With the human population expected to reach 9 billion by 2050, food production needs to grow to meet the demand, bringing with it with it the risk of higher greenhouse gas (GHG) emissions (World Bank, 2023). In 2020, synthetic fertilizer use in land-based crop farming contributed roughly 475 million tons CO₂e to the global carbon footprint (World Resources Institute, 2026).

Seaweed-based agriculture supplements are growing in demand with rising consumer interest in sustainable agricultural practices (Chapman & Chapman, 1980; O’Connor, 2017). Liquid “biostimulant” extracts, meal/mulch, and biochar have been shown to benefit different crops; applied during key growth stages or stress conditions, they can increase yield, enhance nutrient uptake, and strengthen plant immunity and resilience to disease and adverse environmental conditions (Khan et al., 2009). Compared to conventional products made with fulvic or humic acid, microbial, trace element or amino acid mixtures, seaweed-based agricultural supplements have higher efficacy, higher sustainability profiles, and a larger part of the market share (40%), though the knowledge base behind their performance is less understood (Gupta et al., 2023; World Bank, 2023).

This chapter covers the state of the science, market, and policies involved in producing seaweed-based agriculture supplements, specifically liquid extracts, compost/mulch, and biochar, after raw seaweed biomass is dried and up to product use with specific crops. For seaweed cultivation and dewatering/drying, refer to the “Cultivation and Drying Considerations” chapter. After harvest, seaweed biomass is cleaned and dewatered, then kept wet or dried depending on the end product. Wet biomass is composted into meal/mulch for soil amendment, while dried biomass is either extracted (via soaking or heating) into a liquid extract applied as foliar spray/seed treatment/root soak or thermally converted into biochar for soil amendment use.

Pre-treatment for Processing

After being cleaned and dewatered, seaweed biomass is either kept wet or dry depending on the use-case. For example, seaweed compost/mulch performs better when processed wet while biochar production requires dry seaweed biomass. Liquid extracts can be produced with wet or dry seaweed biomass, depending on processing approach.

Processing

The processing stage is where pre-treated seaweed biomass is refined into the final agriculture supplement product and varies according to product.

Liquid extract

Seaweed-based liquid extracts were traditionally made by soaking seaweed in solvents (e.g., ethanol, methanol, water) until desirable compounds in the seaweed have passively diffused; more conventional methods use heating/boiling methods to promote extraction, though at the risk of quality (Rabhi et al., 2025). The final product is applied to crops as a diluted foliar spray, seed primer, or root soak for short- or long-term benefits (Kumar et al., 2020).

Seaweed fertilizer meal/mulch

Seaweed fertilizer meal/mulch is made by compositing wet seaweed and is used as a supplement to existing fertilizer or compost mixes (reviewed in Dang et al., 2023). It can provide long-term benefits due to the remediation effects it has on soil and subsequent plant quality (Battacharyya et al., 2015; Chapman & Chapman, 1980). The final product is packaged and distributed as a seaweed meal or pre-mixed with terrestrial bio-matter.

Biochar

Seaweed biochar is produced by heating seaweed over time. Compared to terrestrial biochar, it has a lower carbon content and surface area but a higher yield; furthermore, its alkalinity and richness in essential trace elements (e.g., nitrogen, phosphorus, and potassium) help it retain more nutrients, sequester more carbon, and even absorb metals from wastewater (Farghali et al., 2023; Roberts et al., 2015). The final product is packaged and distributed as a soil supplement.

Species Selection, Cultivation and Harvesting

Species Selection

There are over 10,000 species of seaweed that vary substantially in chemical composition across geography, life cycle, and seasonality. These factors determine the concentration of bioactive compounds most desirable for product performance specific to crops’ needs. Brown (e.g., Ascophyllum nodosum, Laminaria, Sargassum, Ecklonia maxima, Undaria) and red (e.g., Kappaphycus alvarezii) species are more commonly used due to their fast growth and large size, followed by green species (e.g., Ulva). Table 1 summarizes species used in commercial agriculture supplement production.

Table 1. Examples of seaweed species used in agriculture supplement production and companies.

Harvesting and cultivation

Seaweed used for agriculture supplements are harvested wildly or cultivated, but the method of cultivation impacts final product suitability. For liquid extracts, the timing of harvest matters because the seaweed’s chemical composition—key to product performance—changes with time, meriting cultivated approaches (Craigie et al., 2008; Featonby-Smith & Staden, 1984; Mooney & Staden, 1984). Timing of harvest for seaweed fertilizer meal/mulch and biochar is less relevant, permitting harvest of “waste” seaweed that has washed up on beaches or accumulated around waste management sites. Companies like Carbonwave are taking advantage of Sargassum blooms to harvest seaweed for agricultural supplements, reducing the risk of adverse ecological and economic impacts, while at the same time lowering production costs.

Processing

Processing information is proprietary in most cases, but general processing methods and active R&D are detailed below for each final product covered in this chapter.

