I apologise for the length of this post, but it does fit onto a couple of pages of A4, as promised, and contains all of the essentials.
Fundamentals
Rule 0: Avoid digging. If you do have to dig (say, to plant a tree or create a terrace) be aware that doing so damages the soil and that damage must be repaired.
Rule 1: The soil is alive: never leave it unprotected. Cover every square inch with (in order of preference) living plants, dead plants or compost, scrap paper or cardboard.
Rule 2: Do not kill things unless you are absolutely certain what you are doing. If in doubt, leave it alone.
Rule 3: Value diversity. Grow many different plants, and mix them up. Don’t be afraid of native species (plants and animals) that appear spontaneously among your crops.
These four rules are the basis of natural agriculture. Everything else you do is derived from these. They apply – I am confident – anywhere on the planet. Because these rules are so simple, it follows that there are many different ways of practicing “natural” agriculture – unlike the conventional method, which is done by-the-book with little or no wiggle room. The following assertions follow directly from 0-3:
Rule 0a: Where soil is compacted, plant pioneers and mulch well. Leave the weeds alone. Logs and branches, buried under organic trash and soil, help a great deal – they encourage scavengers/burrowers and release nutrients. It may take a couple of years, but natural processes will return the soil to health. On one troublesome section, I planted daikon, let them rot, and followed with a crop of pigeonpea. That area is now in much better condition. Follow Rules 0-3 religiously and you will never need to dig, because the soil will remain friable.
Rule 0b: There are two forgivable exceptions to Rule 0: use a subsoil ripper or similar if the soil is compacted beyond any reasonable hope of planting in it; or if you are cutting terraces (see rule 5). Note that either of these exceptions will only happen once in the lifetime of your land.
Rule 0c: Since you’re not allowed to plant ‘downwards’, plant ‘upwards’ by putting seeds on the surface of the soil/mulch, and cover with a handful of soil and/or compost. The seedling will root down through the mulch, and it’s easier to see where you’ve put seeds. This mimics the natural process while giving your crops an ‘unfair’ advantage over weed seeds below the much layer. Note: this doesn’t work for some larger plants with weak root systems, like maize, or root crops. You may need to dig into the soil to plant these.
Rule 1a: Mulch everything. Many of the remaining rules are dedicated to producing mulch, which is simply cut and left on the soil surface. You do not need to clean or ‘arrange’ anything: branches, twigs, leaves, grass, vegetable tops – just leave it where it falls. Do not remove more than you have to from the land: take the edible parts, and leave the rest to rot in place. Bring whatever you can (peelings etc.) back to the land. To begin with you may need to ‘import’ mulch, but within a couple of years this will become unnecessary. Make compost heaps if you want, but they’re not essential.
Rule 2a: Don’t interfere with things you don’t understand. Events that appear superficially destructive may have some purpose you are not aware of. If you lose half your crop to disease or drought, collect seeds from the survivors and plant them next year; you’re well on the way to producing your own landrace. Low-level intervention is sometimes wise: if you see the beginning of a viral disease or fungus, it’s probably sensible to destroy infected plants. I picked caterpillars from a moringa tree once, because it had been otherwise healthy. Carrots need weeding. There is a particularly aggressive weed (bidens spp., I believe) that needs periodic uprooting when they threaten to overtake other plants or run to seed, but bees love it. In general, weeds are your friend: they protect and nurture more fragile crops. While it’s true that weeds ‘take nutrients away’, there are plenty of nutrients to go around, and your crops will be forced to compete for survival. This is a good thing. When weeds are (eventually) cut down, the nutrients they have mined are given back to your plants. Also, many ‘pests’ like to eat them - I noticed a particular type of wild solanaceae that was attracting bugs away from my edible ones.
Rule 3a: Let different plants grow together and complement each other. This is not really the same thing as ‘companion planting’ – it’s commonly held that certain sorts of plants are particularly compatible, or that certain combinations don’t work. This may well be true – for example, allium pp. does tend to inhibit legumes, and vice versa – but it’s not a useful concept simply because there are so many edible food plants. You can spend far too much time worrying about exactly what matches what. Combining different species with different habits – different root depths, different nutrition preferences, or different light requirements – will inevitably be better than planting a monoculture, if for no other reason than to keep the soil covered. If in doubt, just let the weeds grow (within reason). Native trees that spring up can be pruned heavily (remember to leave the ‘trash’ on the ground) to make trellises for climbing plants like chayote, passionfruit, grapes, and suchlike.
Rule 3b: encourage native wildlife. 90% of insect species are harmless or helpful; even ones that are traditionally thought of as useless or problematic (eg., ants and beetles) become useful in the context of a natural-agriculture system. The other 10% are self-limiting if those other 90% - their predators – are abundant. You will also find that many ‘pests’ enjoy eating weeds just as much as food crops: another good reason to avoid pulling weeds.
