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My Parents are currently living on a small pacific coral atoll. Coral atolls, for the most part don't have much soil. Mostly they just have coral sands. These "soils" are very well drained (and they get hot during the day), and highly alkaline, with very little carbon or nitrogen. There's some salinity too, but even just 10 meters back from the coast, the water table is fresh enough to drink. Getting anything to grow other than native plants is pretty difficult, although my mum has succeeded with some things, including egg plants, tomatoes, and pumpkins.

Does anyone have any recommendations for quick improvement of such soils? Compost is the obvious one, but the source material is somewhat limited (by how much we eat). Mum is also mulching with leaves of a native that grows pretty fast. I'm also going to try making some charcoal, although the answers here are somewhat discouraging. Are there any other things we could try?

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You could try seaweed which has been used as a soil additive. Just chop it up and wash it with freshwater or compost it before using.

Depending on where you are you should find varieties of palm trees. The fibre found between the hard outer shell and the inner nut is also used as a soil additive.

Animal manure from goats, pigs, sheep or chickens is also used once it has been composted.

Really it's all about organic matter. Increase that enough and you should have success.

I should also note that the Polynesians colonized the Pacific from 1800 BC to ~700 AD. They brought with them or spread the taro plant, breadfruit tree and possibly the sweet potato. These plants should grow well in the garden.

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  • Using fresh water to wash sea weed is not viable on most atolls. For the most part, fresh water comes from rainfall and only minimally provides what is needed to sustain human life. – user3024 Jan 6 '14 at 21:41
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What you need is a fast growing, fast spreading ground cover which is suitable to your environment.

A suitable ground cover has the following benefits:

  • Lock in moisture and nutrients at ground level
  • Provide habitat for beneficial insects (think moist and decomposing versus hot dry and sandy, bugs are going to prefer to stay underground if there is little or no cover)
  • Provide shade, moisture and protection for new plants while they establish
  • Can work to cover a large area with little human intervention to act as a kind of place holder until you are ready to utilise the space

This permaculture video (watch from the 27 minute mark) shows how they are using a type of weed (Singapore Daisy) as a rampant ground cover to do the things I mentioned above. Given it is a different environment and I am not recommending you use Singapore Daisy, but it shows you how you should use a resilient, fast growing ground cover to condition and protect the soil while plants develop.

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If possible, use raised beds, or lined beds, in part to keep the alkalinity out. Composted medium, if there are lots of coconut trees; find dead thoroughly rotted ones that can be added to the mix. Compost is everything; with this method you may need to lower the pH as the pH is a little higher with composted material.

In the atolls I have made gardens for 37 years. I am looking forward to returning to some new life sustaining killer gardens and growing fruit trees.

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The way to improve sandy soil is to add clay. You can substitute organics and make it work for a time, but you're always going to be adding organics and struggling to keep up with the constant decay. Compost and pretty much anything plant-like is mostly cellulose (C6 H10 O5), which decays to CO2 and H2O. What few minerals that are contained in plant-life will just wash away with the rains since there is nothing for the minerals to adsorb to in the sand. The decay is fast in sand because of the high amount of oxygen that can reach the organic matter contained in sand. The amount of humus left is fairly small compared with the giant pile of compost you started with... and then, what holds the humus in the sand against the rains trying to leach it away?

Biochar is good, but consider how much of that you have to make. Then you have to load it with minerals. Biochar is an amendment to soil that is already decent.

Plants need Ca, Mg, K, HPO4, NO3, NH4, SO4, B, Fe, Si, Cl, Mn, Zn, Cu, MoO4, Ni, Se, Na, and air, and water. All of these elements need to be in the elemental form, atoms (except HPO4, NO3, NH4,SO4, and MoO4, which are simple compounds). Clay has a surface area 17,600 times more than silt and 400,000 times more than sand. Source. It's that surface area that retains water (water clings to surfaces) and supplies lots of places for ions to stick.

