Fermented Seaweed Fertilizer
A guest post by Sarah Owens
Greetings, Flavor Freaks! We’re excited to kick off a series of guest posts, featuring voices and talents from around the fermentation world. Rich made #kojibuildscommunity into one of the defining hashtags of this movement, and we firmly believe that fermentation is fundamental to making connections and building the kind of world that we want to live in. Sharing is caring, after all.
Today’s inaugural contribution comes from the mighty Sarah Owens, author of the James Beard Award-winning Sourdough: Recipes for Rustic Fermented Breads, Sweets, Savories, and More as well as Toast & Jam: Modern Recipes for Rustic Baked Goods and Sweet & Savory Spreads and Heirloom: Time-Honored Techniques, Nourishing Traditions, and Modern Recipes. You can find her on Instagram here and her Patreon is here.
We’re excited to bring you this post because it takes fermentation farther up the supply chain than you may be accustomed to—using it to break down seaweed (and nettles) to create a potent fertilizer for plants in the garden. Human health begins with soil health, so if you grow any food at all this is important information.
Here’s Sarah:
I have been using fermentation in my kitchen for transforming all manner of ingredients into tasty foods, including sourdough bread, pickles, and condiments. When I moved to northern California in 2020, I soon learned to forage for seaweed from Strong Arm Farms and began incorporating more of it into my ferments. I found that it responded enthusiastically, and so I tried using it in a fermented fertilizer I was making with stinging nettles. Fermentation not only improves the taste of seaweed in my krauts and pickles, but it also enhances its nutritional value and potential as a fertilizer. I use this preparation at a few key times in the garden, namely for mitigating transplant shock and for giving fruit-bearing plants like tomatoes a boost as they begin flowering.
For fruiting plants, nitrogen, phosphorus, and potassium are especially vital, in addition to micronutrients found in various seaweeds. Phosphorus aids in the production of flowers and fruits, while potassium improves fruit quality, boosts disease resistance, and strengthens the plant overall. Fermentation involves microorganisms breaking down complex organic compounds found in seaweed, like polysaccharides and proteins. This process can free phosphorus and potassium from their bound forms, making them more accessible for absorption by plants (or humans!).
Fermented seaweed fertilizer is not a recent development; it has been utilized in agriculture for centuries, tracing back to prehistoric periods. Initially, coastal regions employed seaweeds as mulch and fertilizer, enhancing soil fertility and boosting crop yields. As kelp industries grew due to the demand for potash, the use of seaweed spread inland. By the 19th century, fermenting seaweed, especially with lactic acid bacteria, started to become increasingly popular.
The recipe below is how I create a 5-gallon bucket of seaweed fertilizer that, once it has completed fermentation, I use in a 1:1 dilution of fertilizer to water. If you live near a coastal area, regardless of the region, look for seaweed that has been recently washed ashore but not yet bleached by the sun. When my nettle patch goes to flowering and seed, I use it as a partial substitution for the seaweed. Other plants can be used in combination with the seaweed, much as you would make a biodynamic prep based upon your particular soil’s needs. Although I use sourdough starter to initiate fermentation and molasses to boost and accelerate the process, you needn’t use the sourdough if you do not keep a culture.
What You’ll Need:
Two 5-gallon buckets (or one bucket and one wash bin)
An instrument for blending (high-speed blender, immersion blender, food processor, etc)
Freshly Foraged Seaweed
170 g unsulphured molasses
100 g sourdough starter (optional)
Method:
Collect the seaweed. Because each phylum of seaweed offers different benefits, collect a variety of seaweeds. You’ll need about ½ to ¾ of a 5-gallon bucket.
Clean the seaweed. When you return home, rinse the seaweed of salt, sand, and grit. Fill the bucket of seaweed with fresh water. Dunk and agitate the seaweed with your hands to loosen the sand and grit. Transfer the rinsed seaweed to a clean bucket and repeat until no grit collects at the bottom of the bucket.
Blitz the seaweed with water. This step can be done in a few different ways, depending on what equipment you have available. If using an industrial-strength immersion blender, simply leave the seaweed in the bucket and add water to fill the bucket, leaving about 3 to 4 inches of space from the rim. The seaweed will not completely break down, but will create a slush. Add the molasses and sourdough starter and continue blending to combine.
Ferment the mixture. Secure a lid and set the bucket aside in a warm place out of direct sunlight. Depending upon the ambient temperature, the mixture will show signs of fermentation after about a week. The mixture will become viscous and somewhat bubbly, and the seaweed matter will float to the top. In 3 to 4 weeks, when bubbling has subsided and the seaweed matter has settled, it is ready to use.
Use and store the fertilizer. I prefer to dilute the fertilizer using a 1:1 ratio of fertilizer to water before applying with a watering can. Store any unused fertilizer in sealed bottles, preferably in cold storage such as a basement or refrigerator. If stored cold, the fertilizer will keep for at least 1 year, but it is best used before the end of the growing season.
Once again, Sarah’s books are:
Heirloom: Time-Honored Techniques, Nourishing Traditions, and Modern Recipes
Toast & Jam: Modern Recipes for Rustic Baked Goods and Sweet & Savory Spreads
Sourdough: Recipes for Rustic Fermented Breads, Sweets, Savories, and More.
You can find them wherever you buy your books:
Sarah Owens (she/her/they) is a horticulturist and celebrated James Beard award-winning cookbook author specializing in fermentation and sourdough baking. Her work seeks the intersection between the natural world and the most celebrated aspects of living. Sarah believes that food is a powerful medium for storytelling that can anchor a community in shared experiences and traditions. After working as the curator of roses at the Brooklyn Botanic Garden and baking professionally for over a decade in Brooklyn and Queens, she now lives in Sonoma County, California where she writes, bakes, and defends her small backyard garden from marauding deer. Sarah's recipes and wholegrain baking workshops can be found on her Patreon platform and website www.ritualfinefoods.com.
Thanks, Sarah! Gardeners, is this method new for you? If you leave near the coast, will you be trying it? Let us know, and send pictures of course.