Seaweed: Why You Should Use It On Your Farm & Garden
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One of the oldest uses of seaweed is in agriculture. The beneficial effects of seaweed on soil and crops have been used by farmers for thousands of years and confirmed by many scientific studies.
Seaweed supplies macro and micronutrients. It contains calcium, increases phosphorus availability, water retention and cation-exchange capacities. It improves soil structure, binds metals and contains compounds that act as chelators which make soil minerals more accessible to plants. Seaweed has proven its worth on every type of crop.
So how do you use it?
Seaweeds are easy to use. Most commercial ones are either powdered or liquid. Powdered seaweeds are often mixed with water for easier application. They can be put through commercial sprayers (such as the Cyclone Multi-Task) or for gardens you can use a watering can. Application rates vary and are always suggested by those that supply you with the seaweed.
You can also make your own. It requires you to gather seaweed from your local beach. You need to be aware of any local regulations regarding seaweed harvesting but once you have it is easy to use.
The simplest way to use your seaweed is to put it in a bucket with water and let it sit for at least two weeks. Some people believe it is a good idea to wash it first, but our experience has found that to be unnecessary.
Another technique is to dry the seaweed first then rehydrate it by putting it in water. The idea here is that this process releases more of the nutrients bound in the seaweed. Also chopping it up, either by hand or through a mulcher or chipper can help with the breakdown and extraction of the nutrients.
Some commercial processers use a much longer extraction cycle, up to a year. This is intended to extract a greater range of nutrients.
New Zealand’s Seaweeds
For those of you liking a bit more detail, here is everything (almost) that you ever wanted to know about Seaweed:
Seaweed Types
Seaweeds can be grouped into three types based on colour; green, red or brown.
They all contain chlorophyll, which is necessary for photosynthesis.
Brown and red seaweeds have additional pigments that enable them to photosynthesise at depths where little light penetrates.
Green Seaweeds are mostly green.
Brown seaweeds can be yellow-brown to dark olive.
Red seaweeds have the greatest range of tone; pink to purple, red, and brown to nearly black.
Where do you find it?
New Zealand has 850 native seaweeds, a third of these are not found anywhere else. Seaweed is particularly plentiful in three areas:
The warm northern waters around the Kermadec and Three Kings Islands.
The Cook Strait–Kaikōura coast region in central New Zealand.
The South, in an area encompassing Fiordland, Stewart Island and the Otago coast including areas around Banks Peninsula.
Green Seaweeds
Green seaweeds are usually found in the intertidal zone (between the high and low water marks) and in shallow water where there is plenty of sunlight. About 140 species have been recorded around the coast. One of the easiest to recognise is sea lettuce (Ulva lactuca), which forms bright green sheets up to 30 centimetres in diameter. It is edible, however, prolific growth often indicates sewage pollution. Sea lettuce can become a problem when large quantities are washed ashore and begin to rot, giving off an offensive sulfurous smell. Gut weed (Enteromorpha intestinalis), a tubular green seaweed, also favours high-nutrient sites. Another common green seaweed is sea rimu (Caulerpa brownii), also edible, and looking very much like the foliage of the large tree, Rimu.
Brown Seaweeds
These medium to giant-sized seaweeds typically grow at depths below the greens and above the reds. Neptune’s necklace (Hormosira banksii) is well known to most people who have visited the rocky shore. Its branching chains of water-filled bladders help it withstand periods of exposure when the tide goes out. Many seaweeds produce mucilage or slime to protect against drying out. Of the brown group, Gummy Weed (Splachnidium rugosum) takes mucilage production to the extreme; its swollen tubes ooze profuse quantities of sticky slime when touched. The largest brown seaweeds are known as kelps and are prominent in the cooler coastal waters of southern New Zealand.
Red Seaweeds
There are 550 species of red seaweed, making them the largest group. In the clear waters around the Kermadec Islands red seaweeds may be found at depths greater than 200 metres.
One of the best-known reds is the edible karengo (Porphyra species), which grows on rocks near high-tide level and resembles sheets of light purple cellophane. It is a close relative of the Japanese nori, used for sushi. Another familiar red is the fern-like agar weed (Pterocladia lucida) which has been harvested for agar production in New Zealand since 1943.
Kelp
Kelp Forests
New Zealand’s largest seaweeds are collectively called kelps and belong to the brown group, they are often called kelp forests. Kelp forests may be over 20 metres tall and support an understorey of smaller brown and red seaweeds as well as rich populations of both grazing and immobile animals.
Bladder Kelp
The country’s largest kelp is Macrocystis pyrifera, known as bladder kelp in New Zealand, but giant kelp elsewhere. It can grow to 50 metres in length and 100 kilograms in mass. Bladder kelp forms large forests in the deep sheltered waters of southern New Zealand and is easily identified by the gas-filled floats at the base of each frond. These help to hold the kelp upright, maximising the amount of sunlight the blades receive.
Common Kelp
In northern waters common kelp (Ecklonia radiata) dominates marine forests. This seaweed is about 1 metre in length and has a bunch of fronds arising from a central supporting stem. It is the preferred food of the sea urchin, kina (Evechinus chloroticus), which sometimes mass together and munch through an entire forest. Following such large-scale habitat destruction, other seaweeds may dominate for a while, but in northern New Zealand, at least, common kelp grows rapidly and soon replaces itself.
Bull Kelp
Bull kelp or rimurapa (Durvillaea species) is the most striking seaweed of the exposed coasts. Its tough, flexible fronds are secured to intertidal rocks by a solid disc-shaped holdfast capable of withstanding tremendous forces when storm waves crash onto reefs and cliffs. The seaweed grows to 10 metres in length and can live for 10 years. Bull kelp forests are highly productive systems, contributing vast quantities of organic matter and nutrients to coastal food chains.
Four species of bull kelp are found around New Zealand, and the most common, Durvillaea antarctica, also grows around the subantarctic islands and southern coasts of South America. In northern New Zealand it grows only on very exposed headlands and becomes more common in the cooler waters south of Cook Strait. Fronds of Durvillaea antarctica have an internal honeycomb-like tissue, full of air, which keeps the blade buoyant.
Asian Kelp
Asian kelp (Undaria pinnatifida) arrived in New Zealand waters in the 1980s and quickly made itself at home in sheltered harbours. Although it is farmed and eaten in Japan, where it is known as wakame, this fast-growing, 2-metre-tall kelp is unwelcome in New Zealand as it can change the structure and composition of native marine communities.
The earliest evidence of green plants was a green seaweed, the fossils of which are about 1 billion years old.
