Article 2-4 Oxygen
The first thing visitors from another world would see upon their approach to Earth is water. Our blue planet sparkles like a jewel bathed in sunlight. Water is apparent from a great distance in all three natural states: liquid, solid (glaciers) and gas (water vapor in the atmosphere).
Life on Earth is based upon water. Just as the majority of the planet is covered with water, all life on Earth is comprised of water. Thus, a comprehension of life necessitates an understanding of water in its relationship to life.
When we study the three states of water liquid, solid, gas-we are studying the "physical chemistry" of water; that is, the relationship between physics and chemistry in water. These three states are in part defined in terms of temperature, In simple and approximate terms, we can say that water which is colder than 32°F (0° C) is solid, or ice. Water which is above 212°F ( 100°C) is a gas, or steam. Between 32°F and 212°F (0° C and 100°C), water is in liquid form.
However, water can exist below 212°F (100°C) as a vapor. If you exhale onto your eyeglass lenses to clean them, you have exhaled a water vapor at a temperature below 212°F. The water that makes up clouds in the Earth's atmosphere is also well below 212°F, so you see there is quite a "grey area" between the defining temperature extremes which separate water into its three states. A limited amount of water may be in the vapor phase as "humidity." This amount is a function of temperature: If it is cold, less water is present at 100 percent humidity than if it is hot. We measure this water in air as a "partial pressure" of water, which can increase with temperature and reaches sea level atmospheric pressure (14.7 psi) at 212°F, the boiling point of water.
Oxygen Life began in the oceans, first in simple forms, algae for example, and later evolving into more complex forms. Common to practically all forms of life on Earth today, is the need for oxygen.
In aquatic environments-oceans, lakes, rivers, etc., most life must draw their oxygen out of tile water, which contains dissolved oxygen. For example, fish take water in through their gills to extract oxygen. Terrestrial animals have lungs that can draw oxygen in with air. In both cases, the oxygen is essential for life, though the mechanism for deriving oxygen is quite different: gills versus lungs.
In the atmosphere, oxygen content is fairly constant, about 18 percent. But in water the oxygen content can vary greatly. This is because oxygen dissolves into water at different rates depending upon variables like temperature and pressure. Put simply, the colder the water, the more dissolved oxygen it can hold and conversely, the warmer the water, the less oxygen it can hold.
Refer to the chart on page 21 for a graphic description of oxygen solubility in water according to temperature. Notice that at 590°F (15°C) water cannot contain more than 10.05 ppm dissolved oxygen. Now notice that at 86°F (30°C) water cannot contain more than 7.51 ppm dissolved oxygen. Therefore, organisms that benefit from high dissolved oxygen levels will suffer in warm water.
The value of high oxygen levels in life-containing water is well demonstrated by comparing the richness of life in Arctic waters to that found in tropical waters. In Arctic waters, huge populations of plankton provide fish, sea mammals and a myriad of other life forms with food. This is possible because of the very high levels of dissolved oxygen in the cold Arctic waters.
Warm tropical waters cannot hold high levels of dissolved oxygen, so only those life forms, which have adapted to lower levels of dissolved oxygen can thrive. Tropical oceans are sometimes described as "underwater deserts" because of the limited life forms they support.
The effect temperature and pressure have on the solubility of gases is best described with the carbonated drink example. When you open a bottle of soda or beer, bubbles of carbon dioxide (CO2) begin to form and rise as the compound is released from the bottle. This is the result of a drop in pressure that occurs when the bottle is opened. If the liquid is very cold, the gas release will be slow, but if it is warm and shaken before opening, the CO2, will surge from the open bottle.
Oxygen behaves very much in the manner as CO2 does with regard to solubility in water-according to temperature and pressure. Water at a temperature of 65°F (18°C) has an oxygen capacity twice that of water at 85°F (29°C).
It is important to also understand that temperature and pressure are not the only factors that can limit dissolved oxygen content in water. As organisms draw oxygen from water, it must be replaced as quickly as they extract it. In aquariums, it is common practice to bubble air through the water to charge it with oxygen. This is not an especially powerful way to add oxygen to water, but it works with fish tanks that hold only a small amount of fish in many liters of water. A far more effective way to charge water with oxygen is to spray the water through the air, which many hydroponic growers do to supply their rapidly growing plants with the large amount of oxygen they need to remain healthy.
Plants Plants can derive oxygen from air or water. In nature, plant roots receive water saturated with oxygen following a rainfall. As the soil begins to dry, air permeates so the roots can breathe and absorb oxygen. During watering, the roots receive both moisture and dissolved minerals. If plants are over-watered, their roots sit in soggy, saturated soil and they can die of oxygen deficiency. Over watering is one of the most common causes of houseplant death.
Some plants have adapted to be able to survive in deficient or stagnant water such as water lilies, rice and some carnivorous plants. Most other plants have a much lower tolerance for oxygen deficiency and cannot sit in over-saturated water for very long.
