What are the benefits of seaweed products in the garden?

Seaweeds and their various derivatives are some of the more interesting organic materials that can be used to help create healthier, more sustainable soils and plants. In order to better understand how such materials work it is useful to ponder the nature of seaweeds.
Seaweeds are marine algae, in other words, algae that live in the sea or in brackish water. Marine biologists often refer to seaweeds as “benthic marine algae”, which simply translated means “attached algae that live in the sea”. There are approximately 10,000 species so far known to science and they are mostly divided into groups according to their colour; red, brown or green.
Algae of one kind or another have been around for more than 2 billion years and are incredibly adaptable organisms as evidenced by their profusion in all sorts of environments. Algae are very simple chlorophyll-containing organisms that are considered by many scientists to be rather primitive plants. It is not surprising then that seaweeds contain a complex mixture of biochemicals including the hormones that regulate growth in throughout the plant kingdom.
Given that there are thousands of different types of seaweeds in the ocean it is obvious that their precise chemical make-up will vary. There are, however, some common characteristics that explain the beneficial effects that seaweed and its derivatives have on plant growth.

The most obvious beneficial effect of seaweed is that it will break down to release nutrients as well as humus that will improve soil structure, water and nutrient holding capacity and encourage the activity of beneficial micro-organisms. However the benefits do not end there. Various types of seaweed are also known to contain biochemicals that act as plant growth regulators (hormones). These substances, although present in very tiny amounts, can have significant stimulatory effects on plant growth.

The seaweed Brown Kelp (Durvillea potatorum) is used in the manufacture of the popular liquid extract product Seasol. An independent analysis supplied by the manufacturer of this product states that this product supplies the plant hormones indole acetic acid (a member of the auxin group of plant hormones) and zeatin and isopentenyl adenine (both members of the cytokinin group of plant hormones). Because of these hormones, seaweed extracts could be considered to be acting as plant ‘€œbiostimulants’€, in addition to conditioning the soil with humus and supplying small but useful quantities of nutrient as well.

Auxins have wide ranging effects on plant growth and are known particularly as ‘rooting hormones’€ due to their ability to encourage the formation of new roots. This would account for the beneficial effect of seaweed extracts on better establishment of new transplants. Cytokinins on the other hand are known to stimulate cell division and new shoot initiation in plants and can be expected to have a generally stimulatory effect on shoot growth.

Seaweeds and their extracts also contain significant amounts of organic matter that adds humus to the soil. This has the effect of binding together soil particles and thereby improving soil structure. The aggregations of soil particles that result from this improvement allow water to pass freely in the pore spaces between them yet store water inside them that can be extracted by plant roots. Humus also enhances nutrient storage in the soil through its ability to attract nutrients and store them until plant roots take them up.

There are many claims made by manufacturers for the various seaweed products available for the gardener including increased crop yields, resistance to frost, increased uptake of inorganic constituents from the soil, more resistance to stress conditions, and reduction in storage losses of fruit. Given the complex nature of the biochemicals present in seaweeds it would seem there is a very reasonable basis for these claims.

If you live close to  beach, you can collect a bag of seaweed from the shore to use as your own soil and plant improve,  but with a few words of caution. It is illegal to harvest living seaweed, and there is a limit of 20 kilograms for washed up seaweed. Check with your local council if in doubt. Seaweed will have some salt attached, which is not something we want to add to our soils or compost heaps. It is recommended that you wash seaweeds before using them in the garden, though I have had good results from not washing it. An excellent option is to compost seaweed with a variety of other organic materials such as dead leaves, lawn clippings and vegetable wastes. It can also be buried to decompose in situ. Chopping it up with a spade will hasten the process of decomposition.
It should also be said that seaweed is part of the ecosystem of the marine environments where it occurs naturally and must not be over collected. We need to be sensitive to this fact and to the presence of any marine conservation zones if we are going to collect seaweed for use in the garden.