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The pH value is a
measure of the acidity of a soil and is based on a
set of numbers ranging from 1 to 10 that are universally recognised
scale. The number 7 has been given to a soil that is neutral,
that is it is neither acid or alkaline. Numbers above 7 indicate
an alkaline soil and numbers below 7 indicate an acid soil. Soil
testing kits are easy to use and can be obtained from most
garden shops or your plant nursery may even offer a free testing service.
The following values are generally accepted as the degree of
Acid Soil. pH 5.0 - 5.5 Plants
for acid soil in this range include Parsley, Potato, Tomato,
Sweet Potato, Maize, Millet, Oars, Tye, Radish, Azalea,
Ferns, Iris, Orchids, Rhododendron, Camellia, Daphne and
Acid Soil. pH 5.5 - 6.0 Plants for a moderately
acid soil include Bean, Brussels Sprouts, Carrot, Choko,
Endive, Kohl Rabi, Peanuts, Rhubarb, Soyabean, Crimson Clover,
Aster, Begonia, Canna, Daffodil, Jonquil, Larkspur, Petunia,
Primrose, Violet and most bulbs.
Acid Soil. pH 6.0 - 6.5 Plants that
prefer this soil include Broccoli, Cabbage, Cauliflower,
Cucumber, Egg Plant, Pea, Sweet Corn, Pumpkin, Squash,
Turnip, Red Clover, Sweet Clover, White Clover, Candytuft,
Gladiolus, Iceland Poppy, Pansy, Rose, Snapdragon, Viola, Wallflower,
Zinnea and Strawberry.
Acid. Soil pH 6.5 - 7.0 Plants that
favor very mildly acid soil are Asparagus, Beet, Celery, Lettuce,
Melons, Onion, Parsnip, Spinach, Lucerne, Carnation, Chrysanthemum,
Dahlia, Stock, Sweet Pea and Tulip.
Lime has the ability to
assist in breaking down heavy clay soils and it will
also reduce the acidity level of a soil. The breaking down
of clay soils occurs becaus the presence of lime in a wet clay
soil causes the fine clay particles to flocculate, that is
they tend gather together in small clumps, and this produces
the crumbly texture that is desired.
However, lime will also
help to release some chemicals from the soil for the
plants to use but it will also tend to lock up others, and this
should be taken into account when adding lime to reduce acidity.
Potash, magnesium, molybdenum and perhaps nitrogen are
more readily soluble in a slightly alkaline soil, while slightly
acid soils assist solubility of iron, copper, zinc and manganese
which may become insoluble and unavailable in an alkaline or over-limed
As we have seen, the lower
the pH reading the more acid the soil, and most plants
prefor a neutral to a slightly acid soil, that is, a pH from
6.0 to 7.0. The addition of lime will raise the pH, and materials
such as sulphate of ammonia, and sulphate of iron will lower
The safest type of lime
for the home gardener to use is ground limestone, also
known as agricultural lime, which has a mild action that may
take many weeks to have an appreciable effect on acidity
or soil structure, but other fertilisrs can be spread,
seeds can be sown and seedlings planted at the same time.
For good results
the lime should not be left on the surface, but raked
in and then lightly forked to a depth of five or six inches.
While the application rate
when adding lime to soil is not critical, a degree of
care should be taken. A level breakfast cup holds about three-quarters
of a pound of lime, and this amount should be adequate for
one square yard of garden bed that has not ben limed for several
years and is to be used for vegetables and flowers that like a slightly
acid soil. An extreemly acid soil could be treated with up to one
and a half pounds to one square yard. Most soils do not need frequent
liming and periodical testing for the pH level should be done to
prevent over liming
In over limed soil it is
common to find plants suffering from chlorosis. They
may show symptoms ranging from paler than normal green foliage
to yellow or cream varigated leaves. While other factors can
cause these symptoms, over liming is the most likely cause when
heavy or too frequent applications of lime has occurred. This
change of leaf coloring is due to an iron deficiency caused by
the locking up action of the lime.
When adding lime to soil it
should not be left on the surface, but raked in and then
lightly forked to a depth of five or six inches.
The addition of sulphate
of iron, or sulphate of ammonia will lower the pH level
and so increase the acidity of a soil, as will the addition
of copious amounts of decayed vegetable matter, compost, stable
manure and straw etc.. When adding soluble sulphates they should
be evenly spread over the soil surface and then well watered
in. While no firm advice can be given to the application rates,
sulphates should be thinly applied, and after watering in and
given light cultivation the pH level should be checked. By repeating
this at say monthly intervals you will learn the characteristics
of your particular garden soil and be able to maintain the required
pH level for the particular plants that you wish grow.
Happy gardening, and the following is an
interesting gardening link for seeds, plants and shrubs that
you might like to check out while you are here.