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Reprint from Nov/Dec 1991 Fine Gardening Magazine

By Delilah Smittle

Seaweed is good for the garden. Mixed in the soil, it slowly releases nutrients that plants need, while improving soil texture. Since it is particularly rich in micronutrients such as iron, copper, zinc, boron and manganese, seaweed offers a natural remedy for soil with a micro-nutrient deficiency. Seaweed also contains large quantities of hormones that stimulate plant growth.  Plants in seaweed amended soil grow faster and larger than plants in soil with a comparable amount of conventional fertilizer.A traditional soil amendment in coastal gardens, seaweed is now formulated in extracts and granular products that you can find on garden center shelves and in catalogs of garden suppliers (see sources on p. 32). Fresh seaweed and dried granular seaweed must break down in the soil to release their nutrients and hormones. A foliar spray of seaweed extract and water makes the nutrients and hormones available to plants faster. Research has shown that plant health can improve within days after the spray is applied. Foliar seaweed sprays rapidly correct nutrient deficiencies, improve fruit set and help a plant endure environmental stress, including drought and frost.

Where it started
Coastal gardeners have long collected seaweed and composted or used it fresh as mulch in their gardens.  In the British Isles, 19th century gardeners grew potatoes of superior flavor in layers of sand and seaweed on bedrock. Traditionally, seaweed is raked from the sea by hand, piled into skiffs and brought to shore. It is time-consuming, heavy work. A small boatload of fresh seaweed weighs 4,000 lb. to 5,000 lb. Not surprisingly, the discovery of synthetic fertilizer sin this century eclipsed labor-intensive and slow-acting organic amendments, seaweed among them. Seaweed's emergence as a tonic for plants began with British experiments with seaweed as a replacement for hemp during World War II. Scientist learned that as a rope substitute, seaweed was hopeless because it dissolved in water. This discovery, however, led to a process for liquidating and concentrating seaweed, making it possible to bottle and to transport economically it's minerals and hormones. Drying seaweed over low heat led to the production of seaweed meal, a source of minerals and vitamins for livestock feed, and a concentrate soil amendment. Today, gardeners can readily find seaweed extract and seaweed meal.

The primal supermarket
Seaweed is a rootless plant in the Fucus family that floats freely or clings to rocks by holdfasts (root-like or disc-shaped plant parts that attach seaweed to rocks but don't absorb nutrients).  Seaweed photosynthesizes the sunlight that reaches it through shallow water and it absorbs nutrients from seawater through its leaves. Since the ocean receives runoff from the entire earth, it contains all known minerals, trace elements and vitamins. This primal supermarket supplies a more complete diet for sea plants that any plot of rich soil or fertilizer provides for land plants. Seaweed contains 60 or more minerals and several plant hormones. It is not, however, a complete fertilizer. It has a fair amount of nitrogen and potash, but very little phosphorus, a major plant nutrient. Only a few seaweeds are harvested commercially. Norwegian kelp (Ascophyllum nodosum) a brown algae, is the seaweed most used in gardening. Norwegian kelp is gathered off the coast of England, Ireland and Norway and both the Atlantic and Pacific coasts of North America where it is called rockweed. Gulfweed (Sargassum) a floating sea plant, is harvested off the coast of North Carolina. Giant kelp (Macrocystis) is collected in the Pacific Northwest.

How seaweed enhances plant growth
Seaweed is constantly worn down by tides and eaten by fish, so it must grow rapidly to survive. Studies at the University of California showed that a frond of seaweed can grow a foot a day, given optimal conditions. The same growth hormones that prompt such rapid growth in seaweed, when applied to plants as a foliar spray, can increase the speed of cell division and elongation in those plants. The hormones also increase root growth when applied to the soil as meal, or when a seaweed extract is used as a root dip.In recent turf test at Virginia Polytechnic Institute in Blacksburg, plots sprayed with seaweed extract had 67% to 175% more roots that untreated plots. Plots treated in fall showed a 38% increase in spring growth over untreated plots and showed 52% more roots.In test at South Carolina's Clemson University, seeds soaked in liquid sea weed extract showed rapid germination and the resulting seedlings and increased root mass and stronger plant growth that seedlings from untreated seeds. They also had a higher survival rate. Soaking plant roots in seaweed extract reduces transplant shock and speeds root growth. Seaweed foliar sprays promote faster, stronger stem and leaf growth and earlier blossoming and fruit set when sprayed on leaves and flowerbeds.

Seaweed as fertilizer
Seaweed improves soil fertility in several ways. Seaweed's nutrients and hormones are directly available to plants. Mannitol, a compound found in seaweed, enables plants to better absorb nutrients from the soil. The rapid breakdown of carbohydrates in seaweed stimulates beneficial soil bacteria that fix nitrogen and make it available to plant roots. These activities reduce the need for chemical fertilizers, and when seaweed is used with them, enhance their effects.Robert Kourik, an organic gardening specialist, suggest using 1 lb. of seaweed meal per 100-sq. ft. of soil or 1/4 tablespoon of liquid concentrate to 1 gal. of water for a foliar spray in intensive vegetable gardens. No matter what formulation is used -- fresh, dried or liquid - don't exceed the recommended quantities because excessive amounts of seaweed can stunt plant growth rather than encourage it.

Seaweed as pest control
Some scientists believe that seaweed has developed antitoxins to fend off bacteria and viruses in the ocean. In the gardens, these antitoxins interrupt the reproductive cycles of some insects and appear to repel others. Seaweed also reduces fungi when applied to plants or soil. In test at the University of Maryland, seaweed meal reduced soil nematodes in turf grass plots. Clemson University studies showed fewer aphids and flea beetles on foliar threatened plants, and other studies showed resistance to spider mites and scab. In Clemson studies, fruits and vegetables treated with seaweed didn't grow mold and thus had a longer shelf life.

Using seaweed
You can apply seaweed as mulch or as a soil additive, or incorporate it in a compost pile (its ability to activate soil bacteria makes seaweed an excellent compost starter). But the preferred method of application is as a foliar feed. For a head start on the growing season, you might want to presoak seeds in diluted seaweed extract for 20 minutes before planting. Then water the seedlings regularly with the same solution until strong growth appears. Apply seaweed meal to the soil as soon as the ground can be worked in spring because the meal needs time to break down. Work the meal in to perennial beds when the plants break dormancy.Apply foliar sprays once or twice a month during the growing season. Spraying in late fall supplies phosphorus and zinc to plant roots and increases the frost tolerance of grass, vegetables, and perennials. A late season foliar treatment can yield a longer harvest of vegetables.A balanced organic fertilizer can be created by mixing fresh seaweed or seaweed meal with manure or fishmeal, both of which supply sufficient phosphorus. Seaweed is also a good soil conditioner and can add as much humus to the soil as manure can.