Plant
Algae comprise several different groups of organisms which produce food by photosynthesis and thus have traditionally been included in the plant kingdom. The seaweeds range from large multicellular algae to single-celled organisms and are classified into three groups, the brown, red and green algae. There is good evidence that the brown algae evolved independently from the others, from non-photosynthetic ancestors that formed endosymbiotic relationships with red algae rather than from cyanobacteria, and they are no longer classified as plants as defined here.
Seaweed fertiliser
Several of the 12,000+ varieties of seaweed in the ocean have been shown to be valuable additions to the organic garden and can be abundantly available free for those living near the coast. However, caution should be observed when collecting seaweed, particularly from areas that are liable to pollution, such as downriver (including estuaries) of industrial activities as seaweed is susceptible to contamination. There are also legal implications relating to gathering seaweed, and concerns about sustainability. In terms of soil structure, seaweed does not add a great deal of bulk, but its jelly like alginate content helps to bind soil crumbs together, and it contains all soil nutrients (0.3% N, 0.1% P, 1.0% K, plus a full range of trace elements) and amino acids. For those who cannot gather fresh seaweed, it is available commercially in a dried 'meal' form or as a concentrated liquid extract which is active in significantly smaller rates. While 'meal' products are limited to soil applications due to their insolubility, foliar feeding or root zone applications through drip lines are possible with soluble extracts.