Most people don’t think of oxygen as something plants need but, rather, something that plants create. Though it is true that plants create oxygen as a by-product of photosynthesis, it may be surprising to find out that plants, more specifically the living cells found within the plants, require oxygen for converting sugars into energy. Plants naturally create oxygen during photosynthesis which means any green portion of the plant (leaves and stems) has access to plenty of oxygen. However, all plant cells need oxygen, including the plant’s cells that are found underground. The cells found in the roots of a plant can’t get oxygen from photosynthesis so rely instead on absorbing oxygen from the environment around them to survive. Buried roots absorb oxygen from small air spaces in the soil. You may have heard the term “drowning” a plant. This refers to a plant that is overwatered and whose roots are unable to receive oxygen. When the cells in the roots do not have access to oxygen, they will die and this can cause the entire plant to die as well. This is why it is so important to make sure the soil and/or nutrient solution is properly aerated to maintain enough oxygen for the plant cells found in the root mass.
Another reason why oxygen around the root mass is so important is that beneficial microorganisms rely on
oxygen-rich environments to live and reproduce. On the other hand, pathogenic organisms do not survive well in oxygen-rich environments. In fact, most pathogenetic microorganisms only thrive in oxygen-depleted environments, which is why they are considered anaerobic organisms. Oxygen around a plant’s rhizosphere directly affects the population of beneficial microorganisms that provide multiple benefits to a plant, including increasing nutrient uptake and protection from pathogens.
Aerating a Soil
A well aerated soil will do two things: maintain high levels of oxygen around the root mass and create adequate drainage so that the soil can be moistened but not overly saturated. Many prepackaged potting soils already have some ingredients added that are designed to help aerate the soil. However, many indoor horticulturists will still amend these prepackaged soils with additional aeration ingredients. For growers who are building a soil from scratch, adding aeration ingredients is crucial to maintaining a healthy amount of oxygen in the soil during the growing process. There are many different ingredients that can be added to a soil for additional aeration. Each has their own advantages and disadvantages. Which aeration ingredient(s) to use will ultimately be determined by the grower’s preference, budget and access to ingredients. The following are some commonly used aeration soil amendments:
Perlite is made from expanded volcanic glass and is probably the most popular aeration additive for soil. Perlite is extremely lightweight which makes it easy to transport and mix. Perlite’s physical composition is very porous and practically repels water while it holds oxygen. Many professional horticulturists will create a mixture of 75% prepackaged potting soil and 25% perlite. This mix is great for any gardener who wants a fast-draining soil. A fast-draining soil can be advantageous for gardeners who like to feed their plants multiple times per week. The biggest disadvantage of perlite is the dust created when mixing (a dust mask or respirator should always be worn when mixing perlite). Wetting the perlite before mixing can also help curb this problem. Another disadvantage of perlite, in outdoor applications, is that it tends to float to the surface after multiple waterings; it is so lightweight that it will blow away in a strong gust of wind.
Pumice is a volcanic rock that is extremely porous. Pumice is much heavier than perlite and, because of this, is rarely used in prepackaged soil mixes. Although it is heavier and a little harder to work with, I tend to favor pumice over perlite. Unlike perlite, pumice provides a small amount of nutritional benefit to the plants. Additionally, pumice doesn’t float which means it stays buried in the soil mix even after extensive waterings.
Vermiculite is a natural mineral that, for horticultural purposes, is pre-expanded by heat. Vermiculite adds aeration to a soil mix and can also help with water retention. Vermiculite is the most popular ingredient of homemade soilless mixes and cloning media. As with perlite, it is extremely important to use a dust mask or respirator when mixing vermiculite; small particles in a grower’s lungs could cause serious health issues down the road.
Coco-coir is actually the inside husk of coconuts and is created as a by-product of the coconut industry. Coco-coir has an amazing ability to hold moisture while also hold a lot of oxygen. Coco-coir has become an increasingly popular option for aeration in a soil mixture. A soil amended with coco-coir will feel spongy and loose which is the perfect conditions for vigorous root growth. The main disadvantage of coco-coir is quality control. All sources of coco-coir are not equal; some require heavy rinsing to remove excess salts while others are ready to go right out of the bag. Generally speaking, you get what you pay for with coco-coir; less expensive usually means a higher concentration of excess salts which will require rinsing before it can be safely used for plants.
