Seaweed aka Blue Gold, Kelp , Maërl & Vraic, Has Been Used In Gardens For Thousands Of Years As A Liquid Or Solid Additive. Harvested Since The Middle Ages For Coastal Gardens & Food. Seaweed Fertiliser Is Very Popular Today.

Seaweed Fertiliser is harvested from the sea of collected from beaches.

Sea veg and seaweeds have been harvested in the UK since around 1200 AD, perhaps before. Most records are in Scotland and Ireland. But by 1607 William Camden gave a detailed account of how Laver (for laverbread) was harvested in springtime along the Pembrokeshire Coast. However Seaweed Fertiliser goes back much further.

Pliny describes how the tribes of Gaul and Britain used seaweed to fertilise their soils. Pliny, mentions the collection of “Margo” (possibly related to maerl, a red seaweed from Ireland and Britain), by tribes of Britain and the tribes in what was then called Gaul. they used it to fertilise their soils. And Palladius described the regular of harvest/ collection of marine algae (seaweed) in March each year.

Seaweeds Used For Fertiliser

Today we see many fertiliser products stating they are enriched with seaweed. And many of us know that seaweed is good for the soil and plants. But that isn’t to say that seaweed enriched products are any good .. well not unless we know how much seaweed is in them.

It’s easy to say that a thimbleful of seaweed extract is enriching a 1000 gallons of fertiliser. But common sense tells us that a thimbleful is so diluted it makes no difference whatsoever.

But, a decent amount of seaweed works wonders. Historically many gardeners used nothing but seaweed to enrich their soils and feed their plants. For example the growers on the Branscombe and Weston Plats near me in East Devon relied 100% on seaweed as a fertiliser for centuries. Their crops were varied and ranged from corn to veg and flowers.

Seaweed has gained popularity in recent years as its an organic fertiliser and means growers and gardeners don’t have to rely on the production of “artificials” ie synthetic fertilisers produced using the Haber-Bosch process and related manufacturing processes.

But go back in history and seaweed wasn’t just used in Devon on the Plats.

From the Romans and ancient Greeks using it to sheep farmers in the Orkneys it has a long history. In the Orkneys and other islands the farmers allowed their sheep to graze seaweed at low tide. And when the tide came in the sheep came on land and did what sheep do .. leaving their droppings wherever they were. These fed the soil and improved it for centuries. This saved the farmers having to harvest the seaweed and move in to the fields. It was a very efficient process.

Archaeological Research: Seaweed Fertiliser

Archaeological research has also found seaweed ash around Scottish settlements so it is likely that the farmers also gathered seaweed and “ashes” it before using it on their fields. Another possible explanation is that they used seaweed to wrap food for cooking in the embers. In much the same way as indigenous people do elsewhere in the world, using bananas leaves out whatever else is available. Seaweed simply replaces banana leaves in this context. Another explanation is that there is the possibility of seaweed being used to wrap roots that are being transported or stored.

Similar evidence of sealed rise is clear from archaeological evidence in settlements around all the coasts of Western Europe. From Scandinavia top Portugal and Spain; from Neolithic times until the 20th current century.

So clearly seaweed fertiliser in the British Isles, Normandy, Brittany, Eire and Channel Isles has been evident for a very long time. the application methods varied from using the seaweed on the soil surface in autumn to the production of ash or liquid extracts.

In Ireland the evidence goes back to the 12th century, but it was very likely used long before that. It fed the nutrient poor post glacial soils and enabled staples such as potatoes to flourish.

In the Channel Islands the seaweed was called vraic or wrack and it’s still used today on the Côtils to grow Jersey potatoes.

The Scottish people of the 17-19th century encouraged the growth of wrack by placing risky substrates in the intertidal zones. Seaweed cant grow on sand, it needs an anchor point. The seaweed was then harvested and trenched to grow potatoes, oats, onions and wheat. Trenching involves digging a trench and laying seaweed in it before trenching the next spit. It’s a long hard process and a lazy system made more sense to some. The lazy system was to place the seaweed on the surface and grow crops through it. It was similar in some senses to todays No Dig method.

However, what Scottish farmers called “Lazy Bed” was far from lazy. They used it to describe the trenching process I describe above. It was carried out on the Runrigs on some of the islands and parts of the mainland. Runrigs are best described as a bit like medieval strip farming where sections of land are allocated to tenants on an annual basis.

Eventually seaweed harvesting became an industry. It was harvested to produce ash which, being lighter, could be shipped inland easier. Plus, as glass and soap manufacture become more common, the seaweed was used to produce the potash needed in its manufacture.

And, of course, potash can be used as a fertiliser in its own right.

For some that worked the wind swept hillside gardens and farms the harvesting of seaweed must have looked a better option to the uncertainty of growing veg!

At one stage the Scottish seaweed industry employed 10,000 families to produce 3,000 tons of seaweed ash a year. It was huge industry. But not without risk to the land and the people. Parts of the Orkneys suffered environmental degradation from the bringing of secede as lang ago as the 19th century. And the people are reported as suffering blindness and various sicknesses due to the work.

