CO2 & Your Crop: Why It’s Important and How to Use It

To pursue an investigation into options for CO₂ enhancement, a bit of background on just how CO₂ gets into the plant would be useful.

Primarily, carbon dioxide enters a plant through the specialized cells that make up the stomata. These are located on the underside of the leaves for most plants.

After entering the plant tissue, the CO₂ becomes useful through a process called diffusion. Stomata are kind of like tiny little mouths that open and close, and when open they allow a transfer of gas and moisture. This is where transpiration occurs; the release of moisture, as well as the exchange of “oxygen out and carbon dioxide in.”

The greater uptake of carbon dioxide by the plant the better its photosynthesis process will be and from this, plant vigor improves. So, with more CO₂ we have greater yield, but to get this vital nutrient into our crop it must both be available and the plant must have active or open stomata.

Factors that influence the stomata are: ambient air and leaf temperatures; light levels; humidity; water stress; and carbon dioxide concentration inside the plant. Stomata close during darkness and in periods of drought stress. The ‘guard’ cells actually do the work of opening and closing the stomata.

There is no value in raising the ambient CO₂ level in the greenhouse or tent when it is dark. The same is true when a plant is in drought stress because the plant is trying to conserve water. The stomata are closed in order to reduce moisture loss. Save your time and resources in either of these cases.

When CO₂ is finally absorbed into a plant it must be diffused into the spongy mesophyll cells. The distance that any gas must diffuse into even a large plant is not far, as every living cell is located close to the surface. Interior cells (used for structure) are no longer living cells.

Gas exchange happens in the spongy mesophyll tissue of the leaf. These cells are covered with moisture and loosely packed, making gaseous movement easier. These aspects help make carbon dioxide diffuse into the spongy mesophyll cells while allowing oxygen to transfer out. Stomata openings are typically less then 20x10 microns.

So, getting the crop to grow more vigorously by adding CO₂ requires that we do this in such a way and time that the stomata are open. High photosynthesis is often occurring during heat and so the plant may be in a drought condition.

In this event, though photosynthesis is very active the stomata are closed or mostly closed and added CO₂ is not benefiting them. Early in the light cycle would seem to be an optimum time for added CO_2. The plant’s photosynthesis is completely active and its tissues are still filled with water moisture. Later in the light cycle this moisture aspect may no longer be true.

How does CO2 help produce vigor?

Biological chemistry! The oxygen from CO₂ gets incorporated into the carbohydrates created by the plant. Most plants produce phosphoglyceric acid (with three atoms–C3), others produce aspartic acid (with four atoms–C4) from the photosynthesis process.

C4 category plants developed as early as 12 million years ago and tolerate heat better and so dominate the tropical regions, while C3 plants developed as long as 3.5 billion years ago and are mostly temperate. Each of these two categories responds to CO₂ differently.

Growth Increase from a 100% Increase in the Level of CO₂

About 95% of all plants are type C3, while C4 make up less than 2%. So, it is likely that the crop being grown by the reader is C3 and will likely experience a 41% increase in growth with a 100% increase in available CO₂. The benefits would seem to be insurmountably obvious.

Again, a major influence and aspect of how much added growth is going to be related to the amount of time during daylight that the stomata are open. Drought tolerant plants (C4) are prone to reduced periods of open stomata and this is why they handle drought better, but would be less affected by increased CO₂.

Enhanced CO₂ also has the effect of narrowing the stomata and so will help (due to a narrow aperture) reduce the pollutants that can get into the plant. Another benefit! Though photosynthesis is vital to utilizing the CO₂ diffused into the plant, the presence of enhanced CO₂ for plants grown in lower light levels is greater than for plants grown in higher light levels.

This is because the CO₂ itself will assist in boosting photosynthesis. The evidence to winning this debate is found in that many greenhouse growers increase the level of CO₂ to three times what is found in the atmosphere.

The benefits from enhanced CO₂ are many and mostly without much debate within the industry. Most every grower wants to provide more of this vital molecule to their crop and are looking for ways to do this; safely and economically.

Now let’s consider a practice that also fascinates me – foliar feeding with added CO₂. With this technique the grower is applying both nutrient and CO₂ (truly a form of nutrient) directly to the leaf tissue. The response can be very fast and dramatic. How to infuse the nutrient with CO₂ has a couple options that are being practiced.

Carbonated liquid nutrient is one way to provide nutrient and CO₂ at the same time. Carbonation is simply CO₂ added and diffused into the liquid. As this carbonated liquid is opened to ambient air, bubbles will be seen coming off from the liquid. This is the release of CO₂ into the air.

Within the liquid nutrient there are millions of tiny bubbles of CO₂ gas, and when exposed to low atmospheric pressure, these tiny bubbles group together, float to the surface and reunite with the atmosphere.

For this reason, the grower using this type of carbonated nutrient will want to keep the lid on the nutrient container or bottle as much as possible in order to keep the level of carbon dioxide as high as is possible for as long as possible.

When a carbonated liquid nutrient (containing carbon dioxide) is applied to living plant tissue both the liquid nutrient and the dissolved CO₂ gas are taken into the plant and then diffuse into the various cells. With a single function performed by the grower, both the CO₂ and nutrient needs of the plant have been satisfied. Two birds, one stone!

The complication with trying to use a pump sprayer pressurized by CO₂ (taking the place of an air hand pump) is that the gas sits atop of the liquid and is not diffused with the nutrient spray when released from the sprayer. The gas needs to be mixed, either in the chamber of the sprayer or through aspiration at the outlet.

Hose end sprayers do aspirate the nutrient but use water force not gas to propel the mixture out of the sprayer and onto the plant. A sprayer that aspirates with gas pressure would work, but I do not know where to obtain these.

Nonetheless, it is a very convenient technique to combine the function of foliar nutrient with CO₂ application however one may accomplish this. Remember; photosynthesis makes the diffusion process possible, so make the application at the right time.

Because photosynthesis itself is boosted through the enhanced availability of CO₂, this in effect will reduce the requirement of light energy to gain the same amount of growth. Obviously, this can be very cost effective. Not only is maximum yield our goal, but optimum economy to go along with this added yield. So, added CO₂ can help save on your energy bill.

CO2 and Your Safety

Using gaseous CO₂ has its risks if you are not careful in what you are doing. In an enclosed area, when the CO₂ level exceeds 5,000 parts per million it can cause dizziness and loss of coordination in humans and levels higher than this can cause death for humans and plant tissue necrosis for some plant species.

Consider that 5,000 ppm is only 0.5%. If one adds 4 cubic feet of CO₂ gas to a room the size of 8x10x10 ft. the total CO₂ concentration would be increased by 4%. That is not that much gas yet our CO₂ level is now borderline dangerous.

Obviously, you will want to use a good quality CO₂ regulator if you intend to provide this vital boost to your crop. Regulators will assure that the gas is applied at the right rate. They generally have built-in timers so that you can avoid feeding CO₂ beyond the quantity desired.

Another important piece of equipment is going to be a good monitor with built-in alarm. The monitor senses the concentration of CO₂ fed at any time. The best monitor/controllers will turn on and shut off when programmed levels are reached as well as shut off when programmed.

You will definitely want this monitor to audibly and visually let you know if the levels have exceeded safety or exceeded the levels that you want. As far as the expense of a regulator and monitor, I would ask you to consider how much your personal health and crop is worth as well as the size of your electric bill for your grow.

As CO₂ can add 40% or more to growth, it can just as well cut your energy costs considerably. Adding carbon dioxide is an investment much more than an expense.