Seaweed for Plants
By: Robert Kourik
Long ago, gardeners who lived near the ocean learned that seaweed was good for their plants. 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.
Robert Parnes in his classic Organic & Inorganic Fertilizers (Woods End Agricultural Institute, 1986; $40) says, "Perhaps the most important merit of seaweed is its content of assimilable organic materials, in particular the growth hormones."
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.
Several university studies have shown that seaweed can produce dramatic results in plants: geraniums produced more flowers per plant; grapes were sweeter; gladiolus corms grew larger; and cucumber yields increased 40 percent and the fruits suffered less often from softening and rotting. 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.
Seaweed for Gardeners
When gardeners talk about using seaweed on their plants, they are usually referring to a brown algae, specifically the one known as knotweed or rockweed (Ascophyllum nodosum). It's common off the coast of Norway but also grows along the American coast from northern Maine to Canada and throughout northern seas. The seaweed industry had an early start in Norway where seaweed supply was abundant, hence the "Norway" on the labels of many seaweed products.
The seaweed product that's been around longest (40 years) is Maxicrop. It is normally sold as a concentrated dry powder that you mix with water and apply to plants as a spray. But it is also available as a liquid concentrate, as are most other seaweeds.
Most liquid seaweed fertilizers are extractions manufactured by hydrolysis, and most of the basic research done with seaweeds (by T. L. Senn, formerly of Clemson University) used this form. You might read about liquid seaweed products that are "cold-pressed" or "enzymatically digested", but little research exists regarding these materials. Seaweed is also available as a dry meal intended for adding to soil. Liquid concentrates cost more than the powders, but they are much easier to mix with water.
Another seaweed fertilizer is derived from California bull kelp (Nereocystis luetkeana). It is offered only by Peaceful Valley Farm Supply, whose catalog claims this fertilizer is more potent than ascophyllum-based products, but I haven't seen any research to back up such claims.