Seaweed contains plant nutrients, including potassium, (up to 12%), but it is low in nitrogen and phosphate. It can contain in the region of 60 trace elements, growth hormones and other nutrients and is particularly rich in iodine and calcium. Spraying seaweed tea on plants is said to increase resistance to insect infestation.
Seaweed has been used as a soil improver for centuries and is still used in the garden as a mulch, liquid feed and in making compost. It is also available commercially both dried and as a liquid.
Seaweed is a commonly used as a constituent in home composting in coastal areas. It acts as an activator speeding up the compost process. If using a cold composting technique small quantities of seaweed can just be added to the bin in the same way as any other nitrogen rich “green”. There are mixed views as to whether the seaweed should be washed to remove traces of saltwater or sand with the consensus being that it is not necessary. However, not all plants tolerate salt so if in doubt the seaweed should be rinsed in fresh water.
If using a New Zealand or pallet bin, in which layers of greens and browns are alternated, the seaweed can be added as a separate green layer or mixed with other green material. Shredded or chopped seaweed cut in 1- or 2-inch will decompose in a few weeks compared with six months or more for uncut fronds so, as with other materials, it is better to cut the seaweed into small pieces. If the contents of the bin are to be turned to aerate the organic material occasionally in cold composting or regularly as in hot composting cutting the seaweed into short lengths will also make it easier to turn during aeration and speed up the composting process.
Seaweed can also be trench composted and used to make liquid feed. Details are available at carryoncomposting.com
If considering collecting seaweed it must be noted that many countries have laws or regulations protecting the marine environment and it is likely that these will cover the harvesting of seaweed. Please check.