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Feeding your Soil with Seaweed

Q. How can we make plant food from seaweed? We have an unlimited supply of a type called sea grass. It comes from the Indian River, which is part of the intracoastal waterway on the east coast of Florida. It seems the stuff should work. The sea cows eat it, so I would be surprised if it wasn't of use in the garden.

    ---Mike in Fort Pierce, Florida

A. Well, seagrass isn't 'seaweed'. As its amazingly thorough Wikipedia entry explains, it does grow in submerged saline environments, but otherwise behaves more like a terrestrial lawn than an underwater plant. It only grows in water that's shallow and still enough to allow it to photosynthesize; and it even pollinates underwater! Those big silly-looking manatees do graze on it, as do a number of other aquatic species. And Wiki points out that it even has history as a fertilizer; the Portuguese, who called it molico, collected it to enhance their sandy soil.

And being in Florida, it's likely that our listener also has sandy soil, but I wouldn't suggest he harvest any molico. The Wiki article goes to great lengths to explain how this grass is an integral part of a very complex ecosystem that supports an amazing biodiversity. These underwater meadows also slow down ocean currents and capture sediment, protecting local residents by breaking the strength of waves and preventing erosion.

So these grasses are important. And they're in decline around the world, so it doesn't sound like something we humans should harvest. But I wouldn't hesitate to add any that washed up out of the water on its own to my compost pile.

Q. In a previous program, you mentioned adding seaweed to shredded leaves for composting. What type of seaweed can be used? And does it need to be rinsed to remove salt before use?

    --- Bob, "near the coast of Maine"

A. Now, this is a plant of a different color—and a great excuse for me to call my dear friends Eliot Coleman and Barbara Damrosch, whose newest book, "The Four Season Farm Gardener's Cookbook" (Workman; 2012), celebrates their amazing success as true four-season growers (half a mile away from the ocean in chilly Maine!) and their love of cooking and eating what they grow.

Eliot and Barbara are the real deal—and it really IS a 'Four Season' farm; I can't personally imagine growing for market over the winter in Maine! But they do it—and with panache.

I called them for this piece because I've known for a long time that they use seaweed, which is locally abundant on their chilly coast. Barbara told me that after a storm, they'll often drive down to the shore and cart a fresh batch home.

And the answer to the 'washing the salt off'' question is No. Eliot says it just never seemed that salty, and he's been using it for decades without seeing any problems. Just the reverse, in fact, as he credits it with supplying the minerals and micro-nutrients that are so essential for healthy, natural plants.

Eliot told me that he feels there are three essential keys to healthy soil—organic matter (the importance of which we stress every week on this show), air in the soil (a topic we really should address more) and minerals—which we mostly talk about when we recommended adding rock dust to the soil, while Eliot depends on seaweed. He calls it his "insurance policy against micro-nutrient deficiencies".

He does add some to his compost piles, but explains that they get the most reliable wash-ups in the Fall, and when that aquatic abundance comes in, he spreads the collected seaweed directly on the soil, lets it break down over the winter and then tills it in in the Spring.

Now, long time listeners (and/or readers) know that I generally don't advocate tilling. But I also don't advocate a backyard gardener with a loud voice (that would be me) telling a hugely successful farmer, writer and organic advocate what to do. And Eliot has always seemed to be able to break the rules and still succeed. Maybe the weeds that tilling is supposed to cause are scared of him.

At any rate, the line that really got me was when he said, "if I didn't live near the coast, seaweed—or kelp—is the only input I would buy." That's high praise coming from one of the masters of sustainable, closed-loop farming.

And seaweed products? Barbara told me that she once investigated the harvesting and processing of the kelp that's used in organic fertilizers and was very impressed with its sustainability. Most—if not all—of the commercial product comes from the North Sea, where there are protections against over harvesting. And the finished product that she specifically reported on was even dried using energy from naturally occurring thermal vents in the sea! How cool is that?

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