Liquid Extracts

Four general techniques exist: alkaline, acid, physical disruption, and cell-burst. All methods aim to release soluble components within algal cells that contain compounds key to product performance; alkaline and acid conversions are unique in that they can impact extracts’ chemical compositions. Alkaline extraction is the most common method, and liquefies the whole seaweed into a slurry (Craigie, 2011; Craigie et al., 2008). Acid extraction and enzymatic and fermentative extractions also occur, the former most commonly as a pre-treatment before alkaline extraction (Battacharyya et al., 2015; Craigie, 2011; Craigie et al., 2008). Physical disruption or cell-burst technology conversions create a “micronized” suspension of fine particles released from the cells ( e.g., ~50 µm in diameter; Temple & Bomke, 1989). This can be achieved with low-temperature milling (e.g., cryo-crushing) or high-pressure forces (e.g., sub-critical water extraction) that rupture the cell walls (Battacharyya et al., 2015; Craigie, 2011; Geelen & Xu, 2020). After extraction, the final product is either dried into a soluble powder or prepared in liquid format. Liquid extract processes are summarized in Figure 1.

Novel cell disruption, cell-burst, and enzymatic extraction methods are being developed to cut down on extraction time, solvent usage, and energy needs (Table 2). Compared to traditional hot‑water extraction, microwave‑assisted extraction (MAE) delivers higher yields (6.17% vs. 5.2%) in a shorter time and with only a third of the solvent required (e.g., Yuan and Macquarrie, 2015; Sharma and Zalpouri, 2022). Furthermore, it can be applied directly in fresh biomass, avoiding one of the most energy-intensive steps in a seaweed-based product’s lifecycle (Zollmann et al., 2019). Ultrasound-assisted extraction (UAE) can also be used with fresh seaweed and avoids impacting heat-sensitive compounds. Furthermore, it can produce extracts with a fraction of the toxic chloride and sulfate concentrations retained in conventional high-pressure water extractions (for example, 12.5 mg/L of sulfate was retained using UAE versus 1520 mg/L using sub-critical water extraction; Zollman et al., 2019; Lewandowska et al., 2023). Enzyme-assisted extraction (EAE) uses hydrolytic enzymes (e.g., cellulase) to break down cell walls, where the enzymes used are specifically tailored to the plant’s cellular wall composition (e.g., cellulose; Choulot et al., 2025). Studies have demonstrated the potential of EAE to improve seaweed liquid extraction using commercial enzymes for terrestrial plants. For example, Naseri et al. (2020) treated Eucheuma denticulatum with Viscozyme® and produced 48.5% extraction efficiency, more than triple the efficiency of water-based extractions.

MAE, UAE, and EAE are desirable for industry-scale use due to their specificity in targeting desirable compounds, higher yields compared to traditional methods, and lower environmental impact compared to current commercial methods. However, each is in varying levels of technological readiness. For example, excessive temperature and microwave power for MAE can degrade polysaccharides and reduce total yield, requiring fine-scale controls (Dobrinčić et al., 2020). UAE requires specialized equipment to control ultrasound waves as well as the surface tension and viscosity of the solvent for optimal performance (Bordoloi et al., 2020). EAE will provide maximum efficiency at scale if cost-effective seaweed-specific enzymes are developed (Matos et al., 2021).

Table 2. Alternate extraction techniques for seaweed liquid extract production, claimed benefits, and technological readiness/status.

Seaweed fertilizer meal/mulch

Meal/mulch processing uses wet seaweed biomass to preserve a seaweed’s nitrogen content, though excessive moisture can impact quality. It is processed through aeration composting or vermicomposting (using earthworms), either alone or with terrestrial bio-matter and/or manure (Ananthavalli et al., 2019; Biruntha et al., 2020; Dang et al., 2023). The seaweed is then ground into a meal or powder through mechanical disruption (Craigie, 2011).

Active R&D is exploring different co-composting methods (i.e., composting seaweed with different terrestrial plant, fecal, or microorganism feedstocks) to produce carbon-to-nitrogen ratios more suitable for crops, decrease salinity and heavy metal concentrations, and boost degradation of polysaccharides and phenols (reviewed in Dang et al., 2023).

Biochar

Seaweed biochar is produced through energy-intensive thermochemical conversion processes like pyrolysis, gasification, or hydrothermal carbonization (Table 3; Dang et al., 2023; Farghali et al., 2023). An active area of R&D is on reducing the energy needs to produce seaweed biochar at a quality beneficial to crops (Dang et al., 2023).

Table 3. Biochar conversion methods, typical conditions and performance, and notable characteristics.

Cascading biorefineries

Cascading biorefineries seek to produce multiple final products from the waste of other workstreams to use the “whole seaweed” and cut on production costs. For example, Pascual et al. (2020) found that using fermented seaweed sludge byproduct from seaweed water production significantly out-performed conventional fertilizer for growing Lactuca lettuce. Processing liquid extracts as a byproduct of existing (e.g., algin, hydrocolloid) and emerging industries (e.g., biofuel) is also being explored (Dang et al., 2023; Farghali et al., 2023; Seghetta et al., 2016)

See below for the Technology Readiness Levels of liquid extracts, fertilizer meal/mulch, and biochar.

