Over the last several years, an increase in the use of air injection systems designed to increase the oxygen capacity or content of stored and utilized irrigation solution has seen major growth. Seems every grower has or is considering the use of these systems in their operations, but what advantage is there? What are the reasons for and against the use of these systems and under what conditions should they be used? An even better set of questions is why are they used and what happens to the solution when they are used?
The basics of water
To begin, water not only holds the water molecule H2O, but additionally it holds dissolved solids that can range from good things for plants like calcium and iron, but also not so good things like sodium and lead. It also can hold dissolved gasses such as oxygen and carbon dioxide. The warmer the water is, the less of these gases it will hold. The higher the concentration of dissolved solids like nutrients, the less gases will be held.
When water moves and is kept in a stirred state so that it is being exposed to the atmosphere, these gases will stay in a fairly stable state. When water sits still, these gases begin to leave the water by rising through the column of water so that there becomes a deficiency at the lower levels but increasing towards the top. This is the process of stagnation. While many changes can begin to occur in this stagnating water column, it is not in the scope of this article or important to it. It is, however, the main reason water is aerated.
Forming of carbonates
While these gases are present in the water, they can affect many things including the physical and ionic states of the elements present in the water and the pH of the water. Carbon dioxide (CO2) as it moves through the water column reacts with ions like calcium and will begin to raise the pH as carbonates form.
Most public drinking water supplies take advantage of this mechanism by adding calcium carbonate to the water which serves as a pH buffer that both provides better tasting water and protects the pipe systems from excess pH issues both high and low. In this case a buffer will offset pH changes from either acid forming compounds or basic forming compounds that might be present in the water so that the pH remains constant for the time the water is stored and delivered to the outlet.
Additional reactions can also occur from other gas in the water including oxygen. Oxygen is an oxidizer and as such, will combine with ions in the water to form new compounds. These new compounds will likely fall out of solution or become unavailable for plant usage. This is very important when these gases are present in ion rich irrigation water that is used to fertilize with.
Why dissolved air is important
The atmosphere is made up of many different gases and some of them will dissolve into water. Dissolved air in water is important because it both sustains and prohibits life. In this situation, the important gases are oxygen, in the diatomic form O2, and carbon dioxide, CO2.
Oxygen has to be in the diatomic form to be useful for life. Oxygen in the form of O2-, the reactive form, also known as free radicals, are detrimental to all carbon based life forms because it is looking for something to combine with and carbon is the optimal partner. Oxygen, as O2, is the oxygen source for life found in the water in the way of both plants and animals. Peroxide compounds do not work here because the oxygen released is the reactive free radical as H2O2 converts to H2O + O2-. Carbon dioxide is, of course, not required by the root system but is required to influence pH by slowing down these fluctuations.
Without oxygen, a nasty form of life that is anaerobic is favored and will develop, and can be the causal agents for stagnation and the associated smells as well as toxins that can be released and a plethora of diseases. Roots from the plants that grow in a medium of water still have to have oxygen, at correct levels, to function properly. Not only roots but also other beneficial micro-organisms require the availability of oxygen to survive and/or thrive. However, the levels that they require can differ from terrestrial plants. Terrestrial plant roots seldom see ambient air concentrations as air must diffuse through the pore structure of the soil, but it does remain much better than oxygen levels typically found in water.
It is very important to note that different gases dissolve differently in water than in air. For instance, CO2 readily dissolves in water but oxygen and nitrogen not so readily. Water will only hold so much total gas, which means that as more CO2 dissolves, other gases such as oxygen and nitrogen are driven out of the system. Also, as temperature or salinity rises, a disproportional amount of the less readily dissolved gases will come out of solution faster than the more readily dissolved gases such as CO2.
When should water be aerated and how?
There are 2 basic ways for air to get into water:
- By dissolving from the atmosphere under normal pressure, or
- By being forced through the water (oxygen diffusion).
While some fish and aquatic plants can extract enough oxygen to survive at concentrations around 5 ppm, terrestrial plants cannot. Plants that are typically land based will require special effort when grown in a water medium. A distinction must be made in whether a plant will be grown in a water medium, or just exposed to water on occasion (deep water culture verses all other).
In deep water culture or Aquaponics, for terrestrial plants, the level of oxygen must be increased beyond what would be absorbed by simple water stirring. Depending on water temperature and salinity levels, this could be a difficult task and the need for oxygen diffusion comes into play. Some risks exist in this system especially if the environment the air is drawn from is CO2 enriched.
Fluctuations in pH
This will cause fluctuations in pH mostly upward as the CO2 combines with calcium. The additional effect is that less O2 will be entrained as CO2 dissolves readily and displaces O2. Care should be taken to pull from an outside source. Watch the pH. It will fluctuate and the more available the nutrient package is, the faster and more pronounced this will occur.
In all other systems in which water is applied and then the supply is stopped while it drains away, including those using clay pebbles, rockwool, sand, soil, peat, coco, or anything else where the roots do not actively sit in water all the time, aeration for oxygenation will not need to take place as intensively as for aquaponics.
The air that dissolves into the water naturally, with perhaps some stirring action on long term tank preparations, might do just fine. This will help avoid stagnation, keep appropriate O2 levels for life, and keep the pH from swinging uncontrollably especially where the tank is located in a CO2 enriched atmosphere.
Additional oxygen for the root health may not be needed because the action of the water draining will pull air into the pores of the medium and provide the appropriate levels of O2 to absorb into the film of water on the root surface. Most of the oxygen in the water will not really be used since it will not stay long enough to be absorbed except from the solution on the root surface.
Also, roots not living in water are not the same as roots that live in water; there are differences in things like the thickness of the pericycle which controls the amount of water that moves into a plant. Exposing roots that are not water developed to water saturation for longer than twenty minutes will kill the root.
In hybrid systems such as ebb and flow (flood and drain), the act of pumping the water up onto the table then the flow back into the holding tank is sufficient to keep enough dissolved gases in the overall system. In environments with supplemental CO2 at high rates, this can still cause a larger amount of CO2 to be dissolved into the water with similar results as air injection. However, this is nowhere near as rapid as the physical bubbling of air through the solution. Care must be maintained for pH issues, and tanks changed more frequently than would probably be required in non-supplemented rooms.
So, in systems that do not involve keeping the roots submerged at all times in water, including systems that allow some water to be held in the medium against gravity but away from the root, it is best to limit air pumps and injection systems, because oxygen in this system will mostly come from diffusion in the medium after irrigation: a simple system that allows for the water in the tank to stir for a few minutes every hour or so, will handle the needs of the system.
This can be as simple as a diverter on the plumbing for tank pump that directs a small amount of the pumped water back into the tank. Other devices could include mechanical stirring equipment such as is employed in the construction industry for paints and other mixing functions.
In true hydroponic systems with an inert medium that holds little water, such as clay pebbles or Nutrient Film Technique (NFT), then having O2 concentrations at or above 40 ppm are required, getting to 60 ppm, which is better, may require much entrainment of air, and as such will have to be monitored closely. However, it does not have to come all from a diffuser. Aquaponics, because the volume of water will not be conducive to allowing appropriate O2 levels naturally, will have to be diffused with O2.
Regulation is critical
Air is an important component in irrigation water for sure, but control is critical to avoid upsetting the balance of the system. The real question is if the effort being made is over-achieving the need and causing more issues than benefits.
The answer for you
The grower should be smart about what is needed, what results can be expected, and what the true costs are. If stagnation is an issue for the grower even with the simpler techniques mentioned here for any system but total root submergence, then the answer may well be a smaller holding tank with more frequent renewals.
