Oxygen (O2)is used in large quantities by plants. If you were to analyse a dried plant you would find that about 45% consisted of Oxygen atoms. Just like humans, plants need fresh air and their cells use O2 in the same kind of quantities that ours do. In air conditions with a low concentration of O2, or where the air is poor, plants do not thrive. Those that do manage to eke out an existence remain poor stunted specimens.
The leaves of a plant have easy access to O2. They make it as a natural bi-product of the process of producing plant sugars
and breathe it out as waste during the process of photosynthesis.
The roots of the plant do not have the same amount of O2 available to them. They have to work a lot harder to find enough for their needs. Insufficient O2 at the roots will reduce the plants root respiration and result in the shutting down of photosynthesis.
A plantís growth and its yield are governed by the size and health of its root system. It can only grow to its full potential if the roots have enough O2 for their needs. In plants grown hydroponically this essential ingredient is supplied dissolved in the nutrient solution.
Dissolved Oxygen in the nutrient solution can be measured by a DO meter
(dissolved oxygen meter).
These are available from all good hydroponics equipment suppliers.
The amount of O2 dissolved in the solution will vary depending on both temperature and pressure. The warmer the water the lower the gaseous content will be. Really cold fresh water has a DO reading of up to 14 ppm or 14mg/litre, while water at 30 degrees centigrade can only hold about 5ppm or 5 mg/l DO.
This DO only amounts to a very small percentage of the roots needs. All water culture systems have to utilise some other form of oxygenation for the roots as well as DO in the nutrient. Root systems that have insufficient O2 available will soon turn brown and become very sick.
We aerate the nutrient in our systems in order to get the best saturation that we can, (from 5ppm to 8ppm) but the main function of this aeration is to kill off the anaerobic bacteria around the roots. Anaerobic bacteria are pathogens that cannot survive in an oxygenated environment; (Anaerobic meaning without air).
Because the DO in the nutrient can only supply about 1% of the roots requirements, the balance must be made up by breathing air. This air is trapped within the soil in conventional gardening and in the growing medium in normal hydroponics systems. This O2 search uses up energy that the plant could better use to produce root growth.
The only type of system where this does not happen is the aeroponics system. The aerated water being sprayed directly onto the roots, allows the plant to take in free Oxygen from the surrounding air, while still keeping the roots moist and supplied with nutrient.
One of the functions of Oxygen is to facilitate the exchange of nutrients and gasses between the plant roots and the surrounding solution. It does this by changing the electrical charges within the water, so allowing the roots to absorb the available nutrients with the least expenditure of energy.
For this reason, if no other, the roots need all the Oxygen they can get.
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