The Science of Seaweed Feed

There are many claims made about seaweed in terms of it's beneficial effects for the gardener. It is known to improve soil structure and invigorate soil organisms, while seaweed extracts are also said to improve seed germination, repel pests, feed plants, boost plant immune systems and improve the overall quality and flavour of your fruit and vegetable harvests. That is a lot!

I have both read and written countless times that seaweed boosts a plant's immune system, but how does it actually do this? What is the process at work? In this article I would like to look, from the layman's point of view, at some of the claims made about seaweed and to try to tie down this magic soil and plant invigorator. 

While we are at it, I thought we should also touch on other seaweed roles - including as a possible carbon capture tool and as a water purifier. That is if you can bear reading all the way to the end! 

Seaweed and Improved seed germination (Gibberellic acid)
Soaking seeds overnight in a diluted seaweed extract has been shown to increase both the speed and percentage of seed germination. While not related to the specific qualities of the seaweed, soaking accelerates the breaking down of the seed coat and subsequent absorption of water, which triggers germination. Soaking will therefore speed up the germination of many seeds, whether you use seaweed extract or not. 

However, tests have shown that soaking in a seaweed tonic further speeds germination because of the phytohormone 'gibberellic acid' that it contains. There are two phytohormones involved in seed germination: abscisic acid (ABA) and gibberellic acid (GA), which slow down or promote it respectively. 

GA also triggers hydrolytic enzymes that are responsible for digesting the seed's food stores - which then provides the energy for early growth. Seeds have their own switching mechanism using ABA and GA, but an increase in GA from a seaweed extract helps to tip the balance into the germination phase. 

Stronger seedling growth - Cytokinins
Seaweed extract also contains another group of plant hormones, 'cytokinins', which are responsible for cell division and cell expansion. Cytokinins are naturally present in all plant tissues, but are particularly concentrated in the main growth areas of the root and stem tips and the leaf margins. They are essential growth initiators and regulators in almost every aspect of plant growth and development. 

As we've said, seaweed builds biomass very quickly. This is partly because conditions are generally favourable (it doesn't need to cope with drought or nutrient shortages), but also because it gets battered about in the tide so often that it needs to regrow broken fronds. 

Cytokinins are the drivers of this fast replacement growth, which explains why seaweed generally contains such high levels of these growth compounds. 

Liquid Seaweed Fertiliser - Better Plants

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Laboratory tests have shown that seedlings grown with a diluted seaweed extract show better root and shoot development, as well as an increase in overall plant vigour. There are a number of factors at play, including seaweed micronutrients (covered next) - but the growth-promoting cytokinins play a key role. 

Anti-ageing properties
Cytokinins are also responsible for controlling leaf senescence (the process by which a leaf yellows and deteriorates with age) by reducing sugar accumulation in the leaf cells. This is because higher sugar levels are one of the main triggers in the dying back process. 

Increasing cytokinin levels (by using a foliar application of seaweed feed) will make the leaves greener as it boosts chlorophyll synthesis, but by slowing sugar accumulation, it will also keep leaves greener for longer: thus prolonging the photosynthetic period. 

The longer a leaf stays efficient, the more sugars it can produce to grow and ripen fruit (tomatoes) or fill energy storage systems (roots in carrots or tubers in potatoes). 

Maxicrop Seaweed Fertilizer 1 litre

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In trials by British professor Gerald Blunden in the 1960s, it became apparent that the most dramatic cytokinin response was associated with plants that store carbohydrates - like beet, potatoes, carrots and parsnips. The twin benefits of enhanced chlorophyll development and stability combined with delayed senescence significantly increased the sugar and carbohydrate levels in these crops. 

Micronutrients
The plants we grow in our gardens need roots to take up water and nutrients from the soil. Seaweed doesn't need roots: it is submerged in nutrient-rich seawater, so it can absorb what it needs from the water that flows around it. 

The reason seawater contains so many nutrients is that rivers mine salts and minerals from the land as they flow from their source to the sea. The accumulation of these salts, combined with evaporation, is why the sea is salty. Excess nutrients from agricultural runoff also end up in the sea; the combination of both is why coastal waters contain more nutrients than the deep ocean. 

Currently there are 17 essential minerals recognised for plant growth: these include the macro-nutrients we all know about like nitrogen, phosphorous and potassium, but also micronutrients like magnesium, boron or zinc. 

As our understanding grows, new, previously ignored trace minerals are added to the list (there used to be 14) so it is not beyond the realms of possibility that more will be discovered. 

Seafeed Organic Seaweed Meal Soil and Plant Feed

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Seawater contains all macro and micronutrients, which means the algae (seaweed) that grow in it also holds the full mineral spectrum. Fruit or vegetables fed with a high quality seaweed feed will also benefit from the wide nutrient profile, whether in terms of healthy growth, improved disease resistance or the quality, flavour and nutritional value of the finished produce. 

The same, of course, applies to us (although hopefully we won't be eaten) in that the resulting high-nutrient food promotes healthier growth and better disease resistance in our own bodies. 

