This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
Various kinds of seaweed have long been used as manure when obtainable in sufficient quantities round the coasts. The commonest kinds are species of Laminaria and Fucus, the latter genus supplying two well-known species met with almost everywhere, namely, F. vesiculosus and F. nodosus. Seaweed is variously known as wrack, bladderwrack, black wrack, and black tang in different parts. During the summer months, after the tide has receded, the seaweed is gathered and laid out to dry along the shores. It is turned over a few times, as if it were hay, and when sufficiently dry is stacked in conical heaps for autumn and winter use. During the winter seaweed cannot be dried and stacked in this way, as it melts away into an oily liquid. It is therefore applied direct to the soil when collected at this season. The value of seaweed is due to the amount of potash it contains - from 30 to 40 lb. in a ton. It also contains about 10 lb. of nitrogen and 10 lb. of phosphoric acid, as well as 11 to 18 lb. of lime to the ton. It is therefore a "complete" manure, but is not so valuable as farmyard manure. For Potatoes, Peas, and Beans it is excellent in light soils, and a good dressing would be from 12 to 20 tons per acre.