Composting Sargassum | The Survival Gardener

Can we compost the seaweed blob?

I have been asked multiple times about composting sargassum, as huge islands of it wash up on shore.

We have used it extensively in the past:

Seaweed grows some very happy crops. As Mizuro comments below that video:

“Dude it worked!!! My plants have grown very well with washed seaweeds! I use or twice every week and it is working awesome! Ive never had such growth before!! wow! Thanks man! God bless you….never listen to those who say negative things on you…You are doing great! God bless you.”

But What About Heavy Metals?

Yet there may be a catch when composting sargassum.

This morning I received an email from Steve with a link to an article claiming there is a danger of heavy metal contamination when seaweed is used as compost, mulch or fertilizer:

“Although, in general, there appeared to be no significant physical differences (shape or quantity of vegetable production) between plants grown with or without the presence of sargassum, samples analyzed at the Radboud University laboratory found that arsenic levels were higher in vegetables grown in soil with sargassum. More specifically, bok choy had 37 times, zucchini 21 times, spinach 4 times and soil 13.5 times more arsenic than their counterparts grown in plain potting soil.  Cadmium levels were also higher in plants grown in sargassum enriched soil, with chemical analysis showing bok choy having 2.5 times, zucchini with 3 times, spinach with 1.3 times and soil with 2.7 times the amount of cadmium than samples without sargassum enrichment.”

This may or may not be a problem, as the article continues:

“The health implications of these findings are still unclear. Arsenic can take several forms, namely organic and inorganic, where organic levels can be much higher before negative impacts are observed in people.  It should be noted that the European Food Safety Authority (EFSA) has not yet set official thresholds for arsenic. In fact, the EFSA Panel on Contaminants in the Food Chain (CONTAM) published data in 2010 which stated that there are no ‘safe’ levels of arsenic.  Long term ingestion of inorganic arsenic has been connected to skin lesions, cancer, developmental toxicity, neurotoxicity, cardiovascular disease, abnormal glucose metabolism and diabetes (CONTAM, 2010). More research is needed to understand impacts of these higher levels of heavy metals and the long -term effects when ingested.”

Pesticides and Herbicides and Toxins, Oh My!

In the past I was sent some articles on the possible uptake of toxic chemicals by sargassum as it drifts past and through the runoff from various Caribbean nations.

It’s a remarkable accumulator, making it excellent at holding onto trace elements… and apparently almost everything else. NOTE: I searched in vain for another link to these studies on toxic chemical residues while writing this post – if anyone has a link and can post it in the comments, I will add it.

But Wait, There’s More!

On the other hand, it has been reported that sargassum can make very high-quality compost:

Sargassum is a particular type of seaweed that is common in coastal regions within the Gulf of Mexico, and is traditionally disposed of by being integrated into dunes along the shoreline or into landfills. But this particular seaweed contains potentially useful nutrients that could benefit plant growth on land. Diverting  this resource into compost could help beautify beaches as well as promote a greater stewardship directed toward minimizing the strain placed on overflowing landfill spaces.

Among the concerns realized by attempting to use sea matter applied to garden growth is the detrimental effect salt content can have on land-based plants. However, this study found that sargassum could be incorporated into compost piles with no detrimental effects because of high levels of salinity. “Since pre-washing of the seaweed did not impact the final compost produced in terms of improved quality, future studies may also attempt to identify the maximum amount and proper ratios of sargassum that can be used as a feedstock for compost creation,” Sembera noted.

The study used 12 cubic yards of sargassum as feedstock mixed with food waste and wood chips to create 72 cubic yards of workable matter. From this, the authors derived 25 cubic yards of stabilized compost. From that, they were able to test the quality of the resulting compost, and discovered sargassum-based compost was of either equal or higher quality than traditional or commonly sought compost; therefore its use in this manner proves to be a sensible way to manage the presence of this invasive species.

Composting can tie up and render certain toxins unavailable.

As Planet Natural notes in an article:

“During the composting cycle, pesticide levels in the feedstock (the material that went into the pile) are reduced by a variety of processes. Some toxins decay into simpler molecules. Some form bonds with other compounds (adsorption). Some volatilize, or escape into the atmosphere. Some leach from the pile, draining away with liquid run-off. Some undergo humification, becoming part of humus molecules. And some undergo mineralization, which Ohio State University Extension (PDF) calls the most “desirable fate” for pesticides.

Mineralization, the preferred end for pesticides, refers to the breakdown of organic compounds into their inorganic (or mineral) and organic constituents. The remaining organic constituents that contain carbon breakdown further into a variety of simple molecules that include carbon dioxide and water. The CO2 volatilizes, or evaporates, the water joins the soil solution, and the inorganic, or mineral part of the pesticide molecule takes its place in the soil environment. The result is that the pesticide has been permanently transformed into non-toxic molecules.”

So – what’s the scoop on composting the sargassum washing up on beaches?

The “safe” thing to do would be to use it to feed non-food plants. But in that case, I don’t see any real use for all the bother of gathering it up and bringing it home.

I would probably use it as a few layers in my compost piles for micronutrients and trust that the composting process would bind up some of the potential toxins. It really does make plants grow wonderfully, but the dark side is that it could also harbor materials you don’t want to stick around in your garden.

Alas, we live in a toxic world. There’s no way to be completely safe and there are sometimes no easy answers. My recommendation for most things is just to do the best you can with the information you have, and pray the Lord gives you wisdom and protects you from what you don’t know.

My Final Thought

I hate to not compost the seaweed, so if I had access to some, I would totally use it, studies be darned. Your mileage may vary.

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