HydroponicsIf a plant's roots are suspended in water, it will absorb oxygen rapidly. If the oxygen content of the water is inadequate, the plant growth will slow in proportion to oxygen availability. Thus the trick is to co-ordinate the supply of water, nutrient and oxygen with the crops' needs according to other environmental factors like temperature of air and water, CO2 levels, ventilation, humidity, moisture capacity of the rooting media, size and type of crop, and day length. This can be difficult in some extreme conditions, but when applied properly the results can be quite dramatic. Hydroponic growers stimulate plant growth by controlling the amount of water, minerals and oxygen in the nutrient solution. These growers work within a narrow realm between irrigating their crop and allowing oxygen into the root zone. Ebb and flow hydroponic systems are based upon the natural principle of irrigation and oxygenation of plant roots. Mineral-rich water is pumped into gravel-filled beds in which the crop is planted. The irrigation ceases and the water quickly drains away. Oxygen follows and fills the gravel bed, allowing the roots to breathe. The roots release CO2, and absorb oxygen. Then the irrigation is repeated and drained away again, basically emulating nature but very quickly. This basic hydroponic method is very reliable; it has been used for decades with different medias such as gravel, sand, wood chips, sawdust, perlite and Rock Wool.
The down side of ebb and flow hydroponics is that the crop is provided with moisture and mineral nutrients at alternating times from oxygen. In other words, when the roots are breathing, they are not being provided with a constant stream of moisture and nutrients. If the media is too absorbent, then the irrigation cycles must be infrequent to allow time for oxygen to penetrate the roots.
"Constant drip" is a more recent irrigation method designed to level out the availability of moisture, minerals and oxygen. Mineral-rich water is constantly provided in a slow drip to plants that grow in a rapidly draining media. The idea is to maintain a constant balance of moisture and minerals without drowning the crop. It can be somewhat tricky to consistently provide a perfect balance. On a hot summer's day, a large plant can transpire a lot of moisture, so water must be provided at a far higher rate than would be required on a cooler day for a small plant.
In recent decades, the leaders in the development of hydroponic technology have moved into "water-culture" methods and away from rooting media. The first and certainly one of the best recognized is the Nutrient Film Technique (N.F.T.), developed in England in the sixties and seventies and made famous by Dr. Alan Cooper. A breakthrough in its day, N.F.T. was based upon the principle of a very thin film of nutrient-rich water flowing slowly over plant roots held within a plastic envelope. The idea is that the nutrient film provides both moisture and nutrients while above it the roots receive a constant supply of oxygen. Today N.F.T. is widely used and well respected by commercial growers and scientific researchers throughout the world. The only drawback is that there is a fairly critical balance between the right amount of moisture and air required in the rooting envelope. If the film is too deep, then the plants will suffer from oxygen deficiency that can lead to root disease. On the other hand, if the pump fails and the film of moisture is interrupted, even for a relatively brief time, the crop can be lost. Because of this drawback, a more reliable and less risky method of water-culture was sought, so "Aeroponics" arose. Aeroponic systems provide roots with a spray of nutrient rich water. Generally, the plant is supported with its roots dangling in the air. A fine mist of nutrient solution is constantly or intermittently sprayed over the roots. This is a great method as long as there is no failure in the pumping system or clogging of spray nozzles...still not completely forgiving or reliable and generally expensive and tricky to set up and run.
The next generation of water cultivation methods was "aero-hydroponics," in which the root zone is divided into two sections. The root tips are immersed in a constantly flowing stream of nutrient solution while the upper roots hang in an air gap and are sprayed or misted with nutrient solution to provide optimum oxygen levels. This is a superb method since a pump failure does not result in water loss to the roots. Generally, aero-hydroponics is more forgiving than the other water-culture methods. Rather than causing dehydration of the crop, pump failure will result in oxygen deficiency from which most crops can recover without a disaster, provided the pump is fixed quickly.
The common link in all of these methods of hydroponic plant cultivation can be found in the oxygen content of the water. As you now understand, warm nutrient is somewhat oxygen deficient, which can have a lot of meaning for a hydroponic grower. Many root diseases, including fungus infestations can proliferate in oxygen deficient environments. I first realized the magnitude of this phenomenon when I observed Pythium destroying crops in Holland growing in Rock Wool. In this case the oxygen deficiency started when the Rock Wool was over-watered. The plants were growing in a saturated sponge. As the Rock Wool dried out, the situation improved, but the next watering led to saturation again. The problem was compounded by the presence of fungus gnats, which seemed to be the vector, or source, of the Pythium. One thing led to another until ultimately, the crop was lost. From this model we learned the importance of deeply analysing problems to learn from experience. Gnat larvae ate and damaged the plants' roots; oxygen deficient conditions caused by high temperature and over-watering stimulated Pythium and the Pythium entered the impaired roots to destroy the crop. In nature, many variables can interact, causing wonderful-or horrible- things to happen. When the plants grow well, there is a lot more things going on than you realize. Similarly, when things go wrong you must look deeper than the obvious to find answers. By better understanding the physical chemistry of water, you can obtain a deeper and richer comprehension of the many phenomena to observe while growing plants.