Difficult Soils in Permaculture

Permaculture Designers Manual

 

CHAPTER 8 – WATER IN PERMACULTURE

Section 8.13 –

Difficult Soils in Permaculture

 

 

CONCRETIONS AND PANS

Several types of concretion or cemented particles occur in soils.

These are commonly the following:

CALCRETE (caliche, platin, kunkar) is a hard, mainly level subsurface concretion about 0.5-1.0 m below a granular or sandy topsoil, typical of coral islands (calcium triphosphate), and the downwind areas of desert borders.

Calcrete must be broken open to plant trees, or the roots will spread out laterally, allowing wind-throw to occur. On atolls, fresh-water deposits develop below the caliche. Broken caliche can be used as a building material and also forms a safe roof for tunnels or dugouts.

Calcium/magnesium concretion, worsened by the addition of superphosphate, is whitish to creamy.

In acid (vinegar), calcrete releases bubbles of carbon dioxide.

 

SILCRETE (cangagua) Is a grey to red shiny hard layer developed below some tropical forest soils, which

gives a glassy surface if forests are cleared. The soil is concreted by silica deposits. If such deposits lie below forests, it Is unwise to clear the forest itself.

 

Durian: Silica-cemented non-wetting horizon, earthy, brittle, found only in volcanic areas. Red-brown hardpan:

occurs in many soil types, not volcanic, semi-arid, and is 10cm-30 m (4 inches to 98 feet) thick.

 

FERRICRETE: Iron-cemented pans and soil layers of varying thickness, sometimes as thin sandy layers of 5 -10mm; also alumina-iron  laterites  (often capping desert hills with veins of silcrete) or iron-manganese nodular horizons in soils.

Ferricrete may lie over pale bauxites, and Is also called ironstone, plinthite, Ortstein occurs in podzols as iron-organic hard 8 horizons.

 

Coffeerock is a thick sandy coffee-coloured horizon, low in iron and easily broken; It is a common horizon in  humic podzols. Duricrusts form hard silica-iron caps on hills in deserts.

 

PLOUGH PANS are usually clay-based com­pacted layers developed below croplands in wet periods; these can be caused by mouldboard ploughs.

 

All of the above need ripping, explosive shattering, or deep mulch pits to establish trees.

Sodium in soils may develop a “collapsed” cemented, greyish, gravelly pan (SOLCRETE) impermeable to water.

Only deep-rooted trees, reduction of salt, and humus relieve these cemented conditions. Deep drainage of 1-2 m is essential for salted soils.

Concreted soil layers (calcium or silica-cemented) are the calcretes (caliche, platin) of dry islands and coasts, or the ferricretes (iron-cemented) of deserts. Any or all may form duricrusts (hard layers) in eroded areas.

Under some tropical rainforest (e.g. in Ecuador), an iron-silica pan which follows hill contours, locally termed congoguo, lies 3 m below the forests; it is a daunting sight to see this glassy and impermeable surface after  the removal of forest and a consequent loss of topsoil.

Where cangoguo is known to exist, perpetual forests used for products other than their wood (honey, fruits, medicines) are the only sustainable use of land.

 

NON-WETTING SANDS AND CLAYS

Some classes of very fine blackish sands, and sands invaded by hydrophobic soil fungi, are difficult to wet; the water sits on top as droplets.

 

There are several remedies for this in gardens:

Ridge soil to make basins.

 

Every square metre, core out sand and drop in a loam or clay-loam plug (4-10 cm by 30cm deep).

Compost thoroughly and build up organic material to 8% of surface soil.

Add a handful of bentonite per square metre, or powdered clay from clay pans.

Mulch thoroughly, and plant. Keep surface mulch supplied.

On the broad scale, deep ploughing in autumn (to 45 cm) is used, followed by rotary plough or chopper, mixing of the top non-wetting profile of 10-20 cm (4-8 inches) with subsoils.

A cover crop is immediately sown to prevent erosion, and this used as a cover crop or green manure for deep-rooting crop or tree species.

The successful establishment of trees permanently curbs the problem.

 

CLAYS which seal on the surface in light rains are often sodium-rich and “melt” in rain.

Remedies are:

Make low banks across run-off.

Add gypsum at 2-3 handfuls per square metre, and if possible push out with fresh water (removing sodium sulphate).

If practical, place sand over the surface to 4 cm deep.

For deep cracking clays and lumpy soils add a sand layer, scatter gypsum at a handful per square metre, and mulch.

For acid or deep sllica sands it is best to add clay and mulch, and to lay plastic at 0.5 m deep in garden beds (Figure 8.7)

 
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