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A
Brief history
Brown seaweeds have been used
as a cheap and readily available source of fertiliser for hundreds
of years. Throughout coastal Scotland and the Isles, farmers
and crofters would take the cast plants brought in by the storms
during winter, or cut the weed from the rocks using sickles. After
being laid out to ‘mature’, the seaweed would be dug into the soil
and oats, wheat, barley, potatoes and brassicas planted.
Crofting
On the crofts, the seaweed
would be laid out next to trenches to rot in the piles of earth beside
them. Once this composted mixture was ready (after three weeks or
so), it would be seeded and turned into the trenches using a cas-chrom,
(a hand plough). An alternative was to lay the seed/ seaweed/earth
mixture on bare rock, thus claiming otherwise unusable land.
This practice is referred to
as the ‘lazy bed’ system as, once the crops had been planted, the
seaweed would provide all the nutrients that were required for their
growth. It was said that a crofter with a considerable portion of
his land abbutting the sea could grow oats on the same land for
two years running, with no need for rotation. The linear tracks
of the trenches can still be seen on the islands and nearby mainland
- excellent examples are to be found in Rum’s Harris Glen.
The Lowlands
In the farms of the central belt and those in Ayrshire
in particular, seaweed was in great demand in the 18th and early 19th
Centuries. Servants of estates were often put to work on the gruelling
task of collecting the seaweed. However, the increased availability
of chemical fertilisers and objections from the collectors led to
a decline in use over the next hundred years. The last significant
harvests of raw seaweed for laying on the land were in Ayrshire and
around North Berwick, in the 1960s.
Although the use of raw seaweed as a fertiliser is
not as extensive as it once was, many gardeners, smallholders and
a number of National Trust for Scotland properties still use seaweed
fresh from the shore. Overall, however, the availability and convenience
of processed seaweed-based fertilisers has seen and end to a once
widespread practice.
Maerl
beach on the Isle of Skye
Composition
and Properties (how does it work?)
Modern applications of seaweed and seaweed extracts
as fertilisers make use of the same properties of the plants that
historical uses did. Although there has been a considerable
body of scientific research aimed at understanding why seaweeds make
good fertilisers, the conclusions are often somewhat hazy and a lot
remains to be discovered.
Seaweeds have very high potassium concentrations in
comparison to other plants. Nitrogen levels are similar, and
phosphate levels are lower. This obviously makes them suitable
for use as fertilisers in low potassium areas. However, the
amount of trace elements (such as Iron, Manganese, Zinc, Copper
and Boron) that seaweeds can provide for a crop are insignificant,
so the mechanisms by which they provide beneficial effects are uncertain.
A variety of beneficial effects for germinating
and growing plants have been discovered:
Major:
- Increased resistance to frost damage and attack
by parasites (bacteria, worms and fungi). When used as a mulch,
the salt content deters slugs from the base of the plants.
- Enhanced respiration in germinating seeds.
- Improved condition of clay soils (probably due
to alginates helping clay particles to 'crumb', giving a soil
with good water retention properties).
Minor:
- Enhanced efficiency of fertiliser use.
- An increase in the soluble solids in tomatoes
and grapes.
- Improved shelf-life in peaches.
- Alginates act to protect root hairs when applied
as a gel to the root system prior to transplanting.
- Increased crop yields in some plants (e.g. cabbage
and cauliflower).
Using
Seaweed Yourself
Although there are a wide range of excellent
seaweed-based fertilisers (composts, foliar sprays, liquid foods
and rooting media) available on the market, it is possible to use
seaweed collected yourself. Although it is a smelly process, an
estate car load of cast-up seaweed can be gathered in a day, if
the beach is bountiful enough. This should be enough for a medium
garden or a few allotments through the course of a year.
1: Wash the salt from the seaweed and leave it spread out thinly,
if possible for three weeks to a month. If the mass of weed is too
dense, anaerobic rotting may occur, giving some (even more) unpleasant
smelling chemicals.
2: Once the seaweed is ‘matured’, it can be:
- Added to the compost heap as an activator, for
use on most plants (but be wary of using it on soft fruit).
- Placed round the base of roses and other shrubs,
as a mulch.
- Dug into the soil as a fertiliser for beets, brassicas
and other sturdy crops.
There are two minor problems associated with using
seaweed on the garden: The smell (which is not actually overpowering)
and the fact that some members of the Chenopodiaceae (such as Chenopodium
album - the ‘Fat Hen’) are coastal weeds which may be transported
back to the garden in the seaweed and can grow well in it.
A thank
you and a disclaimer:
Thank you to everyone (too numerous to list) who
provided the information and advice on seaweed as a fertiliser.
G. Kenicer cannot, unfortunately, accept responsibility
for the death of prized plants resulting from this document (as
he lives in a flat, two stories up, and doesn’t have a garden to
test it himself!)
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