Exactly how it works is difficult to pin down, but scientists have found in seaweeds a veritable soup of plant-growth stimulants, vitamins, chelating agents, trace minerals, enzymes, and amino acids, all of which influence the growth of plants in different ways.
Seaweeds contain small amounts of nitrogen, phosphorus, and potassium.
On a dry-weight basis, seaweeds contain up to 1.2 percent nitrogen, 0.2 to 1.3 percent phosphorus, and 2.8 to 10 percent potassium. Improved yields after seaweed treatments were measured in potatoes, sweet corn, peppers, tomatoes, apples, strawberries, okra, and oranges. Better frost tolerance, increased seed germination, and greater capacity to absorb trace elements were other documented benefits for plants. It's common off the coast of Norway but also grows along the American coast from northern Maine to Canada and throughout northern seas. It is normally sold as a concentrated dry powder that you mix with water and apply to plants as a spray.