Researchers propose designer soil to sequester CO2.

A team from Newcastle University aims to design soils that can remove carbon from the atmosphere, permanently and cost-effectively. This has never previously been attempted anywhere in the world. . .

The concept underlying the initiative exploits the fact that plants, crops and trees naturally absorb atmospheric carbon dioxide (CO2) during photosynthesis and then pump surplus carbon through their roots into the earth around them. In most soils, much of this carbon can escape back to the atmosphere or enters groundwater.

But in soils containing calcium-bearing silicates (natural or man-made), the team believe the carbon that oozes out of a plant’s roots may react with the calcium to form the harmless mineral calcium carbonate. The carbon then stays securely locked in the calcium carbonate, which simply remains in the soil, close to the plant’s roots, in the form of a coating on pebbles or as grains.

Well, it may never have been tried in a formal research setting but it is one of the very oldest agronomic techniques.

Calcium silicates have many industrial and construction uses so there are many methods of obtaining it. One common source is incinerator ashes, such as those resulting from burning agricultural wastes like rice hulls. It has been proposed that these ashes could be exposed to flue gasses at power plants, a CO2 rich stream, to hasten the conversion of ashes into calcium carbonate. The business model sees profits from disposing of the ashes, credits for carbon capture, and sale of the calcium carbonate to various customers.

For eons growers have used ashes as a source of several primary nutrients and as a soil sweetener (raise PH in acidic soils). It contains potash (potassium) and phosphorous as well as calcium and many trace minerals needed by growing plants. Roughly speaking, when you burn a tree the carbon, nitrogen and water are driven off, leaving everything else that made tree flesh, in a form that is useable by growing trees and other plants.

I use ashes from a co-gen plant to renovate ruined land for subsequent use as pasture. They are free except for hauling charges, though that would change if demand rose. It's cheaper than ag lime and has many of the same benefