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Thread: nutrient oxygen levels and why its so important

  1. #1

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    Default nutrient oxygen levels and why its so important

    after becoming a victim of my own success again, i think i have finally worked out what the problem is and how to stop it in future......

    i've grown a plant so big in an aquafarm, that the water can't hold/supply enough dissolved oxygen for a plant this size. thus the low oxygen levels then cause all sorts of problems with nutrient deficiencies and eventually root rot will set in.
    a silver bullet for root rot is friendly bacteria(i'm using tmc mycorrhydro), but as i've recently discovered, these will colonise on your air stones(if using them) and further reduce the dissolved oxygen in the water by reducing the airflow through the stones.
    unfortunately i'm scrogging atm so i can't monitor my airstones, but luckily i was informed of this by another grower, and have now added an additional airline in each farm without the stone on the end.

    read this and don't be a tit like me


    Oxygenation, Air Pumps, Nutrient Uptake and Temperatures
    Introduction: Why plant roots need oxygen
    Oxygen is an essential plant nutrient - plant root systems require oxygen for aerobic respiration, an essential plant process that releases energy for root growth and nutrient uptake. In many 'solution culture' hydroponic systems, the oxygen supplied for plant root uptake is provided mostly as dissolved oxygen (DO) held in the nutrient solution. If depletion of this dissolved oxygen in the root system occurs, then growth of plants, water and mineral uptake are reduced.

    Injury from low (or no) oxygen in the root zone can take several forms and these will differ in severity between plant types. Often the first sign of inadequate oxygen supply to the roots is wilting of the plant under warm conditions and high light levels. Insufficient oxygen reduces the permeability of the roots to water and there will be an accumulation of toxins, so that both water and minerals are not absorbed in sufficient amounts to support plant growth. This wilting is accompanied by slower rates of photosynthesis and carbohydrate transfer, so that over time, plant growth is reduced and yields are affected. If oxygen starvation continues, mineral deficiencies will begin to show, roots die back and plants will become stunted. If the lack of oxygen continues in the root zone, plants produce a stress hormone - ethylene, which accumulates in the roots and causes collapse of the root cells, at this stage pathogens such as pythium can easily take hold and destroy the plant.

    Oxygen in Hydroponic Nutrient Solutions
    While it’s possible to measure the levels of dissolved oxygen in a hydroponic nutrient solution, it’s not carried out as often as EC and pH monitoring due to the cost of accurate DO (Dissolved Oxygen meters). However, if an effective method of aeration is continually being used, and solution temperatures are not reaching excessively high levels, then good levels of oxygenation in most systems can be achieved One of the most common and effective methods of oxygenation in hydroponic nutrient solutions is with the use of air pumps/machines and air stones.

    Air Pumps and Air Stones
    While there are a number of methods that can be used to introduce oxygen into a nutrient solution, many of these, such as ozone treatment, are expensive and not often used by smaller growers. One of the most practical and inexpensive, yet efficient ways of getting more dissolved oxygen into a plants root system is through forcing air into the nutrient. Air pumps are widely available in a range of sizes, from very small up to very large with capacity to run from one to many `air stones’ each introducing hundreds of tiny bubbles of fresh, oxygen rich air into the nutrient solution.

    Why an Air Stone
    While an air pump tube alone can bubble air into a nutrient solution, oxygenation or the process of getting atmospheric oxygen dissolved into the liquid nutrient, is much more effective where many tiny bubbles of air are created, rather than a slow stream of larger bubbles. The greater the surface contact between the air bubbles and the nutrient, the more oxygen will diffuse into the nutrient solution and smaller bubbles create a far greater surface area than a few larger bubbles will. Air stones simply break up the air flow and distribute along the surface of the porous 'stone' so that many tiny bubbles are rapidly introduced into the nutrient. Depending on the size or dimensions of the nutrient reservoir into which air is being introduced for oxygenation, air stones of different shapes and sizes can be selected. For small rectangular tanks, long thin air stones (some up to 1 foot in length) can be placed on the base of the reservoir to distribute air bubbles and oxygen uniformly. A larger number of smaller, round, cylindrical or oval air stones placed at equal distance inside a nutrient pool or tank also ensure high levels of oxygenation.

    Air stones also have the benefit of acting as 'weights' which remain stable on the base, or in the lower layers of the nutrient tank - the further the bubbles have to travel to reach the surface of the nutrient, the more time oxygen has to diffuse into the liquid and the higher the rates of dissolved oxygen than can be obtained from an air pump and stone set up.

