Carbon can be considered the currency of plants; the element is a main component of plant structure and growth and has an intimate relationship to crop yield. Despite the fact that carbon is so important to the bottom line, however, many growers take it for granted.

After all, CO2 (the only source of carbon to plants) is available in ambient levels at about 400 parts per million (ppm) in the air. But although plants can grow well at ambient CO2 levels, indoor growers cannot achieve maximum yields without providing more CO2 for their crops, especially as ambient CO2 is depleted in the growing environment.

Indoor growers can even have trouble maintaining ambient CO2 levels, since plants are removing CO2 from air and insufficient airflow keeps that CO2 from being replenished. In this case CO2 levels may fall low enough to halt plant growth.

There’s no doubt that supplementing CO2 in indoor farms is beneficial (and sometimes necessary). Fortunately, there are a variety of CO2 enrichment methods that farmers can choose. Before employing enrichment methods, it’s beneficial for growers to understand several things that occur in the relationship between plants and CO2.

First of all, plants fix carbon from CO2 in the air by passive diffusion. CO2 passes from an area of higher concentration (the air) to an area of lower concentration (the plant tissues). Because the plant uses the difference in concentrations to take up CO2, the concentration of CO2 is very important. The higher the concentration, the easier it is for plants to take up CO2, which means that stomata are open less and plant water losses to transpiration are lower.

After being taken up by the plant, the CO2 is transformed into sugars used for plant growth. Ultimately, that carbon makes it possible for plants to grow new tissues and stay strong. If the CO2 levels in a growing environment get below around 250 ppm, the plants stop growing. (Remember that CO2 is the only source of carbon for plants.)

The second thing to understand is that plants remove carbon from the growroom environment permanently. After the plant uses the carbon from CO2 to build plant tissues, it remains there until the plant is harvested. At this point, the carbon, now tied up in plant tissues, is effectively removed from the immediate atmosphere.

To maintain a high level of carbon in a growroom environment, growers must replenish that carbon via CO2 enrichment. If CO2 levels are too low, it becomes the limiting variable; it won’t matter how much light and nutrition you have. Your yield will bottleneck. In this case, you’re essentially wasting the other inputs in your system.

For indoor growers, replenishing CO2 will require that the grower actively puts it back into the atmosphere. For most growers, just a few dollars a day in CO2 can boost plant yield by about 30 per cent.

Determining How Much CO2 Your Plants Need

The first step is determining your current CO2 level. Most environment control systems have built-in sensors for CO2. There are also several hand-held or wall-mount sensors that will tell you what the CO2 is for your facility. Most indoor growers should have levels between 800 and 1,200 ppm.

Some growers have used closer to 1,500, but there is a law of diminishing returns at that point; for most people, 1,200 is the highest they should go. In the 800-1,200 ppm range, the exchange process is very quick and easy on the plant, and the plant also becomes more water-efficient. (That’s right, higher CO2 can reduce water use in your system.) Once you’ve decided to supplement CO2, you have three options.

Three Options to Increase the CO2 Level

1. Burn a clean-burning fuel like natural gas or propane. As you’ll see in the calculations coming up, these fuels release a lot of carbon when burned. The downside of burning a fuel for CO2 is that they produce not only CO2 but water vapor and heat you’ll have to deal with.

2. Release CO2 from a tank at a given rate using a regulator. Releasing pure bottled CO2 is a simple method that doesn’t produce water vapor and heat like fuels do. Surprisingly, pure CO2 contains less carbon than either of the fuels (propane and natural gas) does, so this can be less cost-effective.

3. Use a decomposition process. CO2 by decomposition is often done with fungi and detritus bags. This method tends to be costly due to shipping the material, and could possibly raise issues with compliance, but can produce a good deal of CO2 depending on the product being used.

Growers have experimented with other CO2 supplementation methods like using dry ice or fermentation, but the three mentioned above have been found to be the most cost-effective, popular methods today.

Next, let’s calculate plant CO2 consumption and the supplementation necessary to boost yields. Don’t forget that even though understanding the math behind these numbers is important, there are easy-to-use calculators that growers can use to save some time.

We don’t recommend doing all of the math yourself on CO2 supplementation because there are reliable and easy tools out there that will do it for you in much less time. However, understanding the logic behind these calculations can help you understand how CO2 enters and is used in your farm.

Calculate CO2 Supplement Amounts

To calculate CO2 supplement amounts, there are three things you need to understand about the physics and the economics of supplementing CO2: carbon density, direct costs and indirect costs.

Carbon Density – Different compounds have different carbon densities. As you’ll see below, propane has a higher percentage carbon in one molecule than methane does. This means that one pound of propane is not the same as one pound of natural gas or one pound CO2. The carbon from each feedstock has to be calculated differently.

Direct Costs – Direct costs are costs that are directly associated with the goal. In this case, a direct cost of supplementing CO2 would be the cost of the fuel or CO2, or the cost of the burner. If you know the cost of the feedstock and the carbon density of that feedstock, you can get the cost of the carbon.

Indirect Costs – Indirect costs are costs that result from your goal. Supplementing CO2 often involves burning, and results not only in CO2, but water vapor and heat. This means that indirect costs are things like dehumidification and air conditioning. To understand these costs, you need to know the British Thermal Units (BTUs) generated and the amount of water being generated.

Calculating CO2 Needs

We can break the process of calculating the amount of CO2 you need to supplement into steps. Those seeking a simpler process can use a free calculator like those at able.ag.

1. Calculate the carbon density of the crop

For example, one crop might have a carbon content that is 50 per cent of dry weight. Most crops are 40-50 per cent carbon (dry weight). This changes between crops and production methods, and more specific numbers can be found in the literature on the topic, but it’s usually close to 45 per cent.

2. Calculate carbon removed

This basically describes how much volume of crop is removed on a particular basis. Combined with the carbon density of the crop, the volume removed tells us how much carbon we need to supplement. You’ll probably measure this in pounds per time unit. For example:

Farmer Joe harvests 12 pounds of basil and 40 pounds of lettuce every week in dry weight. At 47 per cent and 45 per cent carbon (respectively), Joe is removing [12(.47)] + [40(.45)] lbs of carbon per week. That comes out to 23.64 lbs/wk.

3. Calculate the carbon content of a particular feedstock

You’ll need to calculate the carbon content of the top three types of feedstock used:

  • Natural gas (methane), (CH4)
  • Propane, (C3H8)
  • Carbon dioxide, (CO2)

You might be tempted to say, “Since I have 10 pounds of CO2, I have 10 pounds of carbon.” That’s not how it works; we need to look at the weight of each element and how many atoms are in the feedstock compound.

For CH4Carbon’s molecular weight is about 12, and hydrogen’s molecular weight is close to 1. The total molecular weight is 12(1 carbon)+1(4 hydrogen)=16. The carbon density for CH4 is carbon/other = 12/16 = 75 per cent carbon.

For C3H8Three carbon atoms makes 36, and 8 hydrogen atoms makes the total molecular weight 44. The carbon density is 36/44, or 82 per cent.

For CO2The atomic weight of oxygen is close to 16, so the carbon density is 12/44, or 27 per cent.

As you can see, you get the best carbon density from propane and natural gas. While you do have to burn both of those, they’re much more efficient at getting CO2 into your system. Each of the fuels results in CO2 and other products. Burning one CH4 molecule, for example, results in one CO2 and two H2O, along with heat.

Now you know how much of a feedstock you need to burn/release to supplement a certain amount of CO2. (Note on units: Natural gas is measured in cubic feet, while CO2 is measured in pounds, and propane is often measured in gallons.)

You want your estimate for CO2 supplementation (from Step 2 – the amount of carbon removed) to match the actual supplementation (from Step 3 – the amount of carbon you’re putting back in) in the unit of measurement used.

Each feedstock is measured in a different unit (cubic feet, pounds or gallons) that growers will need to convert to order correctly. Conversion numbers can be tricky, so if you’re unsure, use a CO2 calculator to calculate the amount of fuel required and the cost of burning it.

4. Factor in efficiency of facility

Plants take up CO2, but there are additional ways CO2 can be lost from an environment. This loss could be due to ventilation, a leaky room or diffusion from the facility. Unfortunately, there’s no easy way to calculate this. Growers can only estimate efficiency based on air turnover, airflow through open windows, cracks under doors, etc. In a completely sealed environment, a grower might expect 80 per cent efficiency (20 per cent of air volume being lost).

Finally, multiply gross amount of CO2 supplemented by efficiency to get cost. For example, Farmer Joe is paying US$21.91/week to supplement. He knows, however, that about 20 per cent of that CO2 is lost due to ventilation and air leaks, so he plans on burning through US$26.29 per week.