When the prices dropped out in 1822 the industry slumped and didn’t prosper again until seaweed started being used for iodine production in 1845.

Food manufacture also gave it a boost in the early 1900s when alginates from seaweed were used. s thickening agents.]

Forms Of Seaweed Fertiliser

As can be seen from above there are several ways in which seaweed can be used as a fertiliser.

Fresh Seaweed Direct From The Sea

This is a bulky heavy material not suited to selling in garden centres or by post! It was fine when harvested by local peoples and moved by donkey or horse and cart to nearby gardens. But useless if it needs sending further.

This is however how the plat farmers, local to my home, used their seaweed. They loaded donkey carried panniers on the beach and ascended the cliffs to the plats on the steep slopes. It was hard work but the distances were small.

Dried Seaweed Fertilser

Dried, crushed or powdered, and much lighter this is seaweed that can be packaged and moved longer distances.

Seaweed Extract Or Liquid Seaweed

Maxicrop, first produced in 1947 by Reginald Milton, was the first industrially produced seaeeed/kelp liquid fertiliser.

Products such as Tomorite now also market their product as containing a seaweed extract.

Seaweed Tea

You can make a seaweed tea in exactly the same way as you can make a nettle, comfrey or weed tea. Just ferment some seaweed in water and use the resulting liquid as a feed.

I’m sure someone will ask me what dilution rate is needed if I don’t include it here.

The truth is I haven’t a clue. AND nor has anyone else. It will depend on how much seaweed and water you used and how long it has fermented. It’s what I dislike about all forms of homemade teas or liquid fertilisers. There is no way to tell what their strength is. Plus the chance of bacterial contamination with some very nasty bacteria is always a risk.

Liquids and teas made on an industrial scale are produced using high temperatures that kill bacteria.

Seaweed Fertiliser Nutrient Levels

Seaweed is rich in NPK and many trace elements. Or at least that what we are often told.

But that isn’t the same as the seaweed fertilisers being equally as rich as seaweed. I’ve seen some products claiming a plant food ration of 11:11:11 and others at only  0.9-0.6-0  (this for product with the words SeaFeed Extra in their name.

It’s not that I don’t believe either of these. Both can be correct, depending on the product. But it shows how each product needs to be examined in details before we buy them. We need to compare like with like when comparing prices and efficacy.

So my advice is to look at labels very carefully. Its easy to be ripped off by claims about seaweed.

Of course the people that harvested direct from the beach and put the seaweed on their soil, or dug it in, didn’t see any such claims and knew that what you get with a fresh product, is what you get, and it does the job if you use enough of it! Only experience tells you what is enough.

Additional Compounds

Seaweeds or marine macro-algae are rich in diverse compounds like lipids, proteins, carbohydrates, phyto-hormones, amino acids, osmoprotectants, antimicrobial compounds and minerals.

Impact of seaweeds on agricultural crop production as biofertilizer
E. Nabti, B. Jha, A. Hartmann

Calcified Seaweed

Calcified seaweed is sometimes promoted as an alternative to lime to drop reduce acidity. It is a natural product which comes from natural beds of calcified algae. It works, but is much more expensive than lime.

Seaweed’s Effect on Soil Biosphere

Though a lot of research has demonstrated the benefits of seaweed in enhancing the plant growth and productivity, not much research has been carried out into the impact of the soil flora and fauna.

In Perspectives of Seaweed As Organic Fertiliser in Agriculture the authors content that seaweeds are known to be a promising soil conditioner, protect the plants under abiotic and biotic stress and increase plant resistance against pest and diseases.

And though personally I suspect that seaweed application is likely to be beneficial to the total soil biosphere, I’ve no evidence of this and the above paper doesn’t actually say that. It does however make several references to improved crops and crop health. But that’s not the same thing.

Reference is made to the fact that seaweed can improve the numbers of nitrogen fixing bacteria in treated soils.  

What is however clear is that seaweed application can improve soil porosity, aeration and water retention. Several additional papers are referenced on this point.

The use of seaweed on clay soils is also discussed and reference is made to the porosity improvements due to the alginates provided. This makes sense to me.

Seaweeds: A Local Perspective

The ability of seaweed and seagrass to reduce the force of the sea is well known. They can reduce the energy in the waves and reduce the impact of the waves on beaches and manmade structures.

Seaweed Fertiliser in Sidmouth?

With this in mind consideration is being given to a kelp biosphere being anchored some way off the beach at Sidmouth in Devon. Such a structure could reduce the energy in the waves, sequester signicnat carbon and act as a nursery for fish, shellfish and other marine life.

Research References

Zodape, S. T. (May 2001). “Seaweeds As A Biofertilizer”. Journal of Scientific and Industrial Research60 (5): 378–382.

Impact of seaweeds on agricultural crop production as biofertilizer
E. Nabti, B. Jha, A. Hartmann

Image Attribution Image by Nichole Bohner from Pixabay
Tag: Seaweed Fertiliser

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