Seaweed 鈥� The Free Fertiliser
Seaweed has been used as a fertiliser for grass and crop production for many years and provides听coastal crofters听with听a valuable source of nutrients that can be used to supplement bagged fertiliser.听It听contains听nutrients, organic matter, and trace elements that make it听a very useful听fertiliser for those fortunate enough to have a source available nearby.听
Like all organic manures,听seaweed has听a variability in nutrient content. Therefore, to get the most benefit from seaweed as a fertiliser, it is advisable to take a representative sample to have analysed prior to application.听Analysis carried out by SAC听Consulting听found听typical nutrient values for fresh seaweed as shown in the table below.听听
Unlike bagged fertiliser, not all these nutrients are available from seaweed in the year of application, and it is important to bear this in mind when planning applications.听听
Approximately 5-15% of the听Nitrogen听(N)听will be available, 50% of the听Phosphate听(P), and 80% of the听Potash听(K).听Nitrogen is available as听organic N (where the nitrogen is contained in organic forms that break down slowly to become potentially available for crop and grass uptake over a period of months or years) or as readily available N (available for rapid crop uptake).听
In comparison,听Farmyard Manures (FYM),听seaweed听contains听slightly higher levels of readily available听nitrogen听which can be lost easily if applied at the wrong time of year when there is not a crop need.听
Approximately听50% of the phosphate in seaweed is available to the crop in the year of application with the听remainder听becoming available to the crop in following years.听Seaweed is a useful source of听potash听and听approximately听80% is available to the crop in the year of application with the听remainder听adding to the soil reserves.听Seaweed also听contains听an appreciable amount of听sulphur and听magnesium and amounts of the听micronutrient鈥檚听boron,听cobalt,听copper,听iodine,听manganese,听selenium听and听zinc which can be beneficial to听both听crops and livestock.听So听that nutrients can be fully utilised,听it is essential that the soil is at the听correct pH of between 5.8 and 6.0 for grassland.听Also, fertiliser requirements will depend on whether the grass is for grazing livestock or cutting for silage.听Relatively little听will be听required听for extensive grazing, whilst intensive silage production results in a lot of nutrient听offtake, which needs to be replaced.听
Seaweed directly adds the nutrients detailed above,听but they听have听also听been shown (particularly in the horticultural sector) to act as plant听bio stimulants.听Containing听hormones that trigger increased growth and听improve听nutrient uptake in plants听and听seaweed听can听also听impact听various elements of the听plant鈥檚听successful growth.听听
The addition of seaweed can also have beneficial impacts to soil structure, water retention, and nutrient holding capacity. This is due to the organic matter being added to the soil, and to the presence of听alginates听in the seaweed, which听have structural听influences听on soils.听
A recent study听has found that potassium deficiency in agricultural soils has the potential to听be a听major听threat to future agricultural production.听Volatility in the price of potash, plus more general question marks over the sustainability of mining has听led听to potash being underapplied to agricultural land. Seaweed has the potential to fill this gap, for those with ready access, as a sustainable source of the nutrient that does not suffer price volatility, or a high carbon footprint.听听听
5听top tips听
- Like all organic manures,听the听nutrient content听of seaweed听is variable.听Analysis听prior to application听is听advised.听听
- Not all nutrients are听immediately听available to the crop, but they do help to build soil reserves. Remember to account for this in your fertiliser use.听
- Assuming application is carried out at the听optimum听time of year, seaweed听is听a very useful听source of potash, so it can be听a great help听to potash hungry crops such as grass silage.听听
- Seaweed听contains听other nutrients such as听sulphur,听magnesium听and听varying听amounts听of various micronutrients, which can be beneficial to crops and livestock.听
- Seaweed should be allowed to breakdown fully before grazing or cutting.听听
Useful links听
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David MacKay, Consultant, David.Mackay@sac.co.uk
Posted by SAC Consulting on 11/03/2026
