Canals can be of variable lengths, from one to
tens of metres (Figure 4.71). In general, their
length is not an issue, as seen in the NFT, because
the large volume of water enables adequate
nutrient supply. Optimal plant nutrition in very
long canals should always allow for adequate
water inflow and re-oxygenation to ensure
that nutrients are not depleted and that roots
can breathe. As far as the width is concerned,
it is generally recommended to be the standard
width of a sheet of polystyrene, but it can
be multiples of this. However, narrower and
longer canals enable a higher water speed that
can beneficially hit the roots with larger flows
of nutrients.