Low Island and Coral Cay Strategies in Permaculture

Permaculture Designers Manual




Section 10.14 –

Low Island and Coral Cay Strategies in Permaculture

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All coral sand cays and many low atolls lie within 28° of the equator, as do coral reef areas and sandy alkaline coasts.


As many people live on atolls, very careful design approaches are needed to avoid the known risks of:

  • Hurricane erosion and damage to plantations and coasts;
  • Water table (water lens) pollution; and
  • Poor nutrition due to a limited diet (usually high in carbohydrate and oxalic acids);
  • Additional design input is needed;
  • To extend vegetable and fruit crop;
  • To extend water storage and to conserve water;
  • In conserving natural vegetation and unique birds, reptiles and lagoon or reef fauna;
  • To use shallow marine waters for aquaculture and pond fish; and
  • In developing energy resources locally.
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On atolls, we can expect soil pH values of from 8.0-9.5, sand abrasion and basic mineral deficiency (especially iron, zinc, molybdenum, boron) in soils and plants.

Elemental sulfur, iron sulfates and humus added to garden soils and planting holes lower the pH.

Humus sources are palm fronds, coconut husks, tree trunks and leaf litter from such pioneer species as Casuarina equisetifolia, sea grape (Coccolobus unifera), coastal shrubs (Scaevola, Tornefortia, Pemphis), mangroves and Barringtonia trees.

All yield abundant litter for garden and tree holes.

We also can expect thin coralline sand over a hardpan of caliche or calcrete.

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Calcrete is worsened by the application of superphosphate.

Usually the guano from seabirds provides sufficient phosphates if colonies of terns, gulls, boobies, frigate birds and shearwaters are protected or encouraged.

Failing this, domestic pigeon, quail, pheasant, geese, ducks and chickens can be kept as such predators as feral cats and foxes are usually absent.



The sole natural sources of water on low Islands are those biological storages such as we find in baobabs and coconuts (about 12 coconut trees provide for a person’s drinking water for the year) and that water trapped in the sand below the caliche, floating on the seawater – the water lens of Figure 10.49.

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As rain falls on the islands, it quickly absorbs in surface soils and leaks through the caliche to the water lens, which itself leaks slowly to sea between or below tide levels.

As the beach berm is 2-4m above sea level, and the inner atoll about 2-3m, the delicacy of the situation in the event of water pollution is obvious.

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For this reason, settlement, toilet areas, processing, livestock and even gardens should be kept to the periphery of the island and the interior devoted to dense natural stands of food and native trees, kept free of pesticides or industrial pollutants and fuels.

It is obvious that a polluting source at the center of the island can affect the whole water lens, all plants and all people.

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By what methods can we increase the supply of potable water?

First; by using run-off water to house tanks, the latter above or below ground and made of a reinforced cement-coral-sand mix, which is widely used in construction on all but remote atolls.

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Secondly; and of most importance to plants, by using deep mulches and by growing ever-abundant coconut, banana, arrowroot, papaya, legumes and like plants to provide leaf and trunk materials (or gardens and other tree crop.

Lastly; and often successfully, we can try leaf­ ferment seals (gleys) of gently-sloped coral pots, where a 20cm deep mash or shredded mass of banana, papaya and other soft green leaf is applied to the base and sides of the area, covered with plastic until fermenting (sometimes only 4-5 days) and then pumped full of water from the freshwater lens. (Figure 7.19)

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Made carefully, such surface ponds will also take roof run off.

Success follows careful trials at small scale to get the system working, then scaling up to significant ponds for ducks and garden water, water leaf crop and animal drinking water.

As such ponds are often a source of cross-infection in children, it is sensible to swim in the lagoon and to drink tank water.

Seawater serves also for many toilet uses.



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To plant trees, we need to clear off a patch of topsoil and break open the caliche, making a pit about 40x40cm and 60-80cm deep.

In this pit we make humus pile of domestic and plant wastes, plus some sulfur and the mineral trace elements and plant a coconut or other seedling tree.

The tree roots spread out above the caliche layer and feeder roots go below the caliche to the water lens, which is usually 0.75m-1.0m down and 1.0m-2.0m deep as a saturated sand layer.

Trees are thus well anchored and easily obtain water. (Figure 10.50)

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To plant gardens, we have two or three possibilities:

The first (and best) is to open a pit garden of about 8mx15m and sloped for stability to about 2m deep, thus often damp or wet  on the base  (Figure 10.50).

The sloped sides are stabilized, as steps, with coconut logs or caliche and the spoil banked around the rim.

Mulch is thickly applied to the base and behind the stepped terraces, and when this is rotted, a range of plants are grown.


From base to crown, some such sequence as follows is appropriate:

1. BASE (damp, mulch) watercress, parsley, chives, Brassica, taro, kangkong, and salad greens generally.

2. FIRST TERRACE (18-25cm above base) tomatoes, peppers, taro, sweet corn, beans, peas or taller crop.

3. SECOND TERRACE (25-60cm above base). Banana and papaya, sweet potato and cassava (all provide mulch).

4. THIRD, OR HIGHEST, TERRACE Cassava, sweet potato, banana, dry-tolerant vegetable crop and mulch trees such as Leucaena, Glyricidia, Tipuana tipu, Moringa oleifera and local tree and shrub legumes to provide leaf mulch and partial shade; palms for frond mulch and high shade and vines to climb on  these  (passion fruit, winged bean).

