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Seaweed Fertilizers: Types, Functions, Formulations

Seaweed Fertilizers: Types, Functions, Formulations

Seaweed fertilizers are organic fertilizers made from seaweed, providing an array of macro and micronutrients that can benefit plants and soil. We shared a more detailed article about their features and principles, today let’s go deeper to talk about their components and formulations. Compared to traditional synthetic fertilizers that provide only NPK, seaweed fertilizers contain over 60 naturally occurring nutrients and minerals that enhance overall plant health and development.

Seaweed fertilizers are made from brown seaweeds like kelp, bladderwrack, and Ascophyllum that are sustainably wild-harvested or farm-grown. The seaweed biomass is processed into liquid extracts, meals, powders, and granules using methods like fermentation, enzyme digestion, and drying.

The key benefits of using seaweed fertilizers include improved germination, increased root growth, more robust flowering and fruiting, enhanced resistance to diseases/pests, and higher Brix levels. Seaweed fertilizers work both as a plant nutrition source and a biostimulant due to the presence of plant hormones, vitamins, amino acids, and other bioactive compounds.

Unlike chemical fertilizers that can build up in the soil and cause environmental issues, seaweed fertilizers improve soil health by increasing microbial activity, stabilizing pH, and adding organic matter. With multi-functional benefits for plants and soils, seaweed fertilizers provide a more sustainable alternative to traditional synthetic fertilizer products.

Seaweed Sources and Types

Seaweed Sources and Types

Seaweed fertilizers are derived from various species of seaweed, primarily from the brown algae family. The most common types used in fertilizer production include:


Kelp is a fast-growing brown algae that contains high levels of nutrients. Giant kelp and bull kelp are commonly harvested and processed into seaweed meal or liquid extracts. Kelp is rich in potassium, magnesium, iron, and other minerals that provide nutritional benefits to plants.


Ascophyllum nodosum is another brown algae species that thrives in cold, northern waters. It contains plant growth hormones like cytokinins and auxins that can stimulate root and shoot development. Ascophyllum is commonly used to make liquid seaweed fertilizers.


Bladderwrack (Fucus vesiculosus) is a type of brown algae that contains alginates and mannitol. It is high in potassium, zinc, magnesium, and iron. Bladderwrack extracts are used to boost plant immunity and resistance.

Other Brown Algae

Additional brown algae species used to produce seaweed fertilizers include Ecklonia maxima, Laminaria digitata, Sargassum, and Fucus serratus. These nutrient-dense seaweeds contain a wide array of minerals, vitamins, amino acids and plant growth hormones.

The combination of multiple seaweed species provides a complete nutritional profile to enhance soil quality and plant growth. Manufacturers carefully select and blend seaweeds to create balanced organic fertilizers.

Manufacturing and Processing

Seaweed fertilizer manufacturing

Seaweed fertilizer manufacturing is a multi-step process that converts raw seaweed into usable fertilizer products. Here are the key steps:


The process starts with harvesting seaweed from oceans and seas around the world. Common species used include kelp, bladderwrack, and other brown algae varieties. Seaweed can be harvested wild or farmed via aquaculture operations.


After harvesting, the seaweed undergoes drying to remove moisture. This is done either by sun drying or using industrial dryers. Drying condenses nutrients and extends the shelf life.


Dried seaweed is milled into smaller pieces to make it easier to process. Hammer mills, impact mills, and attrition mills are commonly used. Milling increases the surface area for further processing.


Milled seaweed undergoes extraction to remove soluble components like alginates, mannitol, and plant hormones. Hot water, alkaline solutions, or solvents can be used for extraction.


Extracts are concentrated through evaporation to increase nutrient density. Concentrates may be dried into powders or kept as liquid formulations.


Seaweed extracts and meals are blended with other ingredients like micronutrients to create specialized fertilizer products. Liquids, granules, or powders can be produced based on needs.


