Agriculture Reference
In-Depth Information
effect of rainfall (carbonic acid), there are several other
factors that increase the rate at which soils become
more acid:
X Acid rain (polluted rain and snow) is directly
harmful to vegetation and also contributes to the
fall in soil pH.
X Organic acids derived from the microbial
breakdown of organic matter, e.g. humic acids,
also lead to an increase in soil acidity.
X Fertilizers . Some fertilizers such as ammonium
sulphate increase the rate at which soils become
acid.
X Crop removal . Some plant nutrients such as
calcium, magnesium and potash are bases so
when they are taken up by plants but not recycled
the soil acidity can increase.
should be sampled separately. Finding a satisfactory
target area in a garden or allotment can be more
difficult because of the variable treatment of all the
small areas leading up to the test: different history of
crops/plants, fertilizer and organic matter additions.
Nevertheless, there is merit in testing the soil when:
X taking on a new allotment or garden
X establishing a new lawn
X establishing plantations (soft fruit, cane fruit,
orchards)
X planting valuable specimens
X planning to grow plants that require an unusual soil
pH range to survive, e.g. calcifuges or calcicoles
X there appears to be a nutritional problem, e.g.
defi ciency symptoms or general poor growth.
Adjusting soil pH
Soil testing
Soil testing kits are available for the gardener which
can give a useful guide to the nutrient status and soil
pH with guidance as to how to use the results for
liming, fertilizer and/or manure application. Growers
of valuable crops would more normally use laboratory
testing.
The testing of outdoor soils is normally undertaken in
the autumn (but not within a few months of applying
nutrients or lime/sulphur) in readiness for the following
year. Nutrient testing is usually limited to determining
phosphate and potash levels; 'nitrogen' testing is of
limited value at this time because so much of the
soluble nitrogen (nitrate) is leached from the soil
during the winter.
Soil pH can be measured accurately in the laboratory
with pH meters. Gardeners usually use either very
simple pH meters (sticks) or colour-indicator methods
to do their own pH testing, but these methods are
not usually better than a half unit either side of the
correct value. A method of testing soil pH is given on
the companion website. Above all, the usefulness of
any soil analysis depends on the degree to which the
sample taken is representative of the area from which
it is taken (and to be treated).
The variability of the soil makes it difficult to obtain
a result on which to base any calculations for nutrient
or lime application. That is why it is recommended
that several cores of soil are taken (ideally 20 cores
down to 15 cm) over the area to be treated, such as
'the lawn', but avoiding any abnormal areas, such as
a new area of the lawn that was formerly being used
to grow vegetables (this should be tested and treated
separately). On a large scale, areas with different
textures of soil (sandy loam, clay loam and so on)
There is merit in selecting plants that will grow in
the soil without adjusting it especially if it is at either
of the extremes of the pH range. However, soil pH
does change over time (see above) and plants may
suffer as a result. More particularly, growing fruit and
vegetables requires having soil in the appropriate pH
range in order for the plants to be productive.
Soil pH can be raised ('sweetened') by the
addition of lime normally as ground chalk or
ground limestone (both are calcium carbonate,
garden lime). An alternative commonly bought from
garden centres is hydrated lime (calcium hydroxide,
slaked lime, builder's lime). All should be applied as a
fine powder because coarse material takes too long
to affect the soil. Care should be taken to protect the
eyes as the powders are easily blown around and
gloves should be worn to handle hydrated lime as it
can cause skin irritation and chemical burns. Wood
ash, the result of burning organic matter, is rich in
potash, which has a similar effect as calcium and
magnesium so it can also be used to help raise the
pH.
Lime requirement is the quantity of calcium
carbonate required to raise the soil pH to pH 6.5.
A rough guide to how much lime to add can be
found in Table 14.4. This table gives the quantities
of calcium carbonate required to raise the pH of
different soils to pH 6.5 (the recommended level to
return mineral soils when its pH is too low for a future
planting of 'normal plants'). 'Over-liming' a soil must
be avoided because this can reduce the availability of
plant nutrients; easily done on very sandy soils.
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