Soil Pores and Crumb Structure in Permaculture

Permaculture Designers Manual



Section 8.10 –

Soil Pores and Crumb Structure in Permaculture

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The structure of soil (whether compact or open) depends on the soil composition itself, the way we use it, and the presence or absence of key focculating or ionic substances (synthetic or natural).

Crumb structure in well-structured soils permit good gaseous exchange and free root water penetration without the creation of excessive anaerobic conditions by waterlogging.

In free sands, and in the kraznozems developed over deeply-weathered basalts, crumb structure is either not a factor, or else is so well developed that it permits leaching (to immobile clay sites) of almost all applied fertilizers.

However, in most other soils, we would like to see a good crumb structure develop as in our gardens or crop soils.

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Where crumb structure is poor, we can use a great variety of coulter and rip-tine machines, soil additives, and deep-rooting plants such as trees or lucerne to re-open and keep open the soil structure, which in tum allows adequate water penetration and drainage, and eventually develops the oxygen­ethylene processes that make bound nutrients available.

Soil crumbs of 0.2-2 mm diameter can form as little as 10% of the total soil volume, and still produce crop; below this, crop is greatly reduced and impoverished. The same soils can, when not ploughed lifeless, contain 92-95% of such crumbs (leeper, 1982).

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We destroy crumb structure by destroying permanent vegetation, flooding soils for long periods, using high-speed or heavy vehicle cultivation, stocking with sheep or cattle in especially wet periods, using fertilizers that deflocculate the soils (e.g. too much potassium where soil sodium levels are already high), or by burning in hot periods.

The whole set of disasters out lined above can collapse soils to the cemented, dusty, hydrophobic, salted and desertified areas typical of wheatlands on desert borders.

It is a question of improper use, disastrous planning (or no planning), and a total lack of applied goodwill to earth.

Given a good structure, pores develop for the diffusion of gases and exchange of ions, provided that we make the transition to perennial, low cultivation systems of forest and crop.

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Colloids are stable aqueous gels or suspensions of day, organic, or long-chain polymer particles in a finely diffused aqueous slate in soils. These are particles so fine that they stay in suspension unaffected by gravity, and become active sites for ionic bonding and interchange.

Colloids also form gels which hold soil water reserves, and are in part formed by or derived from natural (and more recently, artificial) substances some of  which  form hydroscopic gels by water they can become available to plant roots, which attract them by producing negative (H ) charges.

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With too much flushing by sodium, the calcium and metallic ions can be lost to leaching processes and carried to streams, hence to seas or lakes.

Colloids used in water softening capture calcium ions and release sodium ions, allowing soaps to lather. Ferric oxide colloids have positive charges and give water a brown stain, typical of waters issuing from acid peats and pine forests.

The colloid particles can be flocculated (aggregated) and settle out if aluminum salts (sulfates) are added; and this may happen as a result of acid rain, or can be induced with the acid forms of any negative particles.

Salt also flocculates colloids up to the point where excess sodium defocculates clays, with calcium and other ions, which is the effect of high salt concentrations in dryland soils (over  1-5 ppm salt).

Burning destroys the colloidal properties of surface clays, and is another reason why desert soils leach out after fires. Burnt clay particles no longer form colloids, and become poor in mineral nutrients, which are then found only in deep soil profiles. Organic humus forms black slimy pools on top of the collapsed soils.

It is the clay particles and colloids (organic and inorganic colloids) that bind water and nutrient in soils. In the tropics at least, most of these colloids are in the biomass of the soil as cellular gels, or are produced as sheathing material by soil bacteria.

Without colloids, soil minerals rapidly leach out and become poor in nutrient. Fire, clearing, ploughing, and cultivation destroy such colloids and soil structure, as does excess sodium ions.

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Thus, to hold and exchange nutrients, we as gardeners need to develop natural or artificial colloid content in soils, from where plant roots and soil fauna or flora derive their water and essential nutrients.

Very little clay is needed in sandy gardens to create a colloidal soil environment, and life forms are encouraged and developed by humus, mulch, and perennial plant crops. Good soil structure (to hold the colloids) is developed by careful earth husbandry, together with flocculating additives such as gypsum and humus.



The water content of soils is a soup of free-living organisms, dissolved gases and salts, minerals, gels, and the wash-off from throughfall in trees (waxes, frass, tree “body wastes“), organic and inorganic particles are held in soil water.

Soils have a widely variable water-holding capacity dependent on their composition and structure, so that sands absorb and retain water more quickly than clays, but clays hold more water per unit volume.

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Available water thus varies from 2% (surface sands) to 40% or more of soil volume. We can assist the quantity held in dry sites by swaling, contouring or terracing, loosening soils, adding flocculants, introducing artificial or natural gels (seaweeds or plastic absorbers), or by placing a clay or plastic sheet layer 30 cm below the surface of gardens.

In soils that become waterlogged, we can resort to raised beds or deep drains to reduce infiltrated water build-up, or plant trees to keep active transpiration going and so reduce soil water tables and re-humidify the air.


Two factors at least affect soil water availability:

The strength of molecular bonding of water to particles in the soil (ionic bonds). Plant root hairs cannot remove this bound-water at pressures above 15 atmospheres.

The salt content of the water. Too many salts, and the soil water exerts reverse osmotic pressures on plant  roots, and will take water from the roots.

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