Tropical Soils in Permaculture

Permaculture Designers Manual




Section 10.3 –

Tropical Soils in Permaculture

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Special problems arise with tropical soils, in that except in areas of recent volcanism such as Indonesia, soils are old (not renewed by glaciations) and deeply leached.

Most of the silica and calcium is in low supply, in clays, aluminum ions substitute for some silica ions, giving soil particles a net negative charge. (Figure 10.2)

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Especially in the oxalic kaolinitic soils (common in weathered volcanic soils) only kaolin clays and oxides of iron-aluminum remain.

In these soils, the charge or cation exchange capacity (CEC) of the soils is affected by pH.

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Once cleared, the humus particles leach out to about 30% of prior levels and infertility appears in such crops as banana and sugar cane.

There are a few ways to restore the soil’s ability to hold nutrients (Wayne Ralph, “Managing Some Tropical Soils” in Rural Research No. 117, pp. 15-16):

Restore humus with green crop and especially perennials such as Leucaena and tree legumes generally any cultivation loses humus as carbon dioxide, so try to grow plants with intercrop.

Now, add small quantities of superphosphate at frequent intervals so that plants can take it up before leaching.

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If possible, add fine crushed basalt, a scatter of cement powder and use shredded bamboo or cane mulches for silica and calcium.

Increase pH with lime after trees and green crop are growing well.

Whatever is added or available as fertilizer, give as a light spread all year at 6 week intervals, until plants are grown.

If at all possible, substitute perennial for annual crop and never practice frequent cultivation.

Basalt, cement powder, coral and bamboo mulch supply essential nutrients and increase soil pH, hence increase the negative charge on soil particles and their ability to hold calcium, sodium, phosphates against leaching.

On coral cays, the calcium-rich sands bind to phosphate to form insoluble calcium tri-phosphate, so that a sort of cement (plantin, or calcrete) forms.

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This may be naturally evolved from the guano of seabirds, but superphosphate rapidly forms the plantin by its greater solubility.

In calcium rich tropical soils, fine rock phosphate yields more slowly and is therefore more likely to provide long-term benefits.

A return of crop wastes as mulch is also essential, which can reduce pH in coral sands (pH 8-9) to a level nearer to pH 6.5 or 7, which is suitable for gardens.

In fresh volcanic areas or areas with volcanic dust deposits, soils are sufficiently rich to sustain intensive agriculture without such aids, but constant cropping will exhaust even these soils.

There are many excellent tropical soils such as the alkaline volcanic soils of Indonesia, which support rich terrace and palm polyculture systems and many tropical high-island soils where dolomite tops or forms a mosaic with, recent volcanics.

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Apart from testing for minor elements, the addition of a mulch-manure mix creates excellent gardens on such soils and plants show only minor nitrogen deficiencies.

These deficiencies can be eliminated by legume intercrop and manures.

In the long term we must rely on tree and ground legumes to keep up soil health in the tropics.

Destructive approaches (now very well demonstrated) combine forest clearing, bare-soil cropping and careless water run-off management to make desolate baked days, brick-like and hostile, out of once-rich tropical forests.

We have (as yet) no categories of “crimes against nature“, but these will prove, in the future, to be some of the worst.

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In soils over rotted granites, such as are found on the high islands of the Indian Ocean, the Ocean in India and where granites are left as inselbergs (domed  hills), a peculiar problem arises in that open, coarse, granitic sands, often very deep, will not retain mulch beyond one growing season.


There are two approaches to these free-draining and low-nutrient soils:

Broad scale

Palms, Albizzia spp., Inga spp., Acacia spp., a general planting of adapted leguminous trees and other native vegetation will establish a light canopy of leaves if small amounts of nutrients are added at regular intervals.

Palms planted in mulch-filled pits will then establish a high crown cover and the detritus from legumes can be used to establish more valuable fruit trees, always retaining a complete root web of legumes.

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Palm trunks are ideal trellis for vanilla and passion fruit crop.

Niches and clefts in the granite mass itself will hold pockets of soil and mulch for valuable trees and vines will establish there to cover the granite slabs, which maintain heat and ripen crop effectively. (Figure 10.3)

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It is the mycelia web of the pioneer legume roots which enables us to maintain the benefit of applied nutrients, to reduce water use and to establish fruiting trees (mango, cashew, pomegranate, persimmon, citrus, tamarind, lychee, custard apple and avocado) in such sparse and drought-prone soils.

Drainage is, of course, excellent and deep-rooting trees thrive. Palms suited to these sites are date, coconut, doum and Borassus palms.



It is effective to excavate long trenches in the loose sand (1-2m wide, 1-1.5m deep), to lay in a sheet-plastic base (upturned at one end only) or to line the trench thickly with cardboard, paper, carpet and leaf and then to backfill with sandy loam.

The deep sealed layer holds water and leached mulch and household waste water can be led into these trenches to provide root water and nutrients. (Figure 10.4)

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Otherwise, we can build log-boxes above ground level, carpet the box base with plastic or thick paper and fill with humus-sand mixes for green crop and vegetables, top-mulching as needed.

Large domestic water tanks, gleyed ponds, solid granite dams and underground plastic-lined cisterns back-filled with sand will hold water.



Termites, ants and some worms are the obvious soil mesofauna of many arid and humid subtropical areas.

Both ants and termites are very active in the transport of rotted rock and subsoil to the surface, in opening up galleries for the infiltration of water, and in the breakdown of woody and leafy plant material.

Some species create large mounds, others build underground compost heaps for fungal culture and all are active burrowers and builders.

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Termites may have a decisive role in the dynamic and delicate balance between the erosion of surface soil and the replacement of the soil by subsoil and rotted rock particles.

They certainly have an important role in plant succession and distribution in savannah areas or where termite and ant mounds are the only well drained or elevated sites in a landscape subject to floods or when impermeable clays underlie thin peats (usually with acid anaerobic soils).

In these situations the spoil heaps present an ideal site for pioneer vegetation or adapted crop planting.

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Harris (1971) records that both the leaf-cutter ant in South America and termite mounds in Uganda assists forest spread and establishes islands or taller vegetation in grasslands on their mounds and colonies.

I have also observed this in the granite country in Hyderabad, India and on acid peat lands in Tasmania. Such mounds protect soils from fire, water logging and poor aeration.

In humid areas, some such sequence as tall grass (Pennisetum, Eragrostis) are followed by shrubs such as castor oil bean, Prosopis and thorny legumes.

Finally, an understory and forest may develop from larger trees such as tamarind, Vitax, Sapium or palms dominant.

Thus, we can start this process or a modest version of it by seeding into termite or ant mounds using similar species; even low ant-heaps may present a site for ground cover pioneers in grasslands.

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Harris records crops such as sisal, cotton and tobacco deliberately cultivated on large mounds in grasslands.

Palms and coffee can have much of their outer bark removed by termites without suffering loss of production.

Termites may greatly assist the primary breakdown of logs, coarse stems and hard leaf material used as mulch in plantations of coffee, tea or bananas.

It is a matter of specifying (by observation and local report) which useful crops or trees are left alone on mounds, which are attacked but remain productive and which actually benefit by association with a local termite or ant species.

Planting in ant or termite mounds is a particular example of niche gardening widely applicable to the tropics.

I have successfully germinated daikon radish in ant heaps in grasslands as part of a changeover to crop production.

Ant and termite mounds present a rich deposit of calcium and potash, better aeration of soil and a faster infiltration of water to release minerals from such rocks as granites, which noticeably rot or erode faster when buried in a free-infiltration soil environment.



As humus in the soil provides a good CEC (Cation Exchange Capacity), we must look to the provision of such humus as a priority.

Some sources are:

Logs and branches of trees. These are sometimes rotted in wet terraces or piled up as rough mulch around new tree plantings. Palm wastes are often plentiful;

Detritus from stands of bamboo, pines Casuarinas;

Aquatic weed mats and emergent water weeds;

Crop wastes and manures, household wastes;

Hedgerow, forage, and specially-grown mulch plants;

Green mulch and ground cover.


Logs and Branches (Rough Mulch)

The rapid breakdown of wood under the combined influence of rain, heat, termites and fungi means that we can lever whole logs together or in line cross-slope to act as planting sites.

This technique is most useful on bare clay soils, eroded areas and isolated atolls (using the trunks of old palm trees).

In Hawaii, a traditional strategy is to rot the logs of the kukui tree (Aleurites moluccensis) in the shallow water of taro terraces.