Boosting immune response
You will often see claims that seaweed feeds boost the immune systems of plants. I have asked (just as often) how exactly this works, but have never found anyone (even those marketing seaweed feed) who can explain it. I am not able to give the definitive answer either, but my understanding for what it's worth is as follows: 

Plant cell walls are made from polysaccharide sugars. Polysaccharides are chains of glucose molecules which can either be linear (e.g. cellulose) or branched (e,g, hemicellulose or pectin). 

Irish Seaweed Tomato and Fruit Feed. 2.5 Litre

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Cellulose chains are long fibres that build an elastic barrier around the plant cell, while branched polysaccharides like hemicellulose and pectin tie the cellulose fibres together to create a very strong barrier. The cell wall keeps the contents of the cell in and forms a barrier to keep disease causing pathogens out. 

A plant can recognise a pathogen (and take action against it) either directly or indirectly. In a direct response, molecules from the pathogen are detected by receptors in the cell wall; while in an indirect response, the receptors pick up on the damage done by the invader. In the case of seaweed immune response, it is the indirect path that we are interested in. 

A pathogen uses enzymes to break up the strong polysaccharide bonds in the cell wall: this is how they gain entry. The glucose molecules from the broken bonds are detected by the receptors, which then alert the cell nucleus and trigger the immune response. This immune response may be the production of anti-microbial substances (phytoalexins), PR proteins, or more polysaccharide sugars to re-build or strengthen the cell wall against existing or future attacks.

Seaweed cells contain high levels of polysaccheride, with different polysaccharides being dominant in seaweed species depending on their colour. The principal cell wall polysaccharides in green seaweeds are ulvans, those in red seaweeds are agarans and carrageenans, and those in brown seaweeds are alginates and fucans. 

As brown seaweeds are more commonly used in Irish and UK produced seaweed feeds (usually Ascophyllum nodosum or 'egg wrack'), and because I use brown seaweed in my garden, I am most interested in alginates. 

When broken down, seaweed alginates form short sugar chains called Alginate oligosaccharides. These short chain sugars resemble the broken chains of plant cell wall polysaccharides (when attacked by a pathogen), and therefore illicit the same protective immune response. Applications of alginate olgiosaccharides to a range of crop plants show protective immune responses against a range of pathogens, including bacterial, fungal, and viral diseases. 

A positive outlook
It may come as no surprise when talking to some people, but plants and animals (including humans) are negatively charged. 

As a rule of thumb, the surface of the earth - and all plants and animals on it - carry a negative charge, while the upper atmosphere is positively charged. If you were struck by lightning, you would probably feel that the charge difference was rather inconvenient; but if you were a bee, it makes your life a lot easier. As a bee flies along, it loses electrons as it 'rubs' against the air: this results in its body becoming positively charged. 

A plant, or more specifically a flower in this case, carries a negative charge - which is why negatively charged pollen grains stick to the positively charged bee. For the bee, it pays to be positive. 

Chelation
Relative positive and negative charges play a part in plant nutrition: especially when it comes to mineral micronutrients, because they tend to carry a positive charge. As we said earlier, seaweed is particularly efficient at absorbing and storing micronutrients from the sea, but it is also able to serve them up in a form which is bio-available to plants. 

The reason for this is a process called 'chelation': where positively charged minerals bond with chelating agents, making them easier to absorb. Chelation is performed as an industrial process to improve the absorption of minerals in plant feeds, but it also happens naturally through the action of a variety of chelating compounds (many of which seaweed contains). 

The process is beyond my comprehension, but essentially a chelating agent bonds with the micronutrient and cloaks its positive charge (as in the diagram above showing a zinc ion). 

The reason that the 'neutralising' effect is helpful is that positively charged micronutrients tend to stick to negatively charged particles in the soil; this locks them up and makes them unavailable to the plant. A chelated nutrient is more stable, so isn't attracted to its surroundings. It therefore passes through the soil and root system intact, and is easily absorbed by the plant cells where it is needed. 

So, not only does seaweed contain the full suite of micronutrients absorbed from the sea, it is also (due to its chelating compounds) able to supply them in an easily absorbable form. 

Granular Seaweed Fertiliser - 5 Litre Bucket

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Improved soil structure
The final benefit I'd like to cover (is this ever going to end?!?) is how seaweed improves soil structure. I have been banging on about this for years, as I use raw seaweed in my garden and have pretty much built my soil with it. 

As a bulky mulch like compost or manure, it is pretty obvious how it adds organic matter to the soil when it rots down - but there is another factor at play here which helps to improve soil structure. 

We've already mentioned the immune boosting role of alginate, but it also works as a gelling and emulsifying agent; this is one of the reasons seaweed is used in the food industry (to emulsify ice cream or make jellies). 

The gelling characteristics of alginate also make seaweed a good soil improver: a gel holds water, thus increasing the moisture-holding capacity of the soil. A soil that holds water well will also hold onto its nutrients better (because they are not washed through as quickly). 