Each of these aeration amendments will help a horticulturist create the same thing: a well aerated soil. But what if you aren’t growing in soil? How can the plant’s roots receive oxygen in a hydroponic system where there is no soil? In many hydroponic systems, a portion of the oxygen needed by the roots can still be absorbed from the air. For example, a flood and drain hydroponic system will automatically draw some air into the root mass as the water drains from the tray. However, a good amount of the oxygen required by the plant’s roots can be provided by the dissolved oxygen found in the nutrient solution.
Dissolved oxygen in a liquid refers to the molecular oxygen found between the water molecules. In other words, it is not the oxygen that is part of the water molecule H2O but rather oxygen that is free to be absorbed by the plant’s roots. Maintaining enough dissolved oxygen in a nutrient solution is crucial for plant health in a hydroponic system. There are two factors that affect the dissolved oxygen content of a liquid: salinity and temperature. For horticultural purposes, salinity is not as important of a factor because, long before the salinity in a solution affected the dissolved oxygen content, it would have already become toxic for the plants. In other words, the plants would show signs of nutrient lockout or toxic shock long before the dissolved oxygen content of the liquid was affected. Temperature, on the other hand, is very relevant to the way it affects the dissolved oxygen content of a liquid for hydroponic gardening. As the temperature of a liquid increases, its ability to hold dissolved oxygen decreases. Put another way, the temperature of a nutrient solution directly affects the way the solution can hold dissolved oxygen.
For most hydroponic systems, the ideal temperature range for the nutrient solution falls between 65-72 degrees F. Temperatures below this range will slow growth rates and temperatures above this range will not be able to hold enough dissolved oxygen to sustain proper growth. There are a few different ways a gardener can maintain a proper temperature. Many horticulturists experience nutrient solution temperatures that are too warm. Simply removing the nutrient reservoir from the garden space can help to maintain a more consistent, lower temperature. Water chillers (much like air conditioners for water) are devices that can be used by indoor horticulturists to help maintain cooler temperatures in the nutrient reservoir. On the rare occasion that the temperature of the nutrient solution is too cold, a grower can incorporate a submersible heater; much like the ones used for aquariums. Before setting up a hydroponic system, horticulturists should give serious consideration to how they are going to control and maintain the temperature of the nutrient reservoir.
The temperature of the nutrient solution determines the amount of dissolved oxygen the solution can hold; however, once that dissolved oxygen is used by the plant’s roots, it needs to be replaced. The most common way to achieve this is with an air pump and air stone which pumps air directly into the nutrient solution. This breaks the surface tension of the liquid and injects air directly into the solution. Assuming the temperature is in the desired range, the air being pumped into the solution will be able to replenish the dissolved oxygen content.
Hydrogen Peroxide (H2O2)
Hydrogen peroxide is another way to replenish the oxygen in a nutrient solution. Hydrogen peroxide is water with an additional unstable oxygen molecule (H2O2). Hydrogen peroxide is a hot topic among gardeners because some believe it works wonders while others warn against using it altogether. I believe hydrogen peroxide can offer horticulturists benefits as long as it is diluted and used in moderation. Hydrogen peroxide can be found in rainwater and has played a significant role in plant evolution. For this reason alone, I cannot say that it is a bad thing. However, when used in too strong of a concentration or when used too frequently, hydrogen peroxide can damage beneficial microorganisms and can become counterproductive. If a gardener takes the time to dial in temperature and aeration, he or she will most likely never need to supplement the solution with hydrogen peroxide.
Oxygen around a plant’s root mass is crucial to the function of the plant’s cells and the microbial world responsible for nutrient uptake. If oxygen levels decrease, plant growth will be hindered. On the other hand, if oxygen levels are maintained, the plant’s cells and the microbes found in the soil will remain healthy. By amending a soil with aeration additives, a horticulturist can avoid “drowning” his or her plants and ensure the plant’s roots have access to the needed oxygen. Hydroponic gardeners who maintain temperatures in the desired temperature range while aerating the nutrient solution will ensure the solution maintains adequate dissolved oxygen content. Maintaining high levels of oxygen around the plant’s roots, either in soil or a hydroponic system, is vital for growing a successful garden.
Eric Hopper resides in Michigan’s beautiful Upper Peninsula where he enjoys gardening and pursuing sustainability. He is a Garden & Greenhouse contributing editor and may be contacted at Ehop@GardenAndGreenhouse.net.