From Sea to Soil
- Isis
- 1 day ago
- 9 min read
Exploring Seaweed, Compost and the Living Soil
How marine biomass might help regenerate soil, without taking more from the sea.
The ocean has always been a fascination to me, the relationship between the ocean and the soil that is beneath my feet. For some, it is as though they operate separately, almost like being two different worlds, however the mycelium is within the ocean deep equally as it presides and communicates throughout the land.
And yet we keep drawing lines.
We call one world "marine" and the other "terrestrial." We study them in separate departments, sell their products in separate categories, and speak of them as though they have never met. But the mycelium does not recognise our taxonomy. The fungi threading through a kelp forest and the fungi threading through a forest floor are part of the same living web, exchanging, cycling, communicating in frequencies we are only beginning to listen for.
Before anything else, I want to sit with that.
Because this post is not a guide to using seaweed in your garden. It is an invitation to pause before you reach for anything, a product, a supplement, a "natural" soil additive and ask something more honest: Do I understand what this is? Do I understand where it came from? Do I understand what I am asking the sea to give up so my soil might benefit?
The ocean is perceived as a saltwater haven with moving tides, currents and seasons that are beyond four. The land is considered to be terrestrial, filled with fungi, roots, minerals, bacteria, organic matter, water and a myriad of other countless organisms that in our perception make soil a living system.
Our perceptions can often create boundaries when it comes to understanding ecosystems.
Everything moves in nature.
Nutrients move. Carbon moves. Water moves. Microorganisms connect systems in ways that are often invisible to us, and seaweed is integral within this fascinating exchange.
But here is the question that keeps turning in me: when we decide to move seaweed from the ocean into our soil, are we working with that exchange, or are we interrupting it for our own convenience?
Seaweed is Habitat
Marine algae and kelp forests provide food, shelter and nursery habitat for marine life. Tasmania is currently working to restore its severely depleted giant kelp forests, making the relationship between seaweed and ecosystem health particularly urgent.
Tasmania's Department of Natural Resources and Environment states that native seaweeds cannot simply be taken directly from the ocean. Tasmania does have specific provisions for limited collection of cast seaweed under particular conditions, but living or seabed-attached seaweed cannot be taken under recreational collection rules. (Fishing Tasmania)
This matters. A beach covered with seaweed is not necessarily a resource waiting to be collected. Cast seaweed contributes to coastal food webs and habitat. What looks like waste to us is shelter, food, and nursery ground to something else.
So before we ask what seaweed can do for our gardens, we might ask a different question: what is seaweed already doing where it is?
What the soil shared:
"All is interconnected woven through multiple hyphae threads, both land and sea have fruiting bodies, we welcome exploration, in non-disturbance, flow quieting in your exploration, in this I will share all that services within both land and ocean."
And then, more simply:
"Don't strip the sea to feed me (soil)."
This is the principle everything else here stands on.
Don't strip the sea to feed the soil.
The Product We Are Sold
Walk into any nursery or gardening centre and you will find seaweed on the shelf. Kelp meal. Liquid seaweed concentrate. Seaweed-derived biostimulants. "Natural." "Organic." "Ocean-derived."
The marketing language reaches toward the ocean's wildness and authority. It borrows the credibility of an ancient, complex, living system, and sells you a bottle.
I do not say this to be cynical. I say it because I have bought those bottles. And because buying them, I rarely stopped to ask: which seaweed? Harvested from where? Using what method? Processed how? And what remained in the ecosystem where it was taken from?
Seaweeds are rich in a complex mixture of organic compounds and minerals. Depending on the species and how they are processed, seaweed-derived materials can contain nutrients including potassium, phosphorus, nitrogen, magnesium, calcium and other micronutrients, alongside carbohydrates and other biologically active compounds. (J Exp Agric Int)
But seaweed is more than a bag of fertiliser. It communicates in ways we are yet to fathom, and even be open to the knowing of.
And the mycorrhizal had something to say about our habit of reducing it to an amendment:
"Whether in the land or in the depths of the ocean, our biomass or what you refer to as soil amendments is counter productive as an additive, as the interconnectivity is present even though it is not visible to a human eye as yet."
Think about what that means. The nutrients we are trying to "add" to soil are already in relationship with the ocean. Moving them, extracting them, packaging them, applying them, disrupts a cycle that was already turning without our help. The question is not whether seaweed contains useful compounds. It does. The question is whether taking it out of its context, and placing it into ours, serves the living web or simply serves us.
What Nature Creates — and What We Extract
Here in Talulah, we are exploring what happens when seaweed-derived biomass becomes part of composting and soil regeneration, not as a reason to harvest from wild ecosystems, but as an opportunity to ask deeper questions about responsibly cultivated or appropriately sourced marine biomass and its potential to return nutrients and carbon to terrestrial soils.
But we hold that lightly. Because the other question the one the mycorrhizal keeps returning us to is whether we need to add anything at all.
Nature has been building healthy soil and healthy oceans without our amendments for longer than we have existed. The depletion we are now trying to reverse is, in most cases, a result of our own extraction and disruption. So when we reach for seaweed as a fix, we should hold that honestly: are we restoring something, or are we taking from a second ecosystem to repair the damage we did to a first?
The terrestrial plants offered this, and the seaweed chimed in:
"The methodology of scientific research and instruments utilised in those findings are the limiting factors of what is actually physiologically sharing as an interconnection within all."
Our instruments measure what they are designed to measure. Our studies answer the questions we know how to ask. But the full conversation between sea and soil between marine fungi and terrestrial fungi, between kelp and clay is still largely beyond our listening.
The Interconnected Reality
The mycorrhizal did not distinguish between land and sea. It said:
"We are never separate, land and sea, we are united, sharing, acknowledging our super power is that understanding that we are all one in the grand design. We who are known as mycorrhizal are infinite combinations of varying particle combinations, constantly exchanging our frequencies of light, in this we combine nutrient rich elements that provide sustenance in all of life."
The boundary we have drawn between ocean and soil is, in this sense, a human boundary. Ecologically, the two are one continuous system. The carbon that cycles through kelp returns to the sea floor, enters the water column, is carried by currents, deposited in coastal sediments, and eventually makes its way into terrestrial cycles through rainfall, coastal erosion, and biological exchange. The soil and the sea are not two stories. They are one story told in different registers.
When we lift seaweed out of the ocean to "feed" our soil, we are not so much connecting two systems as we are making a withdrawal from one to make a deposit in another. That might sometimes be appropriate. But it should never be automatic, and it should never be invisible.
The question I want to keep asking is this: how do we participate in the exchange without breaking the cycle?
What Careful Enquiry Looks Like
Rather than rushing toward a seaweed product or a soil amendment protocol, I want to sit with the research as a set of open questions rather than settled answers.
Seaweed species, concentration, extraction process, crop species, soil conditions, and application methods all influence outcomes. A 2026 meta-analysis of 198 peer-reviewed studies found positive average crop-yield responses to seaweed extract-based biostimulants, while also identifying soil chemistry, organic matter, salinity, formulation and application strategy as factors that shape the response. (PubMed)
That variability is not a weakness in the research. It is nature telling us something: that seaweed is not a universal fix. It is a specific living material with specific properties, interacting with specific organisms under specific conditions. The moment we reduce it to a product, something we apply, consume, and repeat we lose that specificity entirely.
Could marine-derived organic matter and terrestrial woody carbon complement each other in rebuilding soil structure?
Perhaps. But the more foundational question is: before we ask whether they work together, do we understand what each of them is doing in its own ecosystem first?
How do terrestrial soil microbial communities respond to marine-derived organic matter?
We don't fully know. And that is an important thing to hold.
Does combining seaweed with terrestrial compost create a different microbial environment than either material produces alone?
Possibly. But observe it first. Don't assume.
Rather than asking, "How many nutrients are in this seaweed?" a much more useful question is:
"What nutrients become available to plants, when, and under what soil conditions?"
And behind even that question sits another: at what cost to the place the seaweed came from?
The Salinity Question We Cannot Ignore
Seaweed originates in saltwater environments. Salt and soluble ions need to be considered carefully when using marine biomass in terrestrial soils. Too much soluble salt in the root zone can interfere with water uptake and create osmotic stress in plants.
