pH and Soils in Permaculture

Permaculture Designers Manual



Section 8.8 –

pH and Soils in Permaculture

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Of the parent rocks of soils, we speak of ACID rocks as containing 64% or more of silica (Si02), INTERMEDIATE rocks at 50-64%, BASIC at 40-50%, and ULTRABASIC at less than 40% silica.

Of soils themselves, we speak of acidity and alkalinity in terms of a logarithmic scale, in which each point is 10 times the concentration of hydrogen ions less than the scale point below it, so that pH 8 (alkaline) is 10 x 10 x 10 x 10 or 10,000 times less acidic than pH 4, and pH 3 ten times more acidic than pH 4.

Table 8.2 serves to portray the availability to plants of some important elements with respect to pH value.

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We commonly speak of “hard” (alkaline) or ”soft” (acid) waters.

The latter used with soap lathers easily, and is desirable for washing; the former (including seawater or other alkaline waters) is difficult to use for washing, as soaps and detergents are themselves alkaline and so do not easily dissolve in other alkalis.

Soaps, based on sodium or potassium (ash) and fats will lather in soft acidic waters, and detergents based on phosphates or sulphur, lather in hard waters.

Hard waters contain calcium (Ca+) or magnesium (Mg+) ions. Soft water contains hydrogen (H+) ions.

The properties of alkali (soaps or carbonates) and acids (vinegar, citrus juice) have long been recognized and used medicinally or in village chemistry.

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The word for deserts In Arabic is khali (“the salt”).

Acids and alkalis arise from the solution of oxides, hydroxides, sulphates, or carbonates of metals and non-metals.


In water and soil water the common rock and soil constituents are:

Metals: Sodium, potassium , magnesium, calcium. and minerals (iron,zinc, aluminium, copper).

Non -metals: Silicon, sulphur, traces of phosphorus, boron, nuorine, chlorides. Carbon is also found in organic soils.

In solution, metals release positive (H) ions. Nonmetals release negative (0H) ions. Chalk, limestone, calcite, dolomite, magnesite, and gypsum are rocks and minerals giving rise to hard water (from air and water which gives carbonic acid).

All of these are carbonates, sulphates, or oxides of calcium or magnesium, or both. e.g. dolomite.


All these below are used to raise pH values in soils:

CHALK and LIMESTONE are calcium carbonates.

GYPSUM – calcium sulphate.

MAGNESITE – magnesium carbonate

DOLOMITE – calcium magnesium carbonate

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In the table of elements, groups I and 0 contain the non-metals of which lithium, sodium, potassium, magnesium, calcium, strontium, and barium compounds give up alkaline (OH) ions to soils.

The usual soluble (solid) bases for alkalis are magnesium, calcium, potassium, barium, sodium, and selenium.

The usual soluble (liquid or gaseous) compounds for acids are silica and sulphur based, or de-rived from humus.

The measurement ok acidity-alkalinity (pH or hydrogen ion concentration} is basic to soil and water science, as it affects the availability (solubility) of other key or trace nutrients, and (at its extremes) the ability of life forms to obtain nutrition or even to live.

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The pH scale ranges from 0 (acid) to 14 (alkaline), although in nature we rarely find readings below 1.9 (lime juice) or above 11.0 (alkali Flats).

In the presence of air, and in ploughed and aerated soils, both metals and non-metals form oxides and these dissolve in water or soil water.

The METALLIC OXIDES (bases) form alkaline solutions. The litmus reaction is blue. There is an excess of hydroxide ions (OH·) present in solution.

The NON –METALLlC OXIDES form acidic solutions. The litmus reaction is red. Excess hydrogen ions (H+) are present in solution.

Common acidic substances are citrus juices and battery acid. Common alkaline substances are sodium bicarbonate (baking soda) and washing soda.

As rain falls, carbon dioxide in the air combines with water to form weak carbonic acid (as in sore drinks); this helps to dissolve metallic oxides in soils, and to bring the minerals of rocks and soils into solution.

Sulphur from industrial processing or from pyrites in rocks or soil ran form sulphuric acids, and these also aid the solubility of metallic oxides.

Phosphorus and nitrogen can form phosphoric and nitric acids with water. Silica in soils dissolves to silicic acids, and chlorine to hydrochloric acid.

Weak nitric acid is a plant growth stimulant (New Scientist 22 May 86).

All of these acids, in moderation are helpful in nutrient supply to plants.

pH is not a constant for soil or water. Not only does it exhibit diurnal or seasonal changes due to rain growth, and temperature changes, but it is essentially a mosaic in soil crumb structure, on the surface or colloids, and at microsites.

Further, pH exhibits vertical soil gradients, being more acid in surface mulches and more basic or alkaline where evaporation, wormcasts, and capillary action draw up bases to the surface of the soils (dry or wet-dry areas).

Mosaics on a larger scale are imposed by slope, and both rock and vegetative types. As long as people are aware of this, and realize that root hairs can both create and seek out ideal pH environments if there are no gross imbalances, then gardens are likely to contain every sort of pH level somewhere in the soil.

Only by grinding mixed samples of soils to damp pastes; or measuring conductivity, can we “average” pH and obtain some ideal net balance, but (as usual) our measuring methods alter the thing we measure.

If we keep the average pH value to between 4.5 and 10, we can grow a wide range of plants and rear aquatic organisms if calcium is present. If we narrow the range to pH 6.0-7.5, most vegetables grow well in gardens.

Outside these ranges (less than pH 4.0 or more than pH 10.0), only specialized bacteria or higher organisms can cope.

Moreover, soil humus is itself a buffer as is calcium (lime); humus will grow plants at satisfactory levels even if the pH changes, so that limed and mulched gardens rarely show plant deficiency symptoms.

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