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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 acidity.
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 Boronia.
Moderately 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.
Very Mildly 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 soil.
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 pH.
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.
Lime to Soil
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
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
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
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.