Seaweed Fertilizer: Applications, Efficacy, and Practical Guidelines

Recently, “natural plant (biological) stimulants” have gained sudden popularity. This category includes alginic acid, humic acid, amino acids, chitin, beneficial microbes, and more, offering multiple benefits such as nutrition supply, growth regulation, disease resistance, plant repair, immune enhancement, improved fertilizer utilization, and increased yields. Among these, seaweed extracts stand out prominently—numerous companies have simultaneously launched a wide range of seaweed fertilizers in the market, resulting in a mixed quality landscape. Today, we will objectively explore the applications and efficacy of seaweed in fertilizers.

Diverse Types of Seaweed

There are approximately 21,000 genera and 27,000 species of algae. Seaweed is classified into multiple phyla such as red algae, brown algae, and green algae, and by water temperature into warm-water, temperate-water, and cold-water species. As primary producers in the ocean, seaweeds accumulate abundant nutrients due to their unique growth environment. Below is a brief analysis of several common raw materials used in seaweed fertilizer production:

1. Surface green algae (e.g., Enteromorpha): Thrive on the sea surface, triggered by coastal eutrophication from human activities. These filamentous green algae grow rapidly in summer with rising temperatures, have a short lifespan, and decompose easily, offering low nutritional value in fertilizers.
2. Kelp: A type of brown algae cultivated in coastal shallow waters, with significantly stronger vitality than planktonic surface algae.
3. Warm/temperate-water deep-sea algae: Such as Sargassum and Macrocystis.
4. Wild cold-water algae from high-latitude deep seas: Such as Ascophyllum nodosum. Grown in low-light, low-temperature environments, they possess exceptional vitality.

Despite being raw materials for seaweed fertilizer, different seaweed species, coupled with variations in water depth, latitude, longitude, and temperature zones, lead to significant differences in growth cycles, quality, biological activity, nutrition, and energy content. Seaweeds growing in deep-sea, low-light, low-temperature, and high-salinity environments exhibit stronger photosynthetic capacity, nutrient enrichment ability, stress/cold resistance, and richer nutrients, functional components, and bioactive substances.

Limited Production and Extraction Technologies

Currently, three main extraction methods are used for seaweed-based fertilizers:

1. Chemical extraction: Involves strong acids (or alkalis) and high temperatures, which destroy some native bioactive substances in seaweed.
2. Physical extraction (mechanical crushing): Causes minimal damage but yields mostly macromolecular nutrients, which are not easily absorbed by crops.
3. Bioenzymatic hydrolysis: Uses multiple biological enzymes to extract bioactive and nutritional components, while degrading polysaccharides into oligosaccharides for efficient crop absorption. This method requires advanced technology and incurs higher costs.

Why Seaweed Extracts Deliver Outstanding Results

1. Adaptations to extreme deep-sea environments: Growing in low-light (1% of surface light at 100m depth), high-salinity, and low-temperature conditions, seaweeds have evolved enhanced photosynthetic efficiency and cold/freeze resistance genes. When applied to crops, they rapidly boost photosynthesis, nutrient absorption, and conversion, while providing freeze protection.
2. Strong vitality in harsh habitats: Thriving under high pressure and extreme tidal conditions, seaweeds impart crops with improved disease resistance, waterlogging tolerance, stress resilience, continuous cropping tolerance, and environmental adaptability, significantly enhancing crop immunity.
3. Exceptional nutrient enrichment and chelation capacity: To cope with nutrient scarcity, low light, high pressure, and low temperatures, seaweeds efficiently accumulate marine nutrients. Seaweed extracts contain unique compounds absent in terrestrial crops, including seaweed polysaccharides (alginate, fucoidan, laminarin), growth stimulants (cytokinins, auxins, gibberellins, abscisic acid, indoleacetic acid, polyphenols), mannitol, betaine, alginic acid, unsaturated fatty acids, organic iodine, vitamins, 14 medium-trace elements, and over 80 minerals. Their unique plant homeostasis-regulating factors activate enzymatic activity in crops, accelerating nutrient absorption and translocation, thereby greatly improving nutrient utilization efficiency.

