"Chris Hogg" wrote in message
news
On Tue, 2 Mar 2010 20:48:07 -0000, "Phil Gurr"
wrote:


"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

From time to time I chuck a handful or two of the magnesium-containing
stuff into the pond to keep it from getting too acid and killing the
snails by dissolving their shells. AIUI dolomitic limestone gives a
slightly less alkaline pH than pure limestone. Certainly the pond pH
stabilises at around 7 when so treated.

Magnesium carbonate is just as alkaline as calcium carbonate, but the
release is so slow that it is counteracted by natural acidity. This is
especially so in a pond where the water always tends toward acidity.

Phil
Northern Highlands of Scotland