Seaweed has been used as a fertilizer and soil conditioner for centuries, especially in coastal communities where it was plentiful. These days, seaweed is growing in popularity as an organic alternative to synthetic fertilizers for home gardeners and commercial growers alike. Using seaweed in your garden or on your crops provides a variety of benefits that can lead to healthier plants and higher yields.

Different Types of Seaweed Used in Agriculture

There are dozens of edible seaweed varieties found along coastal regions around the world. However, only a handful are extensively used in agriculture as fertilizers or soil amendments. These include.

Kelp – Kelp refers to large brown algae seaweeds from the order Laminariales. Species such as giant kelp and bull kelp are frequently used in organic farming. Kelp is collected from nearshore waters and processed into liquid extracts or meal. It provides over 70 trace minerals and elements, including potassium, magnesium, calcium and sulfur. Kelp meal is a popular soil conditioner.

Bladderwrack – Bladderwrack is another brown algae seaweed known for its high concentrations of iodine, minerals and sugars. It stimulates root growth and helps plants withstand stressful conditions. The fronds are dried and ground into meal, or used to produce liquid fertilizer. Bladderwrack may also be applied directly around plants as a fresh mulch.

Rockweed – Rockweed or knotted wrack is a common cold-water seaweed found on northern Atlantic coasts. Ascophyllum nodosum is the main species used in agriculture. Rockweed contains compounds like mannitol, zeaxanthin and betaines that provide anti-fungal and stress tolerance benefits. Dried rockweed pellets are a slow-release fertilizer option.

Dulse – Dulse is a red algae species popular as an edible sea vegetable. When used as a fertilizer, dulse releases nutrients slowly as it breaks down. It is effective applied as a fresh or saltwater-soaked mulch around garden plants. Dulse flakes can also be brewed into compost tea fertilizer.

In addition to these common varieties, local and specialty seaweed producers may offer fertilizers derived from sea lettuce, Irish moss, spirulina or other sea weeds harvested from nearby waters. Look for sustainably wild-grown or farmed seaweed for making fertilizers for your plants.

Nutritional Benefits of Seaweed

So, what exactly does seaweed offer plants in terms of nutrition? Here are some of the key benefits.

  • Macro and micronutrients – Seaweed provides nitrogen, phosphorus, potassium, calcium, magnesium and sulfur which are essential macronutrients needed for plant growth. It also contains micronutrients like iron, manganese, zinc, copper and boron. The variety of minerals in seaweed fertilizer gives plants a balanced nutrient profile.
  • Vitamins and amino acids – Seaweed contains a wide spectrum of vitamins including A, B1, B2, C, D and E. It also provides amino acids like glycine and glutamic acid which boost plants’ metabolic processes. These organic compounds support overall plant health.
  • Alginic acids – Brown seaweeds like kelp are particularly high in alginic acids which help plants retain water and nutrients. They create a gel-like substance in the soil that holds moisture and dissolved minerals around plant roots.
  • Growth hormones – Seaweed fertilizers contain plant growth hormones like auxins, cytokinins and betaines. These stimulate stronger root systems, greener leaves and stems, and bigger blooms or fruits in plants and crops.
  • Beneficial microbes – The polysaccharides and microorganisms present in seaweed fertilizers act as prebiotics for soil microbiome. This increases populations of beneficial bacteria and fungi that boost soil and plant health.

Maximizing the Benefits of Seaweed Fertilizers

To gain the most benefits from using seaweed fertilizers, keep these tips in mind.

  • Purchase high-quality products from reputable sources. Look for seaweed sustainably harvested from unpolluted waters.
  • Apply early in the growing season to establish healthy plants with strong roots and stems. Use again as needed during growth flushes.
  • Combine with compost, manures or other organic matter for enhanced effects. The seaweed nutrients will be retained longer.
  • Use liquid seaweed extracts for immediate nutrient delivery. Meals and granular forms provide slower, steady release of nutrients.
  • Start seeds or take cuttings in seaweed-enriched potting mix for vigorous growth.

Using Seaweed Fertilizer in Your Garden

Seaweed fertilizer comes in different forms – meal, liquid extracts or granules and pellets. It can be easily incorporated into your garden in a few different ways.

  • Mix seaweed meal into potting mix if starting seeds or transplanting seedlings. This gives them an immediate nutrient boost.
  • Work granulated seaweed into garden beds or row crop soil pre-planting. This allows time for the nutrients to integrate into the soil.
  • Side dress growing plants with seaweed meal or liquid extracts throughout the growing season. This provides a nutritional boost during crucial growth periods.
  • Create a compost tea by soaking seaweed meal in water for 1-2 weeks. Use the resulting liquid to water plants – the nutrients will be readily absorbed.
  • Mulch around plants with seaweed meal or chips to suppress weeds and slowly release nutrients as the seaweed breaks down.

When using seaweed fertilizer, little goes a long way. Use approximately 1 cup of meal per 100 square feet of soil. For liquid extracts, dilute concentrate 5:1 before applying. Excess seaweed can make soils too salty.

Growing Seaweed for Agriculture

In addition to wild harvesting, some seaweed varieties are cultivated specifically for use as bio-stimulant fertilizers and animal feed supplements. Seaweed mariculture operations grow seaweed on underwater structures or nets out in coastal waters. Key cultivation benefits include.

  • Allows precise control over nutrient levels based on plant needs.
  • Avoids depleting wild seaweed beds.
  • Reduces water pollution by absorbing dissolved nutrients like nitrogen and phosphorous.
  • Sequesters carbon dioxide from the water.
  • Provides habitat for marine species.

Kelp, gracilaria, eucheuma and porphyra species are especially suitable for mariculture. The fast-growing seaweeds are seeded onto growing structures and can be harvested multiple times a year. Supporting seaweed farming helps take pressure off of wild stocks while also promoting ecologically sustainable agriculture systems.

Final Thoughts

Seaweed, a natural gift from the oceans, has long been a cornerstone in coastal agriculture and is now re-emerging as a sustainable and beneficial fertilizer. Its diverse array of nutrients, combined with its environmentally-friendly attributes, make it an unparalleled choice for gardeners and farmers alike. From its myriad of macro and micronutrients to its unique growth-promoting properties, seaweed truly offers a holistic approach to plant nutrition. As we continue to seek out eco-friendly and organic solutions in agriculture, turning to the proven benefits of seaweed remains both a nod to traditional wisdom and a step towards a greener future.