Overview of Seaweed Fertilizer

Seaweed fertilizer is produced from marine brown algae, often supplemented with nitrogen (N), phosphorus (P), potassium (K), and medium-trace elements. It is primarily available in liquid and powder forms, with a small fraction in granular form.

1. Comprehensive Nutrition

In addition to valuable marine organic nutrients, seaweed fertilizer contains readily absorbable seaweed extracts, green algae polysaccharides, proteins, 18 amino acids, carbohydrates, and plant growth regulators (auxins, cytokinins, gibberellins) that significantly influence plant physiological processes. It also includes alginic acid, humic acid, vitamins, nucleotides, stress-resistant factors, and essential elements (N, S, Ca, Mg, Fe, Zn, Mn). All these bioactive substances are natural extracts from seaweed, free of pungent chemical odors (with a mild seaweed scent) and residues. Compared to chemical fertilizers, it offers distinct advantages in yield increase, stress resistance, naturalness, and safety.

2. High and Long-Lasting Fertilizer Efficiency

Rich in bioactive substances and fast-acting biological activators, seaweed fertilizer interacts with crop roots to dynamically regulate nutrient supply based on crop needs. Application results in sturdy stems, well-developed root systems, dark green leaves, plump grains, improved lodging resistance, delayed senescence, enhanced quality, and higher yields.

3. Disease and Pest Resistance

Seaweed fertilizer inhibits pathogens, prevents seedling death, and exhibits significant control effects on diseases such as cotton verticillium wilt, red leaf blight, wheat yellow leaf disease, and rice blast. It enhances crop resistance to various pests and diseases.

4. Soil Improvement

By promoting full nutrient absorption and improving fertilizer utilization, seaweed fertilizer prevents soil compaction, loosens and activates soil, and enhances soil fertility.

Key Bioactive Substances in Seaweed and Seaweed-Derived Plant Growth Regulators (SWC)

Marine brown algae contain diverse bioactive compounds. The main researched components in seaweed and SWC are as follows:

1. Cytokinins: A class of physiologically active purine derivatives that accelerate cell division and promote plant growth. Commercial SWC products have been detected to contain trans-zeatin, trans-zeatin riboside, isopentenyl adenosine, and other cytokinin glycosides (Jennings, 1969).
2. Auxins: Stimulate root development and cold resistance, improving cutting survival rates. Common auxins include indoleacetic acid (IAA). SWC products may also contain indolecarboxylic acid (ICA), N,N-dimethyl-β-indoleethylamine, indoleacetaldehyde (IAId), isoindole, and 1,3-indole (N-hydroxyethylphthalimide).
3. Gibberellins: Promote seed germination, plant growth, flowering, and fruiting. While fresh SWC products show gibberellin activity, their content in commercial products is often undetectable due to processing degradation.
4. Abscisic Acid (ABA): A plant growth inhibitor that induces organ abscission by maturing abscission zone cells. It antagonizes gibberellins (antagonism refers to the inhibitory effect of certain elements/hormones on the absorption or function of others).
5. Ethylene: Reduces growth rate and accelerates fruit ripening by promoting RNA/protein synthesis, increasing cell membrane permeability, and enhancing respiration. Limited international research exists on ethylene in seaweed.
6. Betaines: Derivatives of amino acids or imino acids that significantly increase chlorophyll content at low concentrations. Approximately 18 betaines have been identified in seaweed, mainly glycine betaine, β-alanine betaine, and γ-aminobutyric acid betaine.
7. Polyamines: Plant growth factors similar to auxins (not classified as plant hormones) that promote fertilization, flower bud differentiation, and embryo development, delay senescence, and aid post-harvest storage. No reports on polyamines in commercial SWC products are available.
8. Alginic Acid: Reduces water surface tension, stabilizing active ingredients for easy storage and use. It enhances nutrient absorption and maintains nutrient availability.
9. Seaweed Polysaccharides: Chelate heavy metal ions and improve soil aeration, reducing soil erosion by wind and water. Their unique stress resistance minimizes pesticide usage.
10. Quinones and Polyphenols: Participate in redox reactions in crops with humates, significantly increasing invertase activity and phosphorus-containing organic compound synthesis, thereby boosting sugar content in fruits and vegetables. They extend fruit storage life by 10–30 days and flowering periods by approximately 40 days.
11. Mannitol: Enhances crop water absorption, retention, and chlorophyll content, increasing leaf area by over 10%.

