Advice
Lime and Liming Soils
Liming of garden soils is a long-standing horticultural practice, but the rationale behind it is often overlooked.
Liming helps to maintain a balance between the soil's acidity and its alkalinity. This degree of acidity or alkalinity is measured using the pH scale, ranging from 1 (highly acidic) to 14 (highly alkaline). An increase in pH from 6 to 7, for example, records a 10-fold increase in alkalinity. A pH of 7 is neutral, with conditions neither acid, nor alkaline. In garden soils, you are unlikely to experience soil pH values outside a range of pH4-pH9.
The optimum pH for most garden plants and vegetables is 6.5 (slightly acid). If the pH varies significantly from this value, plants may exhibit signs of nutrient deficiency. High pH levels cause a reduction in phosphate and trace element availability, while low pH levels reduce the availability of nitrogen, potassium and magnesium, and discourage earthworm activity.
Lime
provides a source of calcium and raises the soil pH. It is
used to counter acidity brought about by the effects of certain
acidic fertilisers and the action of rainwater, a weak acid.
Soil structure, particularly in clay soils, may also be improved
by lime applications. Reducing the acidity of soil by liming
can encourage improved root development and increase the number
of micro-organisms. These in turn improve the crumb structure
and organic matter levels in the soil ultimately leading to
improved crop growth. Lime can also discourage serious diseases
including club root of brassicas such as kohl rabi.
It
is important to carry out a pH test to determine whether liming
is necessary. If your soil already has a pH of 6.5 or higher,
liming will be of little benefit and may in fact be detrimental.
The autumn is the best season for liming soils, just prior
to digging, as the lime can take effect over the winter months
and will not damage young growth. Lime should not be applied
at the same time as organic matter or fertilisers, when it
may cause the release of ammonia, which wastes nitrogen and
may damage tender growth.
The two types of lime commonly used by amateur gardeners are calcium hydroxide (hydrated lime) and calcium carbonate (ground limestone or chalk). Calcium hydroxide is soluble in water and has a stronger liming action than calcium carbonate, but is less pleasant to handle and easier to over apply. Calcium carbonate is now the most widely available and preferred liming material. It is sold under various names such as carbonate of lime, garden lime, ground limestone or ground chalk.
Application
rates vary with soil type and degree of pH change required.
As a general rule, a loam soil will require an application
of 190g per sq m (6oz per sq yd) of calcium carbonate to raise
the pH by half a unit. A clay soil will require 420g per sq
m (12oz per sq yd) and a sandy soil slightly less - 140g per
sq m (4oz per sq yd). If a greater change in pH is required,
it is better to add the lime in small amounts over a period
of time (rather than all at once).