As logs rot, an edible fungus appears which is taken off as crop. The remaining log is then crushed and spread in the taro terrace. Leaves and branches of kukui and other forest trees were gathered for the same purpose (terrace mulch).

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Marjorie Spears, in Queensland, Australia, has successfully built temporary rough wood terraces across a deforested slope using rejected logs and created a complex and rich garden based on this strategy and green legume mulch.

Logs are available from palms, although fast-growing acacia species can be previously close-planted for this purpose. (Figure 10.5)

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Several plant species (most palms, bamboo thickets, Casuarinas and many Acacia species) provide silica-rich mulch, as do grain and nut husks and residues from copra operations.

This can be applied as shredded or chopped mulch to crop or to the base of newly-planted trees.

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The silica is released for growth, and has the secondary effect, in alkaline island soils, of reducing pH (from 8.5 to 6.5 in my trials on coral islands).

Of particular value are the fronds and spathes of palms, shredded or whole and the stems and spathes of bamboo.

Both have essential structural uses and larger stands can be used to produce mulch.


Aquatic Weeds

Floating aquatics, including the water-fern Azolla (several species), the water-lettuce (Pistia), water hyacinth (Eichornia) and algal mats or fern fronds, plus reeds and rushes gathered from ditches, are excellent crop mulch.

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Azolla has largely replaced kukui (Aleurites) as taro mulch in Hawaii. Pistia has been successfully used in Africa and water hyacinth in many areas.

Azolla and algae such as Anabaena (one species of which “nests” in the glutinous sacs in Azolla) provide nitrogen.

In the dry wet tropics, shallow flood-water bunds collect or produce these plants, which can be gathered as rolls of dry material when the water dries out in the winter period, thus providing garden mulch in abundance from temporarily impounded water.

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John Selman of Cooktown (Australia) has used algal rolls in this way for his garden plants.


Crop Wastes and Manures

The husks of corn, kitchen wastes (including bones) and human and animal manures are invaluable tropical garden mulches and nutrients and most need only burial near tree roots or in growing mounds for safe disposal.

Cardboard and newspaper, where available, are valuable grass suppressing weed mulches and a cover of nut husks completes the job.

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In the Seychelles, cinnamon leaves and branches from pollarded stumps are considered excellent mulch for vegetable crop and the bark is a valuable spice.


Hedgerow and Mulch Plants

All hedgerows (Hibiscus, Casuarinas, banna grass, palms, and leguminous trees such as Gliricidia, Acacia and Prosopis) are almost continual mulch sources.

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The legumes provide in-crop shelter (see later section on avenue cropping).

Lower garden wind break, especially lemongrass (Cymbopogon citratus) and comfrey (Symphytum officinale) are as useful in preventing kikuyu grass intrusion as they are for repetitive cutting for mulch in the vegetable garden.

Many people now use both these species as a combined kikuyu barrier and mulch crop. (Figure 10.6)

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Green Manure and Ground Covers

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In and around gardens and soft herbaceous plants such as nasturtium, comfrey, marigolds, tobacco plants and the tops of mature taro plants and other Araceae not only suppress grass, but provide a constant source of “slash” mulch.

Even more valuable are such soft legumes as Sesbania, Vetch, Haifa clover, Cowpea, Lablab Bean, Soya Bean, Desmodium, Suratro and Centrosema.

These can be slashed or (in wet-dry tropics) interplanted with grains to give a nitrogenous ground cover, aiding in the suppression of grasses.

Lablab dies down just before grains ripen in the winter dry season.

Thus, a combination of growing and gathering mulch enables us to create rich humus for gardens over clays or sands, in loose volcanic cinder, on lava and in loose coral atoll sands.

Each of these situations can successfully produce mulch. (Figure 10.7)

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Some difficult mulch such as hibiscus, Lantana and weeds which tend to re-sprout from cuttings or seed if mulched (several grasses and hedge species) can be routed  to gardens via poultry or cattle pens (where seeds are removed and foliage eaten).

They can also be shredded for anaerobic digestion in biogas plants; bagged in large plastic bales exposed to the sun (where they “cook” to a weed-free silage); or simply bundled and immersed to rot in covered water pits.

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In fact, some such re-routing is ideal for the primary processing of plant wastes that promise to infest gardens if untreated.

