What is
'Relative Humidity' (RH)?
Humidity is best described in terms of
'relative humidity' (RH). RH is simply the ratio, expressed as a
percentage, of the amount of water in the air to the amount that it can
hold when saturated. For example, 1 cubic meter of air at 30oC
can hold a maximum of 30ml of water. Therefore when it contains 15ml it
has a RH of 50% and when it contains 30ml of water, its RH is 100%.
Note, at 10°C, one cubic meter of air holds 10ml of water. With each 10°C
rise in temperature air holds a further 10ml. |
For many species, relative humidities of around 70% are an ideal
compromise between satisfactory growth rates and at a value where fungal
diseases can be controlled.
Transpiration is the process responsible for
distributing water and nutrients throughout the plant. Therefore to
maintain active growth a plant must be able to transpire easily. High
humidity restricts the transpiration rate because the air is less able to
absorb the water vapour that is trying to escape (i.e. transpire) from the
leaves. Once the air's capacity to absorb water is reached it is
'saturated' (i.e.
100%
RH) and the plant essentially stops transpiring. When this occurs water
and nutrient supplies fail to be replenished. In cucumbers and tomatoes
this results in symptoms such as 'blossom-end rot' which is due to a
calcium deficiency at the plant's extremities.
CAUTION: When transpiration has ceased
(i.e. where RH = 100%) it is common for water droplets to form on the leaf
edges. This phenomenon is termed 'guttation'. This is
able to occur because the plant is able to pump water by root pressure -
but only if the nutrient concentration is low enough and the temperature
is sufficient. If guttation occurs regularly it will become evident
by the formation of small white salt deposits on the edges (or margins) of
leaves. Subsequent osmosis will cause localised burning (i.e. necrotic
spots). These points are vulnerable to Botrytis and many other fungal
diseases - when the humidity falls and transpiration continues the water
droplets are reabsorbed into the plant drawing with them any fungal spores
that may be present.
How to minimise humidity in a grow room or
greenhouse?
There are several collective
points of action that must be adhered to:
1.
Air exchange via ventilation: The very act of transpiration
causes humidity to rise. Therefore to keep the humidity relatively
constant (thus allowing the transpiration rate to continue unhindered) the
growing area must be well ventilated - i.e. the humid air must be
regularly dumped and replaced with drier air. Note that cold outside air
may well have an RH of 100%, but once it is drawn inside and its
temperature made to increase its RH will drop significantly. Good air
exchange between the greenhouse and the outside will also help ensure
oxygen and carbon dioxide levels do not become depleted.
2.
Minimise plant density: The more plants there are the quicker
the humidity will rise. Thus ventilation rates must be increased with
increases in plant density.
3.
Optimise air temperature: As the temperature of air increases
then its ability to hold more water also increases. For example, you can
lower the RH of a 15oC body of air from 100% to 55% by heating
it to 25oC. Also, for a given level of RH, plants can
transpire more freely when the temperature is high. For example, at 25°C
and 80% RH, the plant can transpire 5ml of water per cubic meter of air
before 100% RH is reached. Alternatively, at 15°C and 80% RH, the plant
can only transpire 2ml of water per cubic meter of air before 100% RH is
reached. Obviously to minimise humidity it would be best to have the
temperature set as high as possible, however, in order to optimise
photosynthesis (see “Temperature”) it is necessary to set the
temperature no higher than around 25oC during daylight hours
and then 5oC cooler at night.
4.
Circulate air within the greenhouse: To ensure uniformity of
humidity, oxygen and carbon dioxide within the greenhouse itself, the air
must be circulated by use of a
fan.