Finally, the fertilizer products are packaged into containers for distribution and sale. Common packaging options are plastic jugs, pails, and bags.

Proper manufacturing controls ensure high-quality, consistent seaweed fertilizer products. The end result is a sustainable fertilizer rich in marine nutrients.

Key Nutrients and Minerals in Seaweed Fertilizer

Key Nutrients and Minerals in Seaweed Fertilizer

Seaweed fertilizers contain a wide array of essential plant nutrients and minerals that are vital for healthy growth. The key nutrients typically found are:

Nitrogen (N) – Seaweed has moderate levels of nitrogen, which is key for lush vegetative growth and green foliage. Nitrogen promotes leaf and stem growth.

Phosphorus (P) – Phosphorus encourages blooming, fruiting, and root development. It also helps with seed germination.

Potassium (K) – Potassium, or potash, increases disease resistance and drought tolerance. It produces sturdier plants.

Calcium – Calcium is essential for cell wall structure and fruit development. It prevents blossom end rot.

Magnesium – Magnesium aids in chlorophyll synthesis and activates vital plant enzymes.

Sulfur – Sulfur helps form amino acids, proteins, and vitamins. It promotes nodule formation.

Iron – Iron allows chlorophyll synthesis and is vital for photosynthesis. It prevents leaf chlorosis.

Zinc – Zinc is important for enzyme activation, seed formation, and internode elongation.

Manganese – Manganese catalyzes plant enzyme reactions and metabolism.

In addition to primary and secondary macronutrients, seaweed fertilizers contain a plethora of micronutrients like boron, copper, molybdenum, and cobalt in readily available forms. These provide balanced nutrition for optimal plant growth.

The NPK ratios in seaweed fertilizers range from extremely low (1-1-2) to more moderate (4-2-2). This makes them suitable for regular feeding.

Uses in Organic Farming

Seaweed fertilizer is widely used in certified organic farming

Seaweed fertilizer is widely used in certified organic farming due to its natural ingredients and sustainability. The raw seaweed materials and manufacturing processes meet organic standards, allowing seaweed fertilizer products to be OMRI certified for use in organic production.

Compared to traditional synthetic fertilizers, seaweed fertilizers provide nutrients in plant-available forms while enhancing soil health. Leading organic seaweed fertilizer brands use sustainably wildcrafted seaweeds or responsibly farmed seaweed materials.

Seaweed extracts introduce beneficial microorganisms to the soil, promoting healthy rhizosphere activity. The plant hormones in seaweed also stimulate root growth and nutrient uptake. These characteristics make seaweed fertilizer ideal for organic systems aiming to grow nutrient-dense crops without synthetic chemicals.

Reputable organic seaweed fertilizer manufacturers follow strict quality control and testing protocols to ensure their products are contaminant-free. With a range of NPK ratios available, seaweed fertilizers can be tailored to the specific needs of organic farms and gardens. When used as directed, they provide natural nutrition for optimal plant growth and yields.

Application Methods

Seaweed fertilizer can be applied to plants

Seaweed fertilizer can be applied to plants and soil in various ways depending on the formulation. The most common application methods include:

Foliar Spray

Foliar spraying seaweed fertilizer directly on plant leaves and stems is an effective way to provide a quick nutrient boost. The nutrients and minerals are rapidly absorbed through the leaf pores and foliage. Foliar sprays work well for liquid seaweed extracts, water-soluble powders, and diluted granular fertilizers. Follow label instructions for proper dilution rates. Apply early morning or late afternoon to avoid leaf scorching. The fertilizer is absorbed within a few hours. Repeat applications may be needed every 2-4 weeks. Foliar sprays are ideal for vegetables, flowers, and container plants.

Soil Drench

Pouring diluted liquid seaweed fertilizer directly onto the soil around the root zone of plants is known as soil drenching. This method delivers nutrients directly to the roots for steady uptake. It helps build the soil food web. For best results, water the soil before application to enhance absorption. Liquid seaweed can also be mixed into irrigation water. Soil drenching works well for potted plants, trees, shrubs, and garden beds. Granular fertilizers can also be mixed into the top few inches of soil. Apply every 4-6 weeks during the growing season.

