Fertilizers

There is a long history of coastal people using macroalgae, especially the large brown macroalgae, to fertilize nearby land. Wet macroalgae is heavy, so it was not usually carried very far inland, although on the west coast of Ireland enthusiasm was such that it was transported several kilometres from the shore. Generally drift macroalgae or beach-washed macroalgae is collected, although in Scotland farmers sometimes cut Ascophyllum exposed at low tide. In Cornwall (U.K.), the practice was to mix the macroalgae with sand, let it rot, and then dig it in. For over a few hundred kilometres of the coast line around Brittany (France), the beach-cast, brown macroalgae is regularly collected by farmers and used on fields up to a kilometre inland. Similar practices can be reported for many countries around the world. For example, in a more tropical climate like the Philippines, large quantities of Sargassum have been collected, used wet locally, but also sun-dried and transported to other areas. In Puerto Madryn (Argentina), large quantities of green macroalgae are cast ashore every summer and interfere with recreational uses of beaches. Part of this algal mass has been composted and then used in trials for growing tomato plants in various types of soil. In all cases, the addition of the compost increased water holding capacity and plant growth, so composting simultaneously solved environmental pollution problems and produced a useful organic fertilizer.

Macroalgae meal is dried, milled macroalgae, and again it is usually based on the brown macroalgae because they are the most readily available in large quantities. Species of Ascophyllum, Ecklonia, and Fucus are the common ones. They are sold as soil additives and function as both fertilizer and soil conditioner. They have a suitable content of nitrogen and potassium, but are much lower in phosphorus than traditional animal manures and the typical N: P :K ratios in chemical fertilizers. The large amounts of insoluble carbohydrates in brown macroalgae act as soil conditioners (improve aeration and soil structure, especially in clay soils) and have good moisture retention properties. Their effectiveness as fertilizers is also sometimes attributed to the trace elements they contain, but the actual contribution they make is very small compared with normal plant requirements. A company in Ireland that produces milled macroalgae for the alginate industry is developing applications for macroalgae meal in Mediterranean fruit and vegetable cultivation. “Afrikelp” is another example of a commercially available dried macroalgae, sold as a fertilizer and soil conditioner; it is based on the brown macroalgae Ecklonia maxima that is washed up on the beaches of the west coast of Africa and Namibia.

Like all brown macroalgae, Ascophyllum contains alginate, a carbohydrate composed of long chains. When calcium is added to alginate, it forms strong gels. By composting the dried, powdered Ascophyllum under controlled conditions for 11– 12 days, the alginate chains are broken into smaller chains and these chains still form gels with calcium but they are weaker. The composted product is a dark brown, granular material containing 20–25% water and it can be easily stored and used in this form. Steep slopes are difficult to cultivate with conventional equipment and are likely to suffer soil loss by runoff. Spraying such slopes with composted Ascophyllum, clay, fertilizer, seed, mulch, and water has given good results, even on bare rock. Plants quickly grow and topsoil forms after a few years. The spray is thixotropic, that is, it is fluid when a force is applied to spread it but it sets to a weak gel when standing for a time and sticks to the sloping surface. It holds any soil in place and retains enough moisture to allow the seeds to germinate. Composted Ascophyllum has been used after the construction of roads in a number of countries, and has found other uses as well.

Maerl is the common name of a fertilizer derived from calcareous red algae; maerl beds are characterized by accumulations of living and dead unattached non-geniculate calcareous rhodophytes (mostly Corallinaceae but also Peyssonneliaceae, such as Phymatolithon calcareum and Lithothamnion corallioides). Also known as rhodolith beds, these habitats occur in tropical, temperate, and polar environments. In Europe they are known from throughout the Mediterranean and along most of the Western Atlantic coast from Portugal to Norway, although they are rare in the English Channel, Irish Sea, North Sea, and Baltic Sea. Maerl beds are often found in subdued light conditions and their depth limit depends primarily on the degree of light penetration. In the northeast Atlantic, maerl beds occur from low in the intertidal to ca. 30 m depth; in the West Mediterranean they are found down to 90–100 m, while in the East they occur down to depths of ca. 180 m. Maerl beds in subtidal waters have been utilized over a long period in Britain, with early references dating back to 1690. In France also, maerl has been used as a soil fertilizer for several centuries. Extraction of maerl, either from beds where live thalli are present or where the maerl is dead or semifossilized, has been carried out in Europe for hundreds of years. Initially, the quantities extracted were small, being dug by hand from intertidal banks, but in the 1970s about 600,000 tons of maerl was extracted per annum in France alone. Amounts have declined to about 500,000 tons per annum since then, though maerl extraction still forms a major part of the French seaweed industry, both in terms of tonnage and value of harvest. Live maerl extraction is obviously very problematic with regard to growth rates for replacement. Dead maerl extraction is liable to lead to muddy plumes and excessive sediment load in water that later settles out and smothers surrounding communities.

