Limestone is not recommended as a growing
medium, though it is commonly used (Figure 4.54).
Limestone, a sedimentary rock, is less desirable
than other media because it has a lower surface
area to volume ratio, is heavy and is not inert.
Limestone is composed primarily of calcium
carbonate (CaCO3), which dissolves in water
and affects water quality. Limestone will increase
the KH of the water, which will also increase the
pH (see Section 3.3). Therefore, this material is
better used where water sources are very low in
alkalinity or acidic, as in cases of alkaline water
it would call for constant acid corrections of
incoming waters. Nevertheless, a small addition of limestone can help to counterbalance
the acidifying effect of nitrifying bacteria, which can offset the need for regular water
buffering in well balanced systems. Limestone may not be as comfortable to work
with in terms of planting and harvesting, and it can experience clogging if the proper
granulometry is not chosen. However, it is often the cheapest and most common form
of gravel available. Limestone is only acceptable as a medium if no other media are
available, but be aware of its impact on water quality.