Plants consume oxygen via the roots for the process of ‘respiration’.
For this to occur the oxygen must be dissolved in the water (nutrient
solution). This is achieved via ‘aeration’.
Aeration methods
As seen with stagnant water, simply exposing a body of water to air does not
aerate it. System design generally determines how much oxygen
becomes dissolved in the nutrient. Maximum aeration is achieved by
breaking the water up into as small a particle size as possible via a
tumbling treatment (e.g. waterfall, fountain, etc). In hydroponic systems,
aeration can be achieved by:
+
Delivering the nutrient solution via spray jets.
+
Design the hardware (for re-circulating systems only) such that the nutrient
splashes into the reservoir when it returns from the roots.
In either case though, it is critical to ensure that the air is well
ventilated where the aeration occurs otherwise that air will quickly become
stale (i.e. depleted of oxygen) -
Fig 6.5.
An air stone and air pump can also be used. Air stones have the added
advantage of promoting circulation of the nutrient reservoir to ensure it is
evenly mixed (Fig 6.7). Make sure to locate the pump in a well
ventilated area.
Note, oxygen also aids in keeping the nutrient sterile due to its mild
disinfecting properties.
Minimum
oxygen concentration?
To support optimum plant growth, a nutrient solution generally requires a
minimum oxygen concentration of around 3mg/L. It is generally noted that
super-oxygenation fails to deliver improved growth result.
Also, there is a
common belief that high temperatures cause oxygen levels to become
inadequate. However by referring to the Table 6.20 you can see that
water can hold 7mg/L oxygen when at 40oC (104oF).
Growth problems at higher temperatures could be attributed to
photorespiration, increased bacteriological activity, etc.
Unrestricted
root growth:
Because 'new' roots are the main supply route for oxygen,
if new root growth is restricted then oxygen supplies will be restricted.
Hence when selecting pots / channels, ensure they will accommodate the
likely root volume of the plants when at full maturity. Failure to do
so may prevent the plants from reaching maturity. |
