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.
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