biochars, although the magnitude and direction of these changes depended on the leaching method and biochar type. Distilled-water leaching was effective in reducing EC to levels suitable for use in growing media, but its effect on reducing the alkaline pH of the biochars was limited. In contrast, acidified-water leaching was highly effective in reducing biochar pH to a range suitable for soilless growing media; however, it resulted in a considerable increase in EC, particularly in sugarcane bagasse biochar. However, neither leaching method alone provided an optimal improvement in all chemical properties simultaneously. Therefore, determining the most suitable leaching method and the potential use of leached biochar as a component of growing media requires evaluating the chemical properties of the final growing media at different biochar substitution ratios and plant responses.
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