Brown seaweeds, which are the ones used in agriculture and horticulture, not only contain vitamins common to land plants, but also vitamins which may owe their origin to bacteria which attach themselves to sea plants, in particular vitamin B12. There is still some doubt about this -- B12 may be contained in the seaweed, although in some cases it is in associated bacteria. Vitamins known to be present in the brown seaweeds include vitamin C (ascorbic acid), which appears in as high a proportion as in lucerne. Vitamin A is not present, but its precursor, beta-carotene, is, as well as fucoxanthin, which may also be the precursor of Vitamin A. B group vitamins present are B1 (thiamine), B2 (riboflavin), B12, as well as pantothenic acid, folic acid and folinic acid. Also found in brown seaweeds are vitamin E (tocopherol), vitamin K, and other growth-promoting substances. The unusual nature of the vitamin E in seaweed should be stressed. It has valuable characteristics (put technically, a complete set of isomers) found only in such seed oils as wheat germ oil.

It is therefore clear that while seaweed, in common with all organic matter, is beneficial to soil and plant, it has to be broken down, or decomposed, before its benefits are available, that liquid seaweed extract is not subject to this latent period. The nutrients and other substances it contains are available to the plant at once.