Soil Composition in Permaculture

Permaculture Designers Manual



Section 8.9 –

Soil Composition in Permaculture

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We think of soils as mineral compounds, but the pie diagram in Table 8.6 will alter that impression; it does not, of course, represent any one soil, but is an average sort of figure for a good loam with adequate humus.

Outside the pie, I have noted some of the possible ranges of variation from peats to sands in old dunes.

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Soils in nature can vary from a humus content of 2% to close to 100% (as peats). In gardens, 40% or more humus helps block heavy metal uptake by plants, holding heavy meta s bound in colloids.

Many compost-fed or mulched garden soils contain 10-30% humus (some much more). The effect of adequate soil humus is both physical, in effecting good water retention and in preventing erosion, and chemical via colloid formation.

The breakdown products of humus, including the mineral content of the donor plants, form the readily available and biologically active components of soil that are of value to newly­ established plants.

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If these plants are perennial or (in part) mulch-producing, and if we return food wastes to the garden (including urine and well-composted faeces) then very little humus loss occurs.

On the broad scale, humus can only be provided by the root and the above-ground mass of grasses, trees, and plants. Prairie grasses and broadleaf trees are particularly effective at this job.

When we aerate (plough) soils we turn up humus and oxidise it to carbon dioxide, thence to atmosphere. It is lost. Burning the vegetation is worse, with a host of additional pollutants (terpenes, creosotes, nitrogen, and dust particles), and a rapid loss of soil humus.

Where soils are not tilled or burned, soil humus lasts a long time (hundreds or even thousands of years) and provides for a complex soil life.



Mulching is here defined as covering the soil surface with 15 cm or more or organic material, as a loose (uncompacted) mulch; 8 cm of tight-rolled sawdust does not qualify!

Mulching is more generally applied to loose dust “mulches“, plastic sheet mulches, and so on, and these may have specific local value in soil amendment, heating, sterilization, weed suppression, or pest reduction, but as here considered, the object of mulching is to add plant nutrients, buffer soil temperatures, prevent erosion, promote soil life, and restore soil structure.

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Plastic mulches, soil gels (polyacrylamides), herbicide treated soils, and organic or natural mulches may all achieve the result of preventing erosion and helping soil crumb structure develop.

Only long periods of natural mulches stabilize nutrient supply, and complex the soil life. None can be judged over one or two seasons, as it can take 3-5 years to create a balanced soil under mulch from a compacted or mined-out soil.

Even longer periods are necessary to develop humus in permanent crops assessed for yield on the broad scale, where added mulch is not carried to the site, but derived from tree wastes and specially-sown crop (green manures) produced on the site itself.

Used in areas such as wet tropics and arid lands, or on dry coarse sands, mulches may prove to be ephemeral (even if their effects continue), as ants, termites, and leaching reduce the mulch to humic acids or underground storages in fungi and bacteria.

In particular, water absorption is improved under mulch, both as field crop mulch and imported garden mulch, thus water needs are reduced. Jeanette Conachcer, in Western Australia’s Organic Gardening, 1979, reporting on extension trials in Nigeria, records II% better water infiltration on low- to no-tillage and mulched plots.

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Under mulch, excessive soil temperature ranges are buffered, being cooler by day and warmer at night or in winter. Seed germination is enhanced, and over the long term, major nutrients (N. P. K.) remain at satisfactory levels.

Only under mulch does the population of important soil organisms, such as earthworms, increase.

Mulches need some selection for minimal weed seed, minimal residual biocides, and for best effect on specific crops (tested as row-by-row comparisons).

Plastic mulches (black for heat and weed control, sliver for aphid repellency) have a more limited role, important in the short-term, but often expensive or impractical in poor countries, or rejected by growers who suspect that many plastics release persistent chemical polymers of unknown effect on the soil life.

Mulch is an excellent way to add nutrients to soils; the “cool” decay loses little nitrogen, while stimulating soil life generally.

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There are some problems with compost, where “hot” (aerobic) heaps heat up and nitrogen (ammonia) losses are severe. Cold heaps (pitted or silage) do not lose nitrogen, but neither do they kill weed seeds.

One percent of superphosphate added to a hot compost heap prevents ammonia escape. Chinese scientists get the best of both worlds by first building an aerated heap with bamboo poles as holes to create air tunnels.

This is then covered with mud and the heap heats up to 55-60 C (130-140 F) for a few days. Then all notes are sealed, and the rest of the decay is anaerobic.

With sealed boxes, either hot or cold processes can take place.

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Compost or mulch is critical to preserving soil crumb structure, buffering pH, and (in taste tests) improving sugar content and the flavor of the vegetable product.

The gums and gels produced by soil organisms create crumb structure, aerate the soil, and darken it so that it heats up faster in spring. The humic acids assist root development dramatically even at levels of 60 ppm carbon.

Inorganic (chemical} and mechanical farming can as easily destroy soil structure.

In the U.K. (New cientist  3 Nov. 79), liquid manures sprinkled on straw in silos or tanks, together with a forced air draught, produce compost in about a week (efficient open piles encased in straw need 10 days).

The liquid effluent system plus straw is suited to treatment of a manurial sludge (still full of seeds) such as that we get from biogas digesters.

It is best to use this with dry twiggy or straw material as hot compost to both kill weed seeds and to produce useful heat, after the compost pile has been made.

However, in severe winter areas, such “efficiency” is counter-productive as a slower heat release from large compost piles of I0-50 cubic met (12-59 cubic yards) can provide heat over a long period in winter and greatly reduce glasshouse and house heating costs, while the compost itself is best applied to soil in spring.

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Even using organic residues, or “natural” wastes, soil problems can arise from a concentration of nitrates in manures, or toxic mineral residues.

Leaf material cut and mulched for composted green will contain more nutrients than fallen leaves, although the latter are still useful for humus production.


Kevin Handreck (Organic Crowing, Autumn ’87, Australia)  has identified potential mineral contamination from these sources:

Galvanized, copper, or brass containers containing wet residues of manures, or manurial teas; clay, stainless steel, glass, or iron are preferable.

Zinc and copper are produced in excess from galvanized or copper / brass containers.

Manures from pigs, poultry, cattle, or sewage sludges can add excessive copper, zinc, nickel, boron, lead, or cadmium to soils (and especially acid sandy soils).

Where such elements are naturally deficient, such manures may initially help, but heavy or constant application will build up a toxic soil condition.

Excess ziinc can be built up by using earthworm casts from contaminated pig, sheep, or domestic wastes.

Animals penned in galvanized areas can produce excess zinc in pen wastes.

It is in poultry mixes that such contaminants show up, with excess zinc inhibiting plant growth and health, so that the urban gardener should test and use safe mixes, or proceed via plant trials.

Handreck recommends a limit of 10% worm casts in a potting mix. In such cases, nitrogen and potash can be supplied by dilute urine.

Note that copper, arsenic, and other minerals are commonly added to stock feeds, and therefore try to buy dean natural feeds from organic sources.

After initial soil treatment, and a continuing watch for foliage deficiency symptoms (see Deficiency Key, this Unit), the safest course is to grow our own green manures as the foliage of leguminous hedgerow, windbreak or intercrop, and to use these as mulch and compost materials.

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