Secondly, under some light palm-legume canopies, boxes of palm logs will hold thick mulch and household waste for surface gardens.

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These can be planted as beds of potato, yam, sweet potato and normal vegetable crop. The thicker the mulch, the less watering needed.

Thus, log boxes, pits, thick mulch and high shade canopy are the essentials for good atoll gardens. Staples are coconut, root crop, fruits, and normal salad vegetables plus fish and shellfish from the lagoons or coasts.



Access to atolls is traditionally by boats in reef gaps and today by light planes.

When blowing a gap for a reef entry, or clearing a landing strip for a plane, great care must be taken not to open a wave or wind gap to gales, or any atoll can literally wash away.

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Thus, reef entries are cut on the slant through the reef at the east or west quarters (winds blow southeast to northwest south of the equator, northeast to southwest north or the equator). In fact, reef gaps should be in the most sheltered sector of the reef in any winds and also just wide enough (6-10m) to admit a vessel or barge.

Airstrips are also aligned about 20° off prevailing winds and both ends and sides should be of tall palms and trees, especially those borders on the coast, so that light planes drop in, using their rudders to straighten up below tree crown level.

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Airstrips carelessly made have destroyed whole islands when hurricane winds have cut them in two following the line of the air strip.

For the same reasons, the sandy coasts of all atolls and cays need a sequence of perennial shelterbelt to hold the shoreline against hurricanes.

This starts on the beach as convolvulus (Ipomoea pescaprae) and beach pea, rise on the beach berm to a dense shrubbery of vines, Tournefortia, Scaevola and in sheltered bays mangroves and is backed by a 5-6 tree deep layer of coconut palm, Casuarina, Coccifera, Barringtonia and other hardy beach trees (Figure 10.51).

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It is behind this dense frontline windbreak that we site houses, gardens and productive trees, which will produce in shelter but not as exposed systems.



People who live on small islands and indeed small traditional villages generally, may exist eating a very few starchy root foods plus banana, with fish for protein.

It is quite probable that mineral deficiencies and low vitamin/high carbohydrate diets can impair health.

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Thus, a well-mulched pit garden and a well­ selected introduction of tree fruits (guava, citrus generally, vine fruits, and a polyculture of minor fruits and nuts) greatly extends and buffers the diet.

The addition of (in particular) zinc and iron to mulched soils and periodic tests of leaf content of such minerals serves to eliminate problems due to restricted diets and highly alkaline soils.

Even on high islands, soils can be devoid of, or have, very limited mineral rocks and soils may need trace elements.

Almost every island group has unique plant and animal species, some of great value directly, others of value in that they exist and demonstrate new forms and behaviors.

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Such groups as land crabs (derived from ghost crabs, shore crabs, and hermit crabs) do special work as mulch shredders, scavengers, larval insect eaters and may form a valued food resource.

Giant tortoises are also excellent scavengers of fallen fruits and keep grasses below palms neatly-trimmed, while putting on a considerable annual growth.

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Marine iguanas, giant lizards (the Komodo dragon), flightless or specialized birds and rare plant and animal survivors of older land masses are not only common but usual on islands.

All need careful preservation and assessment for their special values and many provide useful functions in polyculture.

At low tide (even the usual tides of about 1m variation) atolls may almost double their “dry” area.

It is thus possible to modify lagoon and reef for better conservation and feeding of economically useful fish, shellfish, and marine plants such as mangroves, or to consolidate and protect shorelines with coral-block breakwaters.

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There are many such marine impoundments throughout the Polynesian world and new marine breeding techniques are bringing into cultivation such species as trochus, turtles, many inshore mullet, milkfish and edible seaweeds.

Because of the frequent internal (atoll) or external (annular reef) lagoons in or around low Islands, designers and residents have extensive quiet waters in which to try a wide variety of productive mari-cultures, shelters for fish, breeding places and undersea constructs generally.

The daily flux of tides through reef outlets brings a regular fish movement well-known to indigenous people.



Islands in oceanic energy flows behave very much like “bluff bodies” in streams.

Tide, waves generated by winds, winds and the water crashing over low reef areas and flowing out of constricted reef inlets present good opportunities for energy generation locally.

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Reliable biogas technologies now widely used in Asia and the less reliable wind-electric systems can also be used for energy generation.

Solid fuels from coconut husks, fast-grown coppicing legumes, fronds of palms and Casuarina stands are always available on well-planned islands.

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Climates are usually mild and the main fuel needs (for cooking) can be much reduced by a vegetable-fruit garden development.

One area all atolls and islands can develop is that of tide-flow turbines (these can be propellers or egg­ beater”catenary-curve vertical axis turbines).

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Coral and cement provide strong anchors for such turbines at reef outlets. Both tidal and ocean current flow provides dense energy power at 1kW/square meter/second of flow, so that a few such turbines at selected high-flow sites can provide either electrical or pneumatic power for Island workshops and lights.

The above outline should assist island design; but one factor that we cannot design for is that of rising sea levels.

Many of today’s atolls will simply be over-topped or washed away by a very modest rise in sea levels, which is expected to occur over the next few decades.

For these sites, early evacuation is the best action!

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