Liquid extracts (TRL 8–9)

• There is already a liquid extract market with regional/global reach
• Uncertainty in product validation has limited consistent buy-in

Fertilizer meal/mulch (TRL 4–7)

• Using seaweed meal/mulch is an old practice, but traditional methods are slow to activate benefits for crops, limiting scale-up

Biochar (TRL 3–6)

• High-energy needs impede scaled production

In 2020, crop farming generated 2.5 gigatons of greenhouse gas (GHG) emissions; with about 23% (575 million tons) coming from synthetic fertilizer use (World Resources Institute, 2026). Seaweed-based liquid extracts reduce synthetic fertilizer use while maintaining or improving crop yield, operating primarily through two mechanisms: direct fertilizer substitution (applying less synthetic nitrogen thus avoiding process emissions from the manufacture of synthetic fertilizers) and nitrogen use efficiency improvement (achieving the same yield from less fertilizer, thereby reducing field N₂O emissions).

For example, Sharma et al. (2017) tested the efficacy of seaweed extracts on rice yield as a supplement and substitute of conventional fertilizer, finding that using 7.5% seaweed extract with 50% reduced conventional fertilizer usage produced statistically comparable yield to “business-as-usual” practices (i.e., 100% conventional fertilizer usage) while reducing the climate change impact by 35 kg CO₂e/ton of rice produced.  However, it is also important to note that the end-to-end carbon footprint of liquid extract production is unknown, as current LCAs have not quantified every stage of the seaweed’s lifecycle from nursery to product end-of-life and have not always included the carbon sequestered by seaweed during cultivation (see Table 5 for a summary of studies and their system boundaries). The TNC/Bain (2023) model used existing LCAs to determine mitigation from seaweed-based biostimulants equivalent to 0.3-1.2 Mt CO₂e/yr when applied to 3% of global farmland.

Mitigation Potential

Note: The mitigation potential estimates presented here are screening-level calculations to establish order-of-magnitude plausibility under specific adoption scenarios. In this case we used the existing TNC/Bain (2023) model. 

Evidence Base

Calculation

Step 1 — TNC/Bain 2030 anchor and implied mixed-crop displacement rate

The TNC/Bain figure is used directly as the primary 2030 estimate. Working backwards through their numbers also yields the implied mixed-crop displacement rate, which is used to build the 2050 Scenario A on a consistent methodological basis.

Step 2 — 2050 scenarios

Scenario A holds the TNC/Bain mixed-crop displacement rate constant at 1.34 kg CO₂e/t and scales farmland coverage to 30% — ten times the 2030 scenario, with an ambitious scale up of seaweed cultivation 9X to approximately 9M Ha. by 2050 (approximate growth rate of 13% per year)

Table 5. Adapted from Hasselström and Thomas (2022). Comparison of system products, functional units, and system boundaries which include climate impact categories.  *Sharma et al. (2017) did not directly measure metrics and referred readers to Ghosh et al. (2015). Presence and absence of a system boundary is depicted with a check and x mark, respectively. ns : not significant; n/a: not applicable.

Agriculture supplement performance and crop yield

The largest concern to potential users of seaweed-based agricultural supplements is the drop in crop yield if management practices change (Pramanick et al., 2022; World Bank, 2023). Most studies find that adding seaweed extract to conventional fertilizers significantly boosts crop yields compared to using fertilizers alone. For example, multiple studies on global crops like maize, rice, pulses, potato, and sugarcane using Kappaphycus alvarezii and Gracilaria edulis liquid extracts have produced between 8-25% increased crop yield as a supplement, two produced statistical parity between control yield and yield from a substitution of conventional fertilizer with liquid extract, and two increased yield as a substitute (reviewed in Veeragurunathan et al., 2023; Table 7).

Table 7. Impacts on crop yield by using seaweed extract (SWE) as a supplement or substitute for traditional fertilizer (TF). In all studies, the control was a 100% recommended rate of conventional fertilizer and a water spray.

Table 6. State of the Market of global seaweed production and seaweed-based agricultural supplement sectors, including its value and growth rate as of 2023.

Cost-competitiveness and market adoption

Seaweed-based agricultural supplements currently cost more than conventional major fertilizers due to the price of processing and conversion, inconsistent efficacy (measured in price per crop yield), regulatory gaps, and supply chain limitations, but similarly priced as competitor alternate fertilizers like microbial soil supplements and slightly more expensive than humic supplements (Table 8). Cost could decrease with increased seaweed production and extraction efficiency; to reduce competition with non-seaweed supplements, products would need to be differentiated on consistently superior performance as well (Thamrin et al., 2020).

Table 8. Comparative cost ranges and considerations for seaweed-based agricultural products versus conventional fertilizers.