Applicability to the Humid Tropics
The above Rules are general recommendations that will work anywhere. The remaining rules are what engineers call ‘good practice’ – things that have been found to work reliably without too much thought required. They are optimized for monsoon climates with hot summers, and may be modified at will to suit different climates.
Rule 4: Use land with contours. The reason is simple: flat land alternately floods and dries in a monsoon climate. It is completely useless for natural agriculture unless carefully structured (see below).
Rule 5: Observe the shape of your land, notice the microclimates, and plan your planting accordingly. You will get this wrong the first time: observe, and correct, but don’t become obsessed with creating uniform conditions across the whole plot. Instead, make use of the differences to grow a wide variety of plants. You will find that you can grow some things out-of-season, which will have higher market value.
Rule 6: Plant deep-rooted grasses in rows on contour lines, about 4m apart. My favourite is vetiver, which is hardy, well-behaved, and has roots that go straight down. Lemongrass is good, but its roots wander outwards and it will seed itself. There is also an excellent native grass in Taiwan that vaguely resembles sugarcane and will root easily from the leaf nodes. The purpose of the grasses is to contain runoff and provide mulch (all of the grasses mentioned can be cut every couple of months). If you have a steep slope, the soil will eventually erode into terraces - you can help this along if you’re in a hurry and you have the manpower, but it’s not essential.
Rule 7: Plant shrubby perennial legumes. I like pigeonpea, sesbania sesban, and leucaena leucocephala. L.leucocephala grows wild – seeds are easy to get. Sesbania needs starting in pots, because bugs seem to like eating young seedlings. Once established, it’s indestructible. In hotter locations, gliricidia sepium may work well. These and related plants (there are literally hundreds of choices) let plenty of light through, so they cast minimal shade on things growing underneath them, but they still provide protection from the elements and fix nitrogen. Many will produce edible fruit/leaves for humans or animals. Cut them back occasionally and leave the branches on the ground.
Rule 8: Plant productive shrubs. I have pomegranate, banana, moringa, Kaffir lime and Szechuan pepper (both of which make good fencing due to thorns), and miracle fruit (synsepalum dulcificum).
Rule 9: Plant large fruit trees. Obviously, these will take several years to mature, but make sure you put them in sensible locations where they will not seriously interfere with other (faster-growing, shorter-lived) plants. Plan to make use of the denser shade they will cast. I have jackfruit, mango, neem, toona sinensis, cinnamon, and tamarind (still small).
Rule 10: Plant a vigorous perennial understory: I have gingers, taro, shallots, garlic, canna lily, arrowroot, plectranthus spp. Those that die back to rhizomes in the winter will leave a natural mulch; larger short-lived, fleshy plants like banana can be cut down like legumes and left to rot on the ground.
Rule 11: plant perennial climbers: I like lablab purpurea and psophocarpus tetragonolobus; these can be allowed to cover the ground (if that’s what you need), or if you let them climb, they will bear edible beans. Basella, passionfruit, grapes, and chayote are also popular in Taiwan. Choose carefully what you let them climb up. It’s fine to let them climb trees, but not younger ones (1-2 years old) – they will die or be pulled over.
Rule 12: plant whatever annual vegetables you like in the spaces, and plant any remaining corners with annual or perennial groundcover. Borage is excellent (comfrey is similar, but it may spread where you don’t want it). Meliolotus (yellow sweet clover) and mimosas can be found everywhere: collect seeds and scatter them around. Aim for no more than 20% of your land area planted with annuals crops; the remainder should be perennials and groundcover. Nevertheless, you can expect an economic yield roughly equivalent to 50% of an equivalent chemical-fed monoculture – that is, if you have 100m2 of 500m2 planted with annuals, your returns (gross profit) will be the same as if you had planted row crops on 250m2, in addition to whatever yield you have from the perennials.
Rule 13: make use of dry/wet areas. I have pineapple and aloe doing well in a location that is hot and dry. Taros are in an area that gets routinely wet; I may add ipomoea aquatica there this year.
Rule 14: make use of light and shade. Papaya, sweet potato, and squashes like full sun. A surprising majority of plants will grow, even thrive, in shade – they grow more slowly, but are far less vulnerable to water shortage. I have coffee and rubus calycinoides (a high mountain species) growing well in deep shade, and had a large crop of roselle nearby. The taro is also in shade and grows huge and lush, although it takes much longer to mature than you would expect. Adequate shade (in conjunction with water-trapping groundcover and mulch) means you never have to irrigate.
Rule 15: when first establishing young plants on open ground, use deep-pipe irrigation to keep plants alive in hot, dry weather. It encourages strong roots and minimizes water requirements.
Rule 16: keep animals, if you have enough space. They consume biomass (plants and insects) that humans can’t eat, they contribute to the fertility of the soil, and of course they produce eggs, meat, or milk. Animals are an integral part of any natural ecosystem. Even if you don’t introduce your own, you should expect and encourage native species like toads and reptiles.