Soil pH
The pH value is a measure of the acidity of a soil
and is based on a set of numbers from 1 to 10 that are universally
recognised. 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. The
following values are generally accepted as the degree of acidity.
Strongly 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.
Slightly 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.
Acid Soil
Treatment
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.
Adding 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 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.
Lowering Soil
pH
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 grow.
Here are some interesting gardening links
that you might like to check out while you are here.
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