1
History

1.1. The different types of kelp

Seaweed has often been used by coastal populations to improve the physical structure of the soil or to provide nutrients for flower beds. These are often algae washed up on beaches by storms and called wrecked kelp (see Figure 1.1(a)). The latter is very distinct from shoreline kelp (see Figure 1.1(b)) which is present on the foreshore and which is manually harvested during low tides. In addition to these two categories, there is also ground kelp which is harvested by dredging the bottom with boats (see section 2.1). Kelp has often been used as a soil amendment for crops. Depending on the region and time, the algal resource exploited for agricultural purposes has been given various and very different names (see Table 1.1).

Table 1.1. Examples of the names of the algal resource used historically in agriculture according to regions and the time period (from Lami 1941, Blench 1966 and Desouches 1972)

The purpose of introducing algae into the soil is to change the composition and texture of the soil. This practice of soil preparation was used by the first inhabitants of the island of Aran off the coast of Galway (Ireland) to create a layer of humus necessary for the establishment of food crops (potatoes). This practice has shaped the landscape of the island, which appears as a succession of plots protected from ocean winds and dedicated to potato cultivation (see Figure 1.2). The stranded algae is preliminarily mixed with sand and sometimes with manure before being spread in the soil.

Figure 1.1. Wrecked kelp (a) and shoreline kelp (b) (photo credits © J. Fleurence, 2010, 2020). For a color version of this figure, see www.iste.co.uk/fleurence/algae.zip

This process of amendment is notably evoked in the 1934 documentary film by Robert J. Flaherty, Man of Aran1.

Figure 1.2. Former plots dedicated to food crops on the island of Aran (photo credit © J. Fleurence, 2010). For a color version of this figure, see www.iste.co.uk/fleurence/algae.zip

1.2. Historical applications

The direct use of algae or composted algae for crops is often related to a need for soil amendment and crop fertilization. Seaweed manure was used as early as the 1st century CE for growing cabbage (Craigie 2011). The Roman writer Columella recommended that cabbage plants in the six-leaf stage of development be contacted at the root level with seaweed manure, also used to mulch crops. Another Roman, Palladius, in the 4th century CE, also suggested the early spring application of algal manure to the roots of pomegranate and lemon trees (Arzel 1994; Craigie 2011).

On the English speaking island of Jersey, algae have been used in agriculture since the 12th century (Blench 1966). The algal resource used is referred to as "vraic" or "wrack", which appears to be a distortion of the French word varech or the Old English wraec (Blench 1966). The use of this resource by farmers was regulated by a Code of Laws, published in 1771 and supervised by two sworn officers. This code defines, among other things, the number of vraic batches that can be allocated according to the owner's cultivable land area (see Table 1.2).

Table 1.2. Allocation of batches of kelp or wrack (vraic) according to the 1711 Code of Laws of the island of Jersey (from Blench (1966))

In the 19th century, algae were the most important fertilizing agent in the island's cultural practices. They were mainly used in the form of ash (Quayle 1815) and their use was subject to the seasonality of the crop. In the case of wheat, the vraic ash was spread on the soil before winter plowing (November-December). Summer vraic ash was particularly valuable for wheat cultivation, and many coastal residents increased their income by harvesting drift seaweed and burning it to ash. Until the middle of the 19th century, varieties of algae constituting vraic had not been identified. In 1860, Dally classified the algae used in the process into two categories: "sawn" or cut "vraic", and "growing vraic" or washed up vraic (see Table 1.3, Blench 1966). For each of these categories, he established a taxonomic identification of the species concerned. The species making up the sawn bulk were mainly pale fucales belonging to the genera Ascophyllum and Fucus. On the other hand, the algae constituting the washed up vraic were brown algae of the order Laminariales or red algae such as Fucus palmatus (see Tables 1.3 and 1.5).

Table 1.3. Algal species constituting the sawn vraic and growing vraic according to Dally (1860) (from Blench (1966))

* Rhodymenia palmata; see Table 1.5.

In France and more particularly in Brittany, kelp has also been used for centuries in agriculture. In 1681, Colbert published a royal decree to regulate the harvesting of shoreline kelp (see article I) and from wrecks (see article V) with the following articles:

Article I "The inhabitants of the parishes situated on the coasts of the sea, will assemble on the first Sunday of January of each year, at the end of the parish mass, to regulate the days on which the cutting of the grass called kelp or vraicq, sart or gouesmon, growing in the sea in the place of their territory, must begin or finish."

Article V "Nevertheless, let us allow any person to take indifferently at any time and in any place, the vraicq thrown by the waves on the shores and to transport them wherever they want" (Desouches 1972).

In the 19th century, wrecked kelp was a resource often used as fertilizer by coastal populations, whether they were islands or not. The nature of this kelp depended on the algal flora, the topography of the coasts and the strength of the waves, in other words the sheltered or beaten mode which applies to the coastal strip. On the Breton coasts, the wrecked kelp exploited by the populations was mainly composed of Laminaria, Fucus and Ascophyllum. On the other hand, in certain bays with a muddy inlet (e.g. in Belfast Bay), green algae of the genus Ulva constitute, almost by themselves, this type of kelp (Sauvageau 1920). The use of kelp on crops by coastal populations also had a phytosanitary advantage at a time when farming practices could not rely on agrochemicals. Indeed, algal manure, unlike farmyard manure, is free of weed seeds, pathogenic fungi and phytophagous insect larvae. This empirical knowledge of the benefits of using algae as a natural crop protection fertilizer has contributed to the development of many market gardening areas along the European coast. However, the preferred use of wrecked or shoreline kelp may vary by region (see Table 1.4).

Table 1.4. Type of kelp and constituent species used in different regions of Europe in the 19th and 20th centuries for agricultural activities (from Sauvageau 1920) (the names of the algae are those that prevailed at the beginning of the 20th century; some species may have changed names; refer to https://www.algaebase.org/)