This is not a minor footnote. It is a central ecological reality. The ocean's chemistry and the soil's chemistry are different for a reason they support different communities of life. Moving compounds from one to the other is not neutral.
For Australian native plants in particular, which have evolved in low-nutrient, specific-pH soils, the introduction of marine-derived salts and minerals is not automatically beneficial. It requires real observation. Real measurement. Real patience.
Before any seaweed-derived material is used on edible crops, native plant production, or developed as a commercial soil amendment, appropriate testing for salinity, electrical conductivity, pH, nutrients and potential contaminants should be undertaken.
This is not bureaucracy. It is respect.
The Ecological Safeguard
There is a question that needs to sit alongside every experiment, every product, every well-meaning amendment:
Where did the biomass come from?
This may ultimately be more important than what the amendment does. If seaweed is harvested unsustainably, transported long distances, processed intensively, and applied to soil, the environmental balance may not be as positive as the word "natural" suggests.
At Talulah, our approach is grounded in this:
No harvesting of living wild seaweed. No encouragement for people to strip beaches. No collection from protected areas. No handling of marine pests like wakame (Undaria) without appropriate authorisation. Tasmania identifies Undaria as a marine pest species and states that it cannot be handled without authorisation. (Fishing Tasmania)
Instead, what we are exploring is responsibly cultivated seaweed, appropriately sourced processing residues, and circular biomass systems, where nothing is regenerated at the expense of another ecosystem.
That is a different philosophy from what the seaweed supplements market generally asks of us.
Can carefully formulated marine-derived amendments support establishment of native plants without disrupting the soil ecology they are intended to restore?
That is the question. Not "does seaweed work?" but "does this approach serve the whole system?"
Sea and Soil as One Story
Perhaps the most beautiful part of this exploration is that it brings us back to something we often forget: the soil is not isolated from the ocean.
The minerals in the sea. The carbon in kelp. The microorganisms in soil. The roots of a native plant. The rainfall moving through a forest. The food we eat. The air we breathe. All are part of a continuous exchange.
We don't have to force nature into separate boxes. We can learn to see the connections.
And perhaps our role isn't to take more from one ecosystem to repair another. Perhaps our role is to learn how to participate in the exchange without breaking the cycle.
That is what I want Talulah to keep exploring.
How can we return more to living systems than we take from them?
Sea. Water. Soil. Plant. Habitat. People. Return.
The cycle continues, with or without us. The question is whether we move through it with awareness, or whether we take without asking what we owe.
I have been sitting with this question in another form too, through music. I recently released a song called Losing Ground, which came from the same place this post comes from: the rising seas, the ground beneath our feet that is changing, the hope that we can still be the change, and the importance of keeping the songlines alive as we connect back to Country. If any of this resonates, you can listen at wildbytalulah.bandcamp.com. The song and this post ask the same thing, just in different languages.
Wild by Talulah's exploration is centred on ecological restoration, responsible sourcing and learning how marine and terrestrial systems can interact without placing additional pressure on wild ecosystems. The research cited here represents evidence of potential, not universal prescription. Every soil, every species, every context requires its own observation.
References and Further Reading
Dang, B-T., et al. (2023). Current application of seaweed waste for composting and biochar: A review. Bioresource Technology, 375, 128830. (ScienceDirect)
Margal, P.B., et al. (2023). Effect of Seaweed Extracts on Crop Growth and Soil: A Review. Journal of Experimental Agriculture International, 45(9), 9–19. (J Exp Agric Int)
Pérez-Oñate, et al. (2026). Crop yield responses to seaweed extract-based biostimulants depend on application strategy, formulation, and extraction methods: a meta-analysis. Frontiers in Plant Science. (PubMed)
Palansooriya, K.N., et al. Response of microbial communities to biochar-amended soils: a critical review. (the UWA Profiles and Research Repository)
Department of Natural Resources and Environment Tasmania. Seaweed Farming. (Natural Resources Tasmania)
Fishing Tasmania. Seaweed Collection. (Fishing Tasmania)
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