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pH is not constant in soil or water, but varies on a seasonal or even
daily basis due to factors such as rainfall, biological growth within
the soil, and temperature changes. Rather, a map of the pH level is
a mosaic, varying according to soil crumb structure, on the surface
of colloids, and at microsites. The pH also exhibits vertical gradients,
tending to be more acidic in surface mulches and alkaline where evaporation,
wormcasts, and capillary action draw bases up to the soil surface. It
also varys on a macro level depending on factors such as slope, rocks,
and vegetation type. Therefore the pH should be measured regularly and
at various points within the land in question.
Methods of determining pH include:
- Observation of predominant flora. Calcifuge plants (those that prefer
an acidic soil) include Erica (heath), Rhododendron and nearly all
other Ericaceae species, many Betula (birch), Digitalis (foxgloves),
gorse, and Scots pine. Calcicole (lime loving) plants include Fraxinus
(Ash), Honeysuckle (Lonicera), Buddleia, Cornus spp (dogwoods), and
Clematis spp.
- Observation of symptoms that might indicate acidic or alkaline conditions,
such as occurrence of the plant diseases mentioned above or salinisation
of alkaline soils.
- Use of an inexpensive pH testing kit based on barium sulphate in
powdered form, wherein a small sample of soil is mixed with water
which changes colour according to the acidity/alkalinity.
- Use of litmus paper. A small sample of soil is mixed with distilled
water, into which a strip of litmus paper is inserted. If the soil
is acid the paper turns red, if alkaline, blue.
- Use of a commercially available electronic pH meter, in which a
rod is inserted into moistened soil and measures the concentration
of hydrogen ions.
pH is important for the organic gardener for several reasons, including
the fact that many plants and soil life forms prefer either acid or
alkaline conditions, that some diseases tend to thrive when the soil
is alkaline or acidic, and that the pH can affect the availability of
nutrients in the soil.
Adjusting Soil pH
The aim when attempting to adjust soil acidity is not so much to neutralise
the pH as to replace lost cation nutrients, particularly calcium. This
can be achieved by adding limestone to the soil, which is available
in various forms:
- Ground limestone and ground chalk. These are natural forms of calcium
carbonate which are extracted in the UK from areas such as the Mendips
and Salisbury Plain. This is probably the cheapest form of lime for
gardening and agricultural use and can be applied at any time of the
year. These forms are slow reacting, thus their effect on soil fertility
and plant growth is steady and long lasting. Ground lime should be
applied to clay and heavy soils at a rate of about 500 to 1,000 g/m²
(1 to 2 lb/yd²).
- Quicklime and slaked lime. The former is produced by burning rock
limestone in kilns. It is highly caustic and cannot be applied directly
to the soil. Quicklime reacts with water to produce slaked, or hydrated,
lime, thus quicklime is spread around agricultural land in heaps to
absorb rain and atmospheric moisture and form slaked lime, which is
then spread on the soil. Quicklime should be applied to heavy clays
at a rate of about 400 to 500 g/m² (0.75 to 1 lb/yd²), hydrated lime
at 250 to 500 g/m² (0.5 to 1 lb/yd²). However, quicklime and hydrated
lime are very fast acting and are not suitable for inclusion in an
organic system. Their use is prohibited under the standards of both
The Soil Association and the Henry Doubleday Research Association.
- Calcium sulphate, known as gypsum can be used to amend soil acidity
and is also useful for lightening the structure of heavy clays. Gypsum
can be purchased or can sometimes be obtained from old domestic plaster.
The pH of an alkaline soil is lowered by the adding sulphur, although
this tends to be expensive, and the effects short term.
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