"Chris Hogg" wrote in message
...
On Mon, 01 Mar 2010 13:12:48 +0000, Broadback
wrote:

Called into a DIY shop for some lime. They no longer stock it because a
H&S issues, though I think that may be a good cop out. It occurred to me
that builders merchants might be a cheap source. Is their lime ok for
gardens, or does it need more refining?


Most 'lime' available in garden centres etc. these days is ground
limestone, i.e. calcium carbonate, whereas true lime is calcium oxide,
or if hydrated, calcium hydroxide. Calcium oxide rapidly converts to
calcium hydroxide when wetted, with the evolution of a lot of heat.
Calcium hydroxide slowly converts to calcium carbonate in the air as
it reacts with carbon dioxide. It's the mechanism by which lime mortar
sets. Much of the lime you spread on the ground will end up as calcium
carbonate. The only real difference between lime and ground limestone
from a horticultural point of view is that ground limestone is slower
to work.

Much of the ground limestone available in garden centres contains a fair
proportion of magnesium carbonate. Where the proportion is over 20% the
product is often marketed as 'dolomite' lime. Magnesium carbonate is even
less soluble in the soil, so very long lasting, but very beneficial to
plants.

Phil Gurr
Northern Highlands of Scotland