Alginates also improve soil structure because the gel creates a film around small particles and helps prevent them from sticking together. A seaweed mulch is especially helpful to break up heavy clay soils, because it both adds organic matter (which disperses or dilutes the small mineral particles) and provides the alginates to help de-aggregate the sticky clay. 

Possible environmental benefits of seaweed
Seaweeds are among the most productive organisms on the planet, meaning that they can build nutrient-rich biomass very quickly. They operate in much the same way as plants (though seaweed are algae), in that they use the sun's energy to process nutrients and turn them into the seaweed equivalent of stems and leaves (which would be stipes and blades or fronds). 

The difference, obviously, is that seaweed gets its nutrients from the sea whereas plants draw on the resources in the soil. 

Because of its fast growth and the fact that it doesn't require any land (which could potentially be used for agriculture, forestry or us building yet more stuff), seaweed has been touted as a possible carbon capture crop. As we've said, seaweed behaves like plants, in that carbon dioxide is used to gain the carbon needed for growth. The carbon dioxide is taken from seawater (as carbonic acid) but, as carbonic acid levels fall from consumption by seaweed, more carbon dioxide is taken from the air to replace it. 

The problem, of course, is that when seaweed dies and breaks down, the carbon is re-released (it doesn't build long term woody storage material like trees). 

There is a proposal to cultivate large rafts of carbon-capturing seaweed which are bundled up when mature and sunk in the deepest parts of the ocean. The plan is that the decomposition of the seaweed would be so slow at these depths (about 100 years) that it buys time for other renewable energy sources to catch up. Whether it becomes a viable solution or not, I have no idea, but it is just one of many solutions being explored. 

Interestingly, experiments in growing seaweed rafts in the deep ocean have run into problems - because lack of nutrients (particularly iron) is restricting growth. This highlights the fact that seaweed needs waterborne nutrients to grow. 

This is also why it is an effective water purifier. We are all aware of excess nitrogen and phosphorous from agriculture running into rivers, but perhaps we don't think so much about where much of it ends up. While nearly half of our rivers have nitrogen levels which are too high, the same can be said for our coastal waters: with over 20% significantly above normal nitrogen levels. 

Large scale seaweed aquaculture could provide a sustainable solution where seaweed farms act like giant sponges clearing excess nutrients from the water. Seaweeds gobble up nitrogen and phosphorous in large quantities in order to achieve all that fast growth but, as we've said, are also incredibly efficient at mopping up carbon dioxide. 

The aqueous form of carbon dioxide - carbonic acid - is what is responsible for the ocean acidification that threatens coral reefs as well as other sea life that need calcium to build their shells. Acidification is directly linked to CO2 in the air, because the ocean absorbs 30% of carbon dioxide in the atmosphere and turns it into carbonic acid. 

Seaweed therefore may provide a double-pronged solution that offers an effective buffer against both high nutrient and carbonic acid levels in our coastal waters. 

I am no expert on any of this but, going by the success I have had with seaweed in my own garden, I would assume farmed crops could then be used as a soil improver and fertiliser for agriculture. If applied as a meal (rather than a liquid feed) the nutrients would be slow release, with much less chance of finding their way back into water courses.

The other advantage of using seaweed as a slow-release fertiliser and soil improver is that the polysaccharide sugars it contains are beneficial to mycorrhizal fungi. Not only do fungi support the plants that they form symbiotic relationships with, they are also store many gigatons of carbon in their massive underground networks. According to the University of Sheffield, Mycorrhizal fungi are responsible for holding up to 36 per cent of yearly global fossil fuel emissions below ground. 

Of course, there is no point in adding seaweed to assist fungal growth in land that doesn't have any fungi. This is often the case with intensively farmed ground, where there is no woody material for fungi to break down. This is where Agroforestry may be a partial solution, where livestock or crops are farmed in harmony with interplanted trees. In this case, the addition of trees will support mycorrhizal networks that can also support pasture and crops, and help to process and store carbon and nutrients from seaweed. 

I am aware that I am oversimplifying things here and that there are massive holes in my knowledge. I guess I am just posing the question that seaweed may form part of a solution. I imagine environmental planners are scrabbling for pen and paper to take notes or to find my contact details. Naturally, I am delighted to be of service. If anyone has any other global issues they'd like sorted, you know where to find me. 

OK, that is about it for today. I hope you found some of the above helpful or, at least, interesting. I'm not a scientist obviously so my apologies if I am not 100% correct on any of the points, but I think I have the gist of it more or less right. 

As regard seaweed feeds, our new darlings are the cold pressed feeds produced by 'Better Plants' which I have promoted here before. Apart from the fact that the cold pressing technique keeps more nutrients intact than heat or chemical extraction processes, I feel the location from where the seaweed comes from is also important. 

We've covered how seaweed absorbs salts and minerals from seawater but this also means that it can absorb pollutants, particularly heavy metals. Better Plants seaweed is sourced from relatively unpolluted and low-industry Atlantic coastal areas; therefore they are seaweed feeds of the highest quality.