An ideal soil is composed of 45 percent mineral (sand, silt and clay), five percent organic material (humus or plant debris and soil organisms), 25 percent water and 25 percent air.

Source

Of the 45% that is sand, silt, and clay, the mix should be 40% sand, 40% silt, 20% clay.

A must read: It describes everything you want to know about soil and more.

The most influential factor in stabilizing soil fertility are the soil colloidal particles, clay and humus, which behave as repositories of nutrients and moisture and act to buffer the variations of soil solution ions and moisture.

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A colloid is a small, insoluble, nondiffusible particle larger than a molecule but small enough to remain suspended in a fluid medium without settling. Most soils contain organic colloidal particles called humus as well as the inorganic colloidal particles of clays. The very high specific surface area of colloids and their net negative charges, gives soil its great ability to hold and release cations in what is referred to as cation exchange

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Humus is the penultimate state of decomposition of organic matter; while it may linger for a thousand years, on the larger scale of the age of the other soil components, it is temporary. It is composed of the very stable lignins (30%) and complex sugars (polyuronides, 30%), proteins (30%), waxes, and fats. Its chemical assay is 60% carbon, 5% nitrogen, some oxygen and the remainder hydrogen, sulfur, and phosphorus. On a dry weight basis, the CEC of humus is many times greater than that of clay.

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Most of the soil's CEC occurs on clay and humus colloids, and the lack of those in hot, humid, wet climates, due to leaching and decomposition respectively, explains the relative sterility of tropical soils. Live plant roots also have some CEC.

If I found myself on a tropical island wanting to grow vegetables, I would dig down below the sand in search of clay to add to my garden soil. I would search for hardwood trees which manufacture protein and have lots of lignin to make humus. Of course, I would take what wood I could find, chip it, and add it to a compost pile and the garden soil. If I found grass growing anywhere on the island, I would harvest that soil for my garden (grass is high in protein and takes good soil to grow it). I would construct something to limit rainfall to 1/2 inch a week (0.1 inches per day) on the soil. Any more than 0.1 - 0.2 inches per day will result in leached nutrients, destroying all my hard work. I would gather any source of calcium I could find... seashells, bones, fish (calcium is utmost importance). Anything green (leaves, seaweed, etc) goes into the compost pile and garden soil as a source of nitrogen. I would make biochar and add it to my compost pile first, then to my garden soil. Any ash from any fires would be gathered into the garden soil (ash from hardwood trees is 30% Ca, 10-15% K, 7% Mg. Source.

I think it can be done. The first step is knowing exactly what you need to do and why. The rest is just a matter of doing it.

It might be easier to buy clay and water soluble fertilizer to supply all the nutrients in proper ratios for each specific plant. Calcium, magnesium, and potassium will displace sodium.

Al3+ replaces H+ replaces Ca2+ replaces Mg2+ replaces K+ same as NH4+ replaces Na+

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    +1 for the chemical details, but clay's not really an option. There's no clay below the sand on coral atolls, just more sand, then old coral, and then volcanic rock (usually tens or hundreds of meters down). Importing clay for gardening isn't really economically feasible. There is some lagoon silt mud available, which I guess would have better surface area than the sand, so it might be worth trying that... – naught101 Jul 16 '13 at 9:08
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    If it will stay suspended in water (ie mud), it could work. If you put it in a jar and add water, then shake the jar and let it stand, the longer the water remains cloudy, the better the soil. – Randy Jul 17 '13 at 3:57
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Normally, atoll sandy sand is seriously deficiency in 3 mineral, iron (Fe), manganese (Mn) and zinc (Zn). These 3 mineral is the most wanted. Chemical fertilizer is the best way, buy these 3 mineral and use as foliar fertilizer.

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Just a small thought to add, most sands on tropical atolls are derived from coral, as such the last thing the sand needs for plant to grow in it is more alkaline material. I find a mix of sand with compost 50x50 and used in containers will grow tomatoes,eggplant, and peppers with the help of some magnesium sulphate and seaweed tea. .

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