Benefits of using seaweed around the smallholding

[UK] In the March edition of Smallholder magazine Helen Babbs explored the different types of seaweed and its many uses around the smallholding.

Seaweed has a long history of use in agriculture: the spreading of seaweed on coastal farmland is described in ancient Chinese records dating from 2500BC!

In the UK, seaweed has traditionally been harvested along the western coasts of Cornwall, Ireland and Scotland. Growing in the deep, mineral-rich waters of the Atlantic, seaweed has a correspondingly high level of nutrients and minerals.

In addition to major plant nutrients such as nitrogen, potassium and magnesium, seaweed contains over 60 minor nutrients and trace elements, including copper, selenium, cobalt, zinc, boron and iodine.

The alginate gels within seaweed promote microbial action, and when applied as fertiliser to the soil, also help to bind soil particles together into a better soil structure.

Seaweed is also very rich in natural plant growth hormones such as cytokinins. These encourage cell growth and division within plants, leading to both stronger and more rapid plant growth, and subsequently larger crop yields, greater disease resistance and improved frost hardiness.

Fresh seaweed

The most traditional form of seaweed to use is, of course, fresh seaweed gathered from the shoreline.  However, before you rush out and fetch home a trailer-full of seaweed, it’s important to have proper permission.  Like any other plant life, seaweed shouldn’t be collected without permission of the landowner.  If you own a beach or know someone with a private beach, this is fine, but picking seaweed up off a public beach is stealing Crown property!

Although the exact nutrient content varies with the species, fresh seaweed has a similar composition to farmyard manure, and can be dug into soil like manure,while still wet, without needing to be composted first.  Any salt adhering to the seaweed will be leached away by rainfall before new crops are planted, so it won’t scorch their roots.  Fresh seaweed is also excellent for the compost heap, since it will add plenty of minerals, as well as promoting the action of the microbes which break down the compost, so it decomposes faster.  It’s important to mix the seaweed in with more fibrous material such as paper, cardboard or woody prunings, otherwise it will settle into a jelly-like layer and smother the entire heap.

Seaweed meal

For smallholders with inland holdings, many of the same benefits of seaweed can be had from dried seaweed meal. This is simply harvested seaweed which has then been dried and ground into small flakes. It contains the same high levels of nutrients, but is less drippy and a bit easier to handle! Seaweed meal can be of just one species of seaweed, or a blend to give a wider range of nutrients.

The granular structure means seaweed meal can be spread on pasture, not just applied to bare soil, either by hand or with a tow-behind fertiliser spreader. The seaweed acts as a slow-release fertiliser, giving a steady supply of nutrients as it is broken down by the soil bacteria. The alginates and plant hormones improve soil structure and plant root growth, for better long-term pasture health. I’ve found that pasture which has had seaweed meal applied in the spring remains green and growing late into the autumn, long after the other paddocks have become yellowed and wind-scorched. A further advantage of seaweed as fertiliser is that you don’t have to exclude stock from the pasture after spreading, although one or two of your animals may try and lick it up!

In the vegetable garden, seaweed meal can again be used before planting like fresh seaweed, or sprinkled on afterwards. It’s particularly useful if your soil has any mineral deficiencies. If you’ve struggled to grow beetroot, apply seaweed meal several times during the growing season – the boron in the seaweed makes a huge difference to the size of the roots.

Seaweed for livestock

Beside making a good plant fertiliser, with its high levels of minor nutrients and trace elements seaweed is very good for livestock too. The zinc and iodine in particular are good for healthy hoof growth. Many smallholders find that feeding seaweed almost entirely eliminates the occurrence of foot rot in their sheep and goats. The minerals are all in an organic, “bio-available” form, so can be readily absorbed and used by the animals. In ruminants, the mildly alkaline nature of seaweed helps keep the rumen pH well balanced, encouraging better digestion.

Although some animals may enjoy nibbling on fresh seaweed, seaweed meal is the main form used with livestock. This can be either as the dry meal, or as part of a proprietary stock lick. Usually combining seaweed meal with molasses, these licks can be a very useful source of minerals and energy for sheep and cattle grazing out over the winter. If feeding dry seaweed meal, this can be mixed into the concentrate feed or put out in a trough or bucket for the animals to help themselves. Seaweed doesn’t have a high copper content, so is safe for sheep. Pelleted seaweed meal is also available for using with poultry, where it will improve egg laying rate, egg shell thickness and yolk colour.

Seaweed extract

Seaweed extract is made on a commercial scale by various methods including by boiling, soaking it in cold water or treating with a strong alkali such as potassium chloride. The resulting liquid is then bottled for sale, or dried into powder to be dissolved in water just before use. Seaweed extract contains far fewer nutrients, but is very rich in cytokinins and other plant growth hormones. Applied either as a liquid feed to plant roots or sprayed on as a foliar feed, these promote faster growth and balanced nutrient uptake, encouraging plants to make best use of what’s in the soil. I’ve found applying seaweed extract can even help greenhouse tomatoes to overcome the early stages of blight and produce a good crop.

Seaweed extract is sometimes also included in “natural” rooting powders for cuttings, where it stimulates strong, rapid rooting. This can be used for all types of cuttings, but is particularly handy if planting willows or other hardwood cuttings in situ, to make sure you get 100% rooting and no gaps in the rows.

Calcified seaweed

Despite its name, calcified seaweed is not really seaweed but a form of cold-water coralline algae which grows on the sea floor. Like seaweed, it has a long history of use, having been dredged from around the UK coastline since at least the 17th century. It’s still harvested by dredging, nowadays using computerised navigation to locate the sites, then dried and crushed into a powder similar to garden lime. Although the algae do regrow, there are some concerns about whether the rate of harvesting exceeds growth, making it unsustainable. Calcified seaweed is therefore not recommended for use in organic systems.

Calcified seaweed is similarly rich in nutrients and minerals to real seaweed, but also contains high levels of calcium and magnesium.  This means it can be used in place of garden lime to raise the pH of your soil or pasture land, while adding slow-release nutrients at the same time.  It has a slightly lower neutralizing value than ground limestone, so you need to use a little more, but the cost is still a lot less than applying separate lime and fertiliser.  Like any lime product, calcified seaweed has a knock-on series of benefits from raising the soil pH: improving nutrient availability; making the grass more palatable to stock; ensuring soil minerals are in an available form, not locked up; and stimulating beneficial soil bacteria, particularly those which fix atmospheric nitrogen so there is a further “free” nutrient input.  Calcified seaweed can also be fed to livestock, providing a good source of calcium and magnesium along with other minerals such as copper.


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