In very simple terms, and in just a few words, lime is used in gardening to
lower the acidity of the soil.
The acidity of soil is measured by it's pH level. pH is measured on a scale
of 1 to 14 with 7 being neutral. Below 7, the soil is acidic and above 7 it's
alkaline.
Measuring Soil Acidity (pH level)
Different soil types will behave differently so one vital tool for the serious
gardener is a tester for acidity levels.
There are different types of test kits available. With the first type, you
mix a soil sample with water then compare a colour change to a chart. The second
type is an electronic meter with probes that you insert into wet soil.
Whichever kit you use, it will come with instructions and will give you a
reading. You will need to take samples or tests from a number of spots to get a
better general view of your soil’s acidity level.
You can also judge the acidity of the soil by the types of weeds that grow.
Sorrel, creeping buttercup, nettle, dock and mare’s tail are all signs your soil
is too acidic, or becoming too acidic. Reducing soil acidity will help deter
some weeds but lime isn't used as a weed killer.
Each plant species is suited to different growing conditions and slightly
acidic soil can help to keep down blight in potatoes for example. On the other
hand, slightly alkali soil has a higher level of salt present which may not suit
your particular crop. Gardeners generally tend to aim for a neutral PH of around
7.0.
Changing The Acidity Level Of The Soil
To lower soil acidity you raise the pH value by adding lime to the soil.
Lime can be applied throughout the year, but most gardeners will apply it
during Winter or early Spring. Lime is insoluble in water so thoroughly mix the
lime with the top soil. Once moisture is applied the lime will start to
chemically react, so thorough mixing in dry conditions is very important. Don't
forget that different plants thrive in different soil conditions, so make sure
you know which pH level your plant needs before deciding how much or how little
lime to apply.
Sprinkle the lime over the soil surface and rake it into the top couple of
inches, letting it naturally work down to the root zone by spring. Do not till
it deeply into the soil, it will leach down soon enough.
To lower pH and increase acidity you can add sulphate of ammonia or urea
which are high nitrogen fertilizers. Adding manure will also lower pH and make
the soil more acid. Adding lots of manure year after year will actually reduce
soil fertility by making it too acid so the plants cannot access the
nutrients.
Never mix lime and fertilizer. They will at best cancel each other out in an
unpleasant, to the soil, reaction. So never lime in the same year you fertilize
if you can avoid it and certainly not in the same couple of months.
Clay soils tend to become acid more quickly than sandy soils and the amount
of organic matter has an effect as well. Clay soils can also be slow to react to
the addition of lime as well.
The most common lime used in the garden is agricultural lime or ground limestone
(calcium carbonate). It contains about 50% calcium, another essential plant
nutrient . Once available only as a powder, pelletized lime is now offered as
well. It can be dispensed from fertilizer spreaders and isn’t as messy to work
with.
Some gardeners prefer dolomite lime, which contains magnesium as well as
calcium. Magnesium is an essential nutrient, though needed in smaller quantities
than calcium. Alternate the use of agricultural lime and dolomite, to get a
better balance of nutrients.
Never use hydrated or slaked lime, sometime called quicklime. While this
substance has many commercial uses, it is much too caustic for the garden.
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