Farming Carbon

The best things in farming are free. We need to think about managing our soils and crops to optimize the things that are free, like water and carbon dioxide.

We have a very clear understanding of the critical necessity of water, because we can visually see exactly how plants respond when they do not have adequate water. On the other hand, we tend to ignore managing carbon dioxide, because it is out of sight, and therefore out of mind. Carbon dioxide shortages are frequently a crop’s greatest yield and health limiting factor. It is more common to have a CO₂ shortage than a water shortage. Furthermore, the highest yielding soils that produce the healthiest crops are soils that cycle the largest volume of carbon.

Let’s look into a few ways that we can “farm carbon” on our farms.

Sources of Carbon Dioxide

Carbon dioxide can come from three possible sources:

1. the background atmosphere
2. organic matter being oxidized or ‘burned up’ in the soil
3. microbial respiration (breathing) in the soil.

The background atmosphere has CO₂ levels of 350-425 ppm, which are too low for optimal plant growth. In controlled environments like greenhouses, when CO₂ levels are increased to 1100 ppm, we get double the plant biomass and double the yield, after controlling all the other variables. We also get lower nutrient density unless we actively manage it. Interestingly, atmospheric CO₂ has a seasonal peak in the spring when farmers across the midwest are tilling their soils and releasing a surge of carbon dioxide into the atmosphere.

When corn experiences a surge of growth following cultivation, that is a response to a carbon dioxide flush, from CO₂ released from the soil when organic matter is burned off as the cultivator introduces oxygen into the soil. This is not a nitrogen response, as we have long been led to believe, but a  CO₂ response. Imagine what a corn crop might look like if it grew like that all the time. The downside of this approach is that we can deplete soil carbon levels over time if we are not mindful of replenishing them.

The third, and best, source of CO₂ is from microbes living and breathing in the soil. When we breathe, we absorb oxygen and release CO₂. Healthy microbial populations do the same thing. They also release some of the carbon contained in organic matter, but in a steady and sustained way. When CO₂ is released from the soil microbial populations, it slowly diffuses upward from the soil surface. Ideally, the soil is covered with green growing plants that can immediately capture that CO₂, and transfer it back into the soil.

CO₂ can also come from mulch that is on the soil surface. We can add hay or straw mulch in greenhouse environments and spray it with microbial inoculants to speed up breakdown, which has the effect of releasing a lot of CO₂ that can benefit the crop. This is one of the reasons why cover crops that are roller crimped, with the residue left on the surface, can produce such a strong crop response. We work with several farms that have large amounts of biomass on the soil surface from roller crimping. We can add a microbial inoculant such as Spectrum™, or a breakdown agent like Rejuvenate™, just before we want a burst of CO₂. I would recommend adding such products when corn is at about V4 or V5. This will provide a sustained flush of CO₂ release for weeks as the residue breaks down. Using this approach might mean that you have no residue left at the end of the growing season, which is fine for some farming systems, but might not be a fit for others.

There’s a hidden benefit to keeping the soil covered with mulch and cover crops. 78% of the air around us is nitrogen gas (N₂), which has a molar mass of 28. Oxygen gas (O₂) comprises 21% of air and is slightly heavier, with a molar mass of 32. Carbon dioxide is heavier than either of these, with a molar weight of 44. So CO₂ is heavier than 99% of the air. Generally, the wind mixes these atmospheric gasses enough that this isn’t worth thinking about. But where it does matter, and where it is worth thinking about, is right at the soil surface.

When carbon dioxide is released from the soil microbial community, it diffuses outward and upward through the soil, limited by the soils’ air exchange capacity. Once it reaches the soil surface, it diffuses upward into the atmosphere. The speed at which it diffuses depends on how much the soil surface is protected: the relatively heavy carbon dioxide will naturally settle and remain close to the soil surface as long as it not disturbed by wind. If the soil is covered by mulch, residue, or vegetation, the wind cannot mix up CO₂ as much, so it diffuses upward more slowly, and can be more easily captured by green leaves. When you have a lot of mulch or plants that limit air mixing at the soil surface, you get to use more of the valuable carbon dioxide released by the soil microbes.

Building Soil Organic Matter

In order to build soil organic matter well, we need our soils covered with green plants that are in a vegetative state and growing rapidly all season. If we grow corn alone, that corn photosynthesizes in large volumes for 90 days, at most. In reality, it’s closer to 60-70 days, since photosynthesis volume is not substantial at either end of the plant’s life cycle, when the plant is still small, and after it begins senescing. What about the remaining 300 days of the year? What is capturing carbon and solar radiation on your soil then? This is why relay cropping, intercropping, and cover cropping are so important. One plant or crop can begin to increase as another begins to mature and decline, and we can have continuous carbon capture all year round.

When plants capture the carbon dioxide that is released from the soil, they turn it into sugars, and a large proportion of those sugars are transmitted out through the root system back into the soil environment. When these sugars are released into the soil, they are used as a food source by the soil microbial community, consumed, and released into the atmosphere again as carbon dioxide. This is the small carbon cycle in a nutshell. The carbon that is in the soil A horizon (often 4-6 inches deep) cycles relatively quickly. If plant roots returned no carbon back into this zone, the organic matter in this zone would often be depleted in 5-10 years or less. The key is to have plants that capture not just the carbon lost from the soil, but also a little bit more from the background atmosphere. In this way you have a slight net gain, and a net increase of soil organic matter every growing season. The organic matter that is held stable in the soil for decades is the carbon that gets stored deeper in the soil in the B horizon, from 10-40 inches deep.

The Role of Minerals in Carbon Farming

So, the foundational question is, how can you optimize the things that are free, such as carbon dioxide? The primary pieces are:

• keeping the soil covered with green plants all the time
• making sure the soil can breathe
• making sure the soil has good gas exchange

The next step is to work with the mineral nutritional profile and the microbiome profile to increase the crop’s photosynthetic capacity.

Most plants today are photosynthesizing at only 15-20% of their inherent photosynthetic capacity. This number is so low either because of inadequate carbon dioxide, not enough chlorophyll, leaves being too thin and narrow, or any combination of a dozen other reasons. Most of the reasons come down to not having balanced mineral nutrition. In most crops, simply making sure they have enough manganese, iron, and magnesium can produce a significant boost in photosynthesis.

Minerals are the key that is needed to start the engine and optimize the engine’s performance. The only way to get all the free stuff is to have the engine working well so that it can take in all the fuel that the sun and the soil naturally provide. If the engine is limited, it cannot absorb the fuel and use it.

The foundational difference between high-brix and low-brix plants is the amount of photosynthesis that occurs in each 24-hour photoperiod. When you balance minerals, plants automatically absorb more free sunlight and carbon dioxide, and turn it into sugars.

The long-term solution is to increase soil air flow, eliminate compaction, and balance soil minerals. The short-term solution is to use foliar sprays to jump start the engine.