A study published in the journal Discover Agriculture by Elia N. M. Ruben, Simeon S. Hamukoshi, and colleagues systematically investigated how a simple water-soaking pre-treatment could optimize marine biomass for the production of agricultural biochar. The research focused on two abundant seaweed species from the Namibian coast, the brown alga Laminaria pallida and the red alga Gracilariopsis funicularis, aiming to reduce their high salt content while preserving their nutrient value. The resulting biochar can serve as a climate-smart technology, utilizing increased marine seaweed biomass through carbon sequestration and soil fertility improvement. The authors found that cold-water soaking of Gracilariopsis funicularis for six hours yielded the most agronomically favorable biochar, reducing the electrical conductivity decrease from the unsoaked biomass electrical conductivity in cold water treatments^.. This optimal biochar also retained total nitrogen and a moderate pHpH is a measure of how acidic or alkaline a substance is. A pH of 7 is neutral, while lower pH values indicate acidity and higher values indicate alkalinity. Biochars are normally alkaline and can influence soil pH, often increasing it, which can be beneficial More of 9.3, making it highly suitable for soil amendmentA soil amendment is any material added to the soil to enhance its physical or chemical properties, improving its suitability for plant growth. Biochar is considered a soil amendment as it can improve soil structure, water retention, nutrient availability, and microbial activity.   More, particularly in arid regions where salinity managementSalinity is the buildup of salts in the soil, which can harm plant growth. Biochar can help manage salinity by improving soil drainage and reducing the concentration of salts in the root zone.   More is critical.

The researchers employed a factorial design, testing two water temperatures and four soaking durations (0, 0.5, 2, and 6 hours) on both seaweed species. The effectiveness of the soaking treatment was evaluated based on the biochar’s pH, electrical conductivity (EC), and the concentration of key nutrients and heavy metals. Electrical conductivity is a strong indicator of soluble salt content and salinity hazard in soil amendments, generally considered detrimental to sensitive crops. For G. funicularis, cold water soaking progressively decreased the EC over time, with the lowest value of recorded after six hours. This decrease is primarily attributed to the effective leachingLeaching is the process where nutrients are dissolved and carried away from the soil by water. This can lead to nutrient depletion and environmental pollution. Biochar can help reduce leaching by improving nutrient retention in the soil.   More of soluble cations and ionic compounds facilitated by cell wall swelling. However, for the same species, hot water treatment showed an opposing trend, with EC increasing with prolonged soaking, suggesting enhanced solubilization of mineral salts and thermal breakdown of cell wall structures releasing additional cations.

Nutrient retention was another critical finding influenced by the pre-treatment conditions. For both seaweeds, cold water soaking led to an increase in total nitrogen (N) content with prolonged duration, whereas hot water soaking caused N to decrease. The highest total N was achieved in G. funicularis after six hours of cold water soaking. This superior nitrogen retention under cold water likely reflects reduced leaching losses, as elevated temperatures enhance nutrient solubility and volatilization. Similarly, the extractable phosphorus (P) in G. funicularis showed an increasing trend with soaking time, achieving its highest concentration of after six hours of cold water treatment. This increase is thought to be due to the removal of competing salts, allowing P mobilization within the biomass.

The study highlighted species-specific differences, which are crucial for selecting an appropriate feedstockFeedstock refers to the raw organic material used to produce biochar. This can include a wide range of materials, such as wood chips, agricultural residues, and animal manure. More. The brown alga L. pallida yielded biochar with excessively high alkalinity (pH range of 10.98-11.35) and persistently high sodium (Na) concentrations, ranging from after six hours of cold water soaking. This strong alkalinity and high salinity limit its immediate agronomic suitability, particularly for neutral or slightly alkaline soils^{191919191919191919}. In contrast, G. funicularis produced moderately alkaline biochar (pH range of 8.6-9.47) and achieved a much lower exchangeable Na concentration after cold water soaking for six hours, reducing the salinity risk significantly.

Finally, the total concentrations of all heavy metals measured, including Cadmium (Cd), Chromium (Cr), Copper (Cu), Nickel (Ni), and Zinc (Zn), remained below the maximum permissible limits set by the International Biochar Initiative (IBI) for organic soil amendments across all treatments and species^.. The optimal G. funicularis treatment resulted in a notably high Zn concentration which could enhance soil micronutrient availability, suggesting a favorable trade-off for agronomic use.

The overall conclusion is that the simple, water-soaking pre-treatment is an effective strategy to enhance the agricultural suitability of marine biomass-derived biochar. While the residual EC and sodium levels remain a concern, further integration with techniques like vermicomposting or microbial inoculation is recommended to further buffer the pH, leach excess cations, and stabilize the biochar quality for broader application, especially in arid regions.

Source: Ruben, E. N. M., Hamukoshi, S. S., Handura, B., & Mupambwa, H. A. (2025). Optimizing the pre-treatment of marine biomass (Laminaria pallida and Gracilariopsis funicularis) for enhanced production of climate-smart agricultural biochar. Discover Agriculture, 3(268).