Pigs eliminate or eat the nut-grasses, rhizomes, bulbs and sedges that re-sprout from compost.

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All else failing, even plastic sheet mulch has excellent effects on row crop, preventing rain splash and nutrient leaching, and at the same time condensing groundwater at night.

It does not, however, add to the humus content of soils, nor to the cation capacity of soil structure and may even release unwanted chemicals to the soil.

The value of surface mulch in weed suppression is a major factor in lowering garden work.

For this reason, any mulch should be thickly applied 20-25cm (8-20 inches) deep when first establishing home gardens.

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Later mulch can be derived from green herbage and borders or windbreaks.

B. N. Okigbo and R. Lal in Residue Mulches and Agrisilviculture (International Conference on Ecological Agriculture, Montreal, 1978), in mapping strategies to cope with increasing land pressures, found that no-tillage systems maintained or gained yields for maize in Nigeria and increased yields from mulched crop for cowpea, soya bean and cassava.

I have selected out some natural mulches from the more extensive original table given. Mulch trials are compared with bare ground (on the last line of Table 10.1)

Maize had a marked positive response to legume straws or water plant (Pistia) mulch, while the legumes themselves responded well to grass and sawdust mulch and cassava to both legume and grain husk wastes.

Although plastic mulch has a good effect on all crops, it does not add humus to soils and is therefore not as appropriate to a remote village situation where soils must be built up from wastes and from mulch.

However, every type of organic mulch increases yields and we should therefore use all available materials for soil restitution.

Mulch provision is the cornerstone of tropical home gardens and green mulch and tree legumes the essential accompaniment of main crops and tree crops.

Special mulches may be used in tropical areas, grown to provide N, P, K (legumes, comfrey, Pultanea) and to increase or decrease pH.

Pine and legume mulch may benefit the growth of bromeliads (pH 4-5), buckwheat and nut husks serve to raise the pH of garden soils, as do many bark mulches.

For fire control, too, it pays to rake under bamboo and dump canes and re-route the leaf mulch through animal bedding or poultry straw yards.

Branches of legumes and forage trees may also be used in the same way on their path to the garden.

In the rampant grasslands that replace fallen forests, there is little else we can do than to strip-mow and mulch while trees re-establish.

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The timing of slashing is important, as seed-free mulch (called second-cut grass) is best for placing around valuable crop.

Seed-head mulch should not be placed in gardens or areas where it might be a nuisance. All species of weeds and grasses give weed free mulch when not in seed.

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Many useful tree species provide leaf mulch and so are excellent also for inter-planting with crop, for example tamarisk in dry areas, Casuarinas in sand and legumes in all areas.

There is absolutely no excuse for burning any organic wastes in the tropics, as even large logs quickly rot under the onslaught of fungi, termites and beetle larvae.

At the same time, logs provide cross-slope barriers against monsoon erosion, until new trees take hold.

Coconut husks have a variety of uses, not the least of which is as mulch for a valued crop such as vanilla orchids.

Their one drawback is that they hold small sections of water which will breed mosquitoes, but on many islands they will also be available (with palm fronds) to shred to a first-class mulch of high potash value, to burn and steam to activated (filter) charcoal, or to be used as a solid fuel.

Shredded bark and broken shells are ideal mulches for ginger, turmeric and vines.

I have not found any crop or tree suited to the specific locality that does not grow, produce and thrive in mulch, nor any widespread pest that grossly affects a total polyculture yield.

Ginger, taro, beans, bananas, palms, fruit trees, flowers, yams, sweet potato, melons, etc…, have been trialed in thick mulches of straw, fronds, nut husks, cardboard, and sawdust.

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Thick mulch almost totally eradicates kikuyu grass and other persistent grasses.

In the field situation, extensive mulching is often impractical, particularly if it is carted in from off the site.

However, a pioneer crop of quick-growing tree Acacias, bananas, legumes such as lablab, deep-rooting comfrey and a grove of bamboo and palms will provide continuous mulch for gardens and main crops, fruit trees and valued plants.

Growing in exhausted or poor tropical soils is possible, but the early work of rehabilitation takes hard work, seed, essential fertilizer resources, and a strategy or starting small and expanding the system at the periphery.

Dense planting or nucleus areas plus mulch is the key strategy.

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