Benefits for Plants and Crops

Seaweed fertilizer's Benefits for Plants and Crops

Seaweed fertilizer provides many benefits for plants and crops. The key benefits include:

Increased Growth

Seaweed fertilizer enhances plant growth in several ways. The natural plant hormones like auxins, cytokinins, and gibberellins in seaweed promote cell division, elongation, and differentiation. This leads to stronger root systems, more vigorous vegetative growth, and increased yields. The wide array of minerals and trace elements in seaweed also support plant development.

Stress Tolerance

Seaweed fertilizer increases plants’ tolerance to various environmental stresses. The polysaccharides and amino acids help plants better handle stresses like drought, extreme temperatures, salinity, or disease pressure. The anti-oxidants in seaweed also minimize oxidative damage. Overall, seaweed strengthens plants’ natural defenses for dealing with stress.

Liquid vs. Granular Seaweed Fertilizers

Liquid vs. Granular Seaweed Fertilizers

Liquid and granular seaweed fertilizers each have their own advantages and disadvantages. Here’s an overview of the key differences:

Liquid Seaweed Fertilizer


Faster nutrient absorption since already dissolved in water

Can be mixed with other liquid fertilizers and applied through irrigation

Works well for foliar feeding and spraying directly on plant leaves

Provides a quick boost when plants need immediate nutrition

Easy to measure out and apply small amounts

Can apply with sprayers or through drip irrigation


More expensive per volume compared to granular

Can clog irrigation systems if not diluted properly

Higher risk of burning plants if overapplied

Nutrients may leach out of soil more quickly

Requires more frequent applications

Granular Seaweed Fertilizer


Provides a slow, steady release of nutrients over time

Nutrients less likely to leach from soil

Cost effective for large scale application

Can be worked into soil for root uptake

Less risk of burning plants

Can create a residual effect in soil for future crops


Slower to take effect compared to liquids

Difficult to do foliar feeding

Requires working into soil for best results

Harder to make small, precise applications

Can attract pests if left on top of soil

So in summary, liquid seaweed fertilizers offer more versatility and faster effects, while granular options provide longer-lasting nutrition and cost savings. Consider the needs of your plants and your application methods when deciding between liquid vs. granular seaweed fertilizer. Proper dilution and dosage are important with both forms to avoid plant damage.

Using Seaweed Fertilizers

Using Seaweed Fertilizers

Proper storage, handling, and application are key to maximizing the benefits of seaweed fertilizers. Here are some tips:


  • Store in a cool, dry place out of direct sunlight. High heat and humidity can degrade seaweed fertilizers over time.
  • Liquids should be stored above 40°F to prevent freezing and separation. Granular products are more stable.
  • Use containers that seal tightly. Open bags of granular fertilizer can absorb moisture and clump.


  • Wear gloves and wash hands after handling, as the salts may be drying to skin.
  • Avoid inhaling dust from granular fertilizers. A damp cloth can help contain dust.
  • Check for spills and clean equipment after mixing/loading liquids to avoid corrosion.


  • Follow label instructions carefully for amount and timing. More is not always better.
  • Apply directly to soil or as a foliar spray, depending on the formulation.
  • Water in granular products after spreading to help dissolve and activate.
  • Reapply every 2-4 weeks for a continual effect during key growth periods.
  • Combine with compost teas or organic nutrients for an added boost.
  • Keep good records to refine application rates and schedules for your conditions.

Proper use combined with good cultural practices will help maximize the benefits of seaweed fertilizers. Always consult the label or manufacturer for product-specific guidance.

Here at Dora, we have been manufacturing seaweed fertilizers for many years and we have our own formula to better take advantage of the seaweed fertilizers and for your plant’s growth. Contact us if you need them!

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