Seaweed fertilizer is a biological fertilizer made from marine algae through scientific processing. Its core components include natural bioactive substances beneficial to plant growth and mineral nutrients absorbed and enriched by seaweed from the ocean, such as seaweed polysaccharides, phenolic polymers, mannitol, betaine, plant growth regulators (cytokinins, gibberellins, auxins, ABA), and trace elements (Fe, B, Mo, I). To enhance efficacy and chelation, appropriate amounts of humic acid and medium-trace elements are often added.

History and Applications of Seaweed Fertilizer

Alginic acid-based foliar fertilizers were first produced in the British Isles in 1949.

Foliar fertilization, also known as extra-root fertilization, supplements plant nutrients via leaf spraying to regulate growth, correct nutrient deficiencies, prevent premature senescence, and increase yields.

This method is increasingly favored by lawn care professionals, especially for golf courses, due to its rapid nutrient supply, avoidance of soil adsorption/fixation, and high fertilizer utilization. It is particularly effective under stress conditions when root absorption is impaired.

Application Techniques

1. Appropriate concentration: Within a certain range, nutrient absorption rate and quantity increase with solution concentration, but excessive concentration (especially for trace elements) may cause phytotoxicity. Generally, macronutrients (N, P, K, Ca, Mg, S) are applied at 500–600x dilution, and trace elements (Fe, Mn, Zn) at 500–1000x dilution.
2. Optimal timing: Longer leaf wetting (30–60 minutes) improves absorption. Foliar fertilization is best conducted in the evening with no wind. Spraying in the morning with dew dilutes the solution, reducing efficacy. Avoid spraying before or during rain (nutrients are easily leached). If rain occurs within 3 hours of application, re-spray at a reduced concentration on a sunny day.
3. Uniform and thorough spraying: Use fine droplets and ensure even coverage, focusing on upper leaves and leaf undersides (where stomata are more abundant).
4. Adequate frequency and intervals: Due to low single-application absorption, spray 2–3 times with intervals of at least one week. Avoid excessive spraying to prevent harm.
5. Proper mixing: Mixing two or more foliar fertilizers saves time and labor, enhancing yield effects—provided no adverse reactions or efficacy loss occur. Ensure the mixed solution has a neutral pH (around 7) for optimal leaf absorption.
6. Add wetting agents: Crop leaves have a cuticle that hinders penetration. Adding appropriate wetting agents or surfactants reduces surface tension, increases leaf contact area, and improves absorption.

Critical Notes

1. Field experiments demonstrate that foliar fertilization is more effective when plants face growth stress (nutritional, climatic, or water-related).
2. Root fertilization is the fundamental method, while foliar fertilization is supplementary—prioritize root application.

Supplementary Table: Comparison of Common Seaweed Fertilizer Raw Materials

Raw Material Type	Growth Environment	Key Advantages	Nutritional Value	Application Suitability	Source Reference
Surface green algae (Enteromorpha)	Coastal surface, eutrophic waters	Low cost, easy access	Low	Low-demand crops, soil amendment	AlgaeBase (Guiry, 2024)
Kelp (Laminaria japonica)	Coastal shallow waters, cultivated	High yield, stable supply	Moderate	General field crops, vegetables	AlgaeBase (Guiry, 2024)
Sargassum (warm/temperate deep sea)	Deep seas (20–50m), warm/temperate	Rich in fucoidan, strong stress resistance	High	Cash crops, fruit trees	AlgaeBase (Guiry, 2024)
Ascophyllum nodosum (cold deep sea)	High-latitude deep seas, low-temperature	Abundant cytokinins, cold resistance	Very High	High-value crops, stress-prone areas	AlgaeBase (Guiry, 2024)

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