The maerl is marketed mainly for use as an agricultural fertilizer, for soil improvement in horticulture, mainly to replace lime as an agricultural soil conditioner. There are conflicting reports on the benefits of maerl use as opposed to the use of dolomite or calcium carbonate limestone. Other uses include: as an animal food additive, for biological denitrification, and in neutralization of acidic water in the production of drinking water, aquarium gravel as well as in the pharmaceutical, cosmetics, nuclear, and medical industries. These uses are all related to the chemical composition of maerl, which is primarily composed of calcium and magnesium carbonates. It is occasionally used for miscellaneous purposes such as hardcore for filling roads, and surfacing garden paths.

Maerl beds are analogous to the sea-grass beds or kelp forests in that they are structurally and functionally complex perennial habitats formed by marine algae that support a very rich biodiversity. The high biodiversity associated with maerl grounds is generally attributed to their complex architecture. Long-lived maerl thalli and their dead remains build up on underlying sediments to produce deposits with a three-dimensional structure that is intermediate in character between hard and soft grounds. Maerl thalli grow very slowly such that maerl deposits may take hundreds of years to develop, especially in high latitudes. One of the most obvious threats is commercial extraction, as this has led to the wholesale removal of maerl habitats (e.g., from five sites around the coasts of Brittany) while areas adjacent to extraction sites show significant reductions in diversity and abundance. Even if the proportion of living maerl in commercially collected material is low, extraction has major effects on the wide range of species present in both live and dead maerl deposits. Brittany is the main area for maerl extraction with about 500,000 tons extracted annually; smaller amounts are extracted in southwest England and southwest Ireland. Maerl beds represent a non-renewable resource as extraction and disruption far out-strips their slow rate of accumulation. In France, maerl extraction is now considered to be “mining,” which implies more constraints for the extractors and more controls on the impact of extraction. Scientists, managers, and policy makers have been slow to react to an escalating degradation of these habitats such that there is now an urgent need to protect these systems from severe human impacts.



Macroalgae extracts and suspensions have achieved a broader use and market than macroalgae and macroalgae meal. They are sold in concentrated form, are easy to transport, dilute, and apply, and act more rapidly. They are all made from brown macroalgae, although the species varies between countries. Some are made by alkaline extraction of the macroalgae and anything that does not dissolve is removed by filtration; others are suspensions of very fine particles of macroalgae.

Macroalgae extracts have given positive results in many applications. There are probably other applications where they have not made significant improvements, but these receive less, if any, publicity. However, there is no doubt that macroalgae extracts are now widely accepted in the horticultural industry. When applied to fruit, vegetable, and flower crops, some improvements have included higher yields, increased uptake of soil nutrients, increased resistance to some pests such as red spider mite and aphids, improved seed germination, and more resistance to frost. There have been many controlled studies to show the value of using macroalgae extracts, with mixed results. For example, they may improve the yield of one cultivar of potato but not another grown under the same conditions. No one is really sure about why they are effective, despite many studies having being made. The trace element content is insufficient to account for the improved yields, etc.

It has been shown that most of the extracts contain several types of plant growth regulators such as cytokinins, auxins, and betaines, but even here there is no clear evidence that these alone are responsible for the improvements. Finally there is the question, are macroalgae extracts an economically attractive alternative to NPK fertilizers? Perhaps not when used on their own, but when used with NPK fertilizers they improve the effectiveness of the fertilizers, so less can be used, with a lowering of costs. Then there are always those who prefer an “organic” or “natural” fertilizer, especially in horticulture, so macroalgae extracts probably have a bright future.