In 1995 excavations uncovered a strip of peat embedded in marine sediments at Kolhorn in the province of Noord Holland, just to the north of the northern part of the Westfriese Omringdijk. The strip of peat is the last remaining part of an extensive peat bog that was almost completely dug up. This ‘road’ was found to contain many pits along its surface, containing large amounts of charred and waterlogged remains of eelgrass. Accurate dating of these pits proved to be impossible, but they must have been used somewhere between the 12th and 17th centuries AD. These particular remains are interpreted as remains of a salt extraction process that has been mentioned in the literature (Van Geel & Borger 2002; 2005).
Even though the exact method is not quite clear, it is known that in medieval times the inhabitants of the Dutch coast made salt out of peat that had become infused with seawater due to flooding. The peat was probably burned on the spot, the ashes were leached, and the resulting brine boiled down to extract the salt. Apparently this ‘road’ near Kolhorn served as a place where the diggers could burn their peat for its salt-containing ashes.
Presumably the peat digging turned the area surrounding the site into a lagoon, supporting a dense vegetation of eelgrass. Since no more peat could be dug up in the lagoon, the salt makers turned to the eelgrass, continuing the process with dried eelgrass instead of peat. The scale on which this was done seems to have been fairly large (Van Geel & Borger 2002).
The main historical source for this activity is Linnaeus, who discusses the uses of eelgrass in Denmark (Houttuyn 1793, 243-244, 704). The quality of the salt obtained this way was poor, but it could be used to preserve fish and meat. Salt was a very precious and vital commodity in the Middle Ages and Early Modern period, so the practice may have been economically viable or even necessary. Potash (potassium carbonate) could furthermore be obtained from the ashes, an important resource for glass and soap making.
Secondly, the burning of eelgrass may have served as a method to produce fertilizer. In Normandy washed up sea vegetation (called warec) used to be collected and applied either burned or fresh as a fertilizer rich in potash and other minerals (Van Ravelingen 1644, 780-782). Eelgrass would in this respect be inferior to true algae (Laminaria spp., Fucus spp., Ascophyllum spp.), which contain more potash. Yet there is evidence that coastal communities used eelgrass as fertilizer if there was no alternative, although there is no specific record of burning it (Wyllie-Echeverria & Cox 1999; Alm 2003).