    For systems with multiple nutrient reservoirs or tanks, one large air pump with many outlets will allow oxygenation into all systems and it is always a good idea to buy an air machine and air stones larger than currently required so that aeration can be increased under warmer conditions or if the hydroponic system is later expanded.

    Oxygen and Temperature Effects - Effective Aeration
    While forcing air bubbles deep down into the nutrient reservoir generally increases the dissolved oxygen levels in the nutrient, there is one other major factor to consider and that's the temperature of the air being pumped into the nutrient. As the temperature of a nutrient solution increases, its ability to hold dissolved oxygen decreases. So a cool nutrient solution may in fact hold twice as much oxygen at 'saturation level' than a warm solution. For example a nutrient solution at 45 F can hold around 12ppm of dissolved oxygen at 'saturation', (meaning it is the most it can hold), but the same nutrient solution at a temperature of 85 F will hold less than 7ppm at saturation. This means at a solution temperature of 85F there is much less dissolved oxygen available for the plant’s root system to take up. To complicate matters further, the requirement of the plant’s root system for oxygen at warmer temperatures, is many times greater than at cooler temperatures due to the increased rate of root respiration. So warm nutrients mean a very high oxygen requirement from the plant’s roots, but the nutrient can only hold very limited amounts of dissolved oxygen at saturation, no matter how much air is being bubbled into the solution. Ideally, nutrient solution temperatures for most plants should be run lower than the overall air temperature - this has many beneficial effects on plant growth and development. However, if overly warm air from the growing environment is pumped into an otherwise cool nutrient solution, the warm air will rapidly increase the temperature of the nutrient to that of the growing environment. If air is being pumped via an air machine with an intake close to lights or other heat sources then rapid heating of the nutrient will occur. On the other hand, cool air has the ability to reduce the temperature of the nutrient if sufficient levels are pumped in and thus result in a much more highly oxygenated solution for the plant’s roots. If keeping the nutrient solution temperature down seems to be a continual problem, checking the air inlet temperature of an air pump is a good idea. Overly warm nutrient solutions (ideally nutrient solutions should remain below 65 - 75 F) for most warm season, high light plants and well below 69 F for cool season.can have serious effects on the plants root system. Apart from the increased oxygen requirement due to a much higher rate of root respiration which can rapidly result in oxygen starvation, high solution temperatures favour many of the root disease pathogens. Plant roots become highly 'stressed' when experiencing high temperatures, particularly if there is a large mis-match between the air the root temperature. Root stress slows the development of new roots, resulting in reserves inside the root tissue being `burned up’ during respiration faster than they are accumulated, and stress makes the root system in general more susceptible to disease attack. Keeping a check on nutrient temperature is vital, as is ensuring that air machines are not blasting hot air into the solution and cooking plant roots. Aeration is most effective when cool air is bubbled into a nutrient.

    Oxygenation and Nutrient Uptake
    Healthy roots supplied with sufficient oxygen are able to absorb nutrient ions selectively from the surrounding solution as required. The metabolic energy which is required to drive this nutrient uptake process is obtained from root respiration using oxygen. In fact there can be a net loss of nutrient ions from a plant’s root system when suffering from a lack of oxygen (anaerobic conditions). Without sufficient oxygen in the root zone, plants are unable to take up mineral nutrients in the concentrations required for maximum growth and development. Maintain maximum levels of dissolved oxygen boosts nutrient uptake by ensuring healthy roots have the energy required to rapidly take up and transport water and mineral ions.

    Calcium is one important nutrient ion which has been shown to benefit from high levels of oxygenation in the hydroponic nutrient solution Calcium, unlike the other major nutrients is absorbed mostly by the root growing tips (root apex). The root apex has a large energy requirement for new cell production and growth and is therefore vulnerable to oxygen stress If root tips begin to suffer from a lack of oxygen, a shortage of calcium in the shoot will occur. This shortage of calcium makes the development of calcium disorders such as tip burn and blossom end rot of fruit more likely and severe under oxygen starvation conditions. High levels of oxygenation ensure healthy root tips are able to take the levels of calcium required for new tissue growth and development.

    Conclusion
    While providing oxygenation with the use of air machines and stones is an excellent method of increasing the dissolved oxygen (DO) levels in a nutrient solution, the temperature of the air intake and nutrient solution must also be managed to ensure oxygen starvation in the root zone does not occur. Pumping hot air into a nutrient not only creates temperature stress in the root zone, it also results in less oxygen carrying capacity in the solution itself - a recipe for root suffocation that will rapidly affect the top portion of the plant as well. Getting oxygenation right means checking both aeration capacity of the equipment being chosen and temperatures in the nutrient and root zone.
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    Top post Gramps

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    Nice posts pops glad to see u finally solved it and hope to see u full on again now

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    cheers buddy, me too. done all i can now, just keep doin it and prayin

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    read the same article yesterday mate http://www.quickgrow.com/gardening_a..._airpumps.html and I thought the point about sucking in warmer air through the air intake was brilliant .

    Any thoughts how you can prevent the problems especially as you grow your plants so HUGGGGEEEE in them aquafarms? would it be worth adding a res to your farms and trying to get some DO in there as well to help ?

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    yea a sticky

    i've been thinking about it, and i'm leaning toward drilling 5 or so larger holes in my air stones(6 inch discs), as an unweighted airline will just float.
    alas i'm out of room to add another res in.

    i'm also gonna use 3 farms instead of 2 to fill my screen, so they aren't just as big. as my last big plant started going tits up around the same size.

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    Hi grandad good read up as about to start my first ever grow,

    I think I seen u mention u adding an airline to ur farms as the stones got blocked
    By the friendly bacteria, if the line just floats how would that work thanks again

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    mornin immyedin,
    mine is down the blue pipe, which brings it out(in) at the bottom of the tank. if i go in the farms again, i will glue the airline to the bottom of the tank and not bother with the airstones. an airline can also be placed down the drip ring pipe(which unfortunately i have removed, hence the need to run the airline down the blue pipe at the front)

    this is only a problem if using airstones and bennies under a screen. without the screen the plants can be lifted up, so the airstones can be checked/changed or cleaned without too much trouble.

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    Great write up Grandad! A lot of useful information here

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    here's a bit more I found......

    I have done something terrible. I killed my plants. I strangled them, slowly depriving them of the very oxygen they needed to live. I regret my transgressions, and even though I didn’t mean to do it, the fact remains that they are all dead. For the sake of preventing more of these senseless deaths, I will share with you my findings on how plants use oxygen. We all know that plants use carbon dioxide for photosynthesis and produce oxygen as a byproduct of that photosynthetic reaction. Here is where my lack of understanding ended in my plants’ end: plant roots must have oxygen. In a hydroponic environment it is essential to ensure your plant’s roots are supplied with oxygen. Growers using organic soils will never concern themselves with supplying oxygen to their plants; however, hydroponic growers like me must be well versed in the science of dissolved oxygen (from here on out called DO).


    Abundant, small bubbles are the key to increasing DO levels in your nutrient solution.



    While most crops seem to prefer about 9 PPM DO, some crops like lettuce have demonstrated little to no difference in fresh weight or dry weight when grown in DO levels ranging from 2.1 PPM to 16.8 PPM (Albright et al, 1996). The low level of DO necessary for lettuce growth helps explain why the non-circulating Kratky method of hydroponic growing is so successful for growing lettuce. Conversely, some types of hydroponic systems require saturated or even supersaturated DO levels to yield vigorous growth, as is the case with NFT systems where ideal DO levels can surpass 40 PPM.

    Dissolved oxygen levels in tap water are typically between 5-7 parts per million (PPM) at room temperature. Temperature has a powerful effect on how much oxygen can be held by water molecules. The warmer the water is, the less gas it is able to hold. Fully oxygenated water at 68° Fahrenheit will hold about 9 PPM of DO, while the same water at 86° Fahrenheit only holds about 7.5 PPM (over 15% less DO). It should come as no surprise then that the ideal temperature for maximizing root growth in hydroponic systems is 68° Fahrenheit, the temperature at which water naturally holds the 8-9 PPM of DO, which is the very amount of DO necessary to support vigorous plant growth in most crops. Water holds both dissolved solids like fertilizer as well as dissolved gases like CO2 & O2. The higher the PPM (dissolved solids), the less gases water can hold, so be aware that over fertilizing often occurs in conjunction with oxygen deprivation.

    Common methods for increasing DO levels in hydroponics include adding concentrated hydrogen peroxide to the reservoir, powered aeration with air stones, and surface contact with atmospheric oxygen.

    Hydrogen Peroxide

    Adding hydrogen peroxide is a common practice used by many hydroponic growers but is one that is seriously misunderstood. There are 2 forms of oxygen: O2, the diatomic form which can be absorbed by the plant roots, and O2-, the free radical form which is highly reactive. Hydrogen peroxide (H2O2) converts to H2O + O2- when added to water. The O2- that is formed is not desirable because it will indiscriminately oxidize or damage healthy living plant cells. Also, although adding hydrogen peroxide to a reservoir will result in an initial boost in DO levels, those gains are short lived. Hydrogen peroxide is best kept in a grower’s toolbox for cleaning equipment and treating bacterial and fungal infections.

    Air Pumps & Air Stones

    Air diffuses into water whenever water comes in contact with air. This commonly occurs when water on the surface of a reservoir touches the air in a grow room or when air bubbles rise through a column of water in a reservoir. Many growers employ the combination of an air pump and air stone to add oxygen to their fertigation solution (the mixture of water and fertilizer). This is a viable option for many growers and one that is able to increase DO levels fairly inexpensively. An air pump pumps air through food grade plastic tubing to the bottom of the reservoir, where it is attached to an air stone. The air stone then diffuses the air into small bubbles which rise through the water, increasing the DO level as they rise. This also happens to be a great method for keeping one’s water and fertilizer mixed, preventing the fertilizer from settling to the bottom of the reservoir.

    The size of the bubble that an air stone produces makes a significant difference in their ability to add dissolved oxygen to water. If a single large bubble and 8 small bubbles have the same total amount of air inside of them, then the surface area of the smaller bubbles will always be greater. Consider this example: a bubble with a 5 mm diameter has a volume of 524 mm3 and a surface area of 314 mm2. A bubble with a 10 mm diameter has a volume of 4,188 mm3 and a surface area of 1,256 mm2. The 10 mm bubble could be divided into eight 5 mm bubbles, which would have a combined surface area of 2,512 mm3. By producing bubbles that are half the size, the surface area is effectively doubled, doubling the surface contact of bubbles to water therefore doubling the capacity of the air to increase the DO level. Additionally, smaller bubbles are less buoyant and rise through the water slower, allowing them to diffuse more oxygen into the water. Smaller bubbles are produced by high quality air stones made from silicon carbide, which is fired at 1300 degrees Celsius. The high temperature produces a more porous stone which makes smaller bubbles than the less expensive blue air stones which are only fired at 200 degrees Celsius.




    An air stone in the bottom of your hydroponic nutrient reservoir can make all the difference!

    Air pumps and air stones are not without their problems, however. Pumping air from inside a grow room, especially a grow room enriched with CO2, will cause the pH of the fertigation solution to drop over time. As air that contains 400 PPM of CO2 reacts with elements in the water like calcium, carbonates are formed which accumulate and lower the pH of the solution. Additionally, water holds on to some gases better than others, and because water can only hold a set amount of gas at a given time, the water molecules will selectively hold CO2 and allow the O2 to dissipate out of the water. Consider this example: if water can only hold 100 units of gas at saturation and it starts with 50 units of CO2 and 50 units of O2, then as air (which contains both CO2 and O2) is bubbled through the water over time, the water will absorb more CO2 and release the O2, resulting in lower and lower DO levels over time.

    So are all hydroponic growers destined to kill their plants? If we add hydrogen peroxide then we oxidize the roots; if we bubble air through the water then we raise the pH and force the DO level down over time; even if we do nothing, when water stagnates (stands still), dissolved oxygen starts to rise from the bottom of the reservoir and dissipates back into the atmosphere. Don’t give up hope yet! There isa solution to our DO woes – adding a mixing or stirring pump to the reservoir.

    Stirring Pumps

    A stirring pump is a submersible pump placed at the bottom of a reservoir attached to little or no tubing that either continuously or on a timed cycle mixes the solution, similar to how the oceanic currents keep the water in the oceans perpetually moving. Mixed or moving water will allow the already dissolved oxygen in the water to remain stable in its concentration and also provide increased DO as the water mixes with air on the surface of the reservoir. Moreover, the movement of the water will prevent stagnation and reduce the potential of the growth of harmful anaerobic bacteria which cause unpleasant smells, toxins, and disease.

    Although there are a few methods of increasing the DO level I have not touched upon like commercial air diffusers and electrolysis, these methods are not practical for most gardeners. In a nutshell, it is best to use the air stone and air pump combination for hydroponic methods like Deep Water Culture, and if you choose to use it in another type of hydroponic system make sure to place the air pump in an uninhabited, well-ventilated room that is not enriched with CO2 (people hanging out in an enclosed space increases the CO2 levels). Dissolved oxygen levels in systems other than DWC are best maintained with the addition of a stirring pump. If you follow these recommendations you may save your plants (and your conscience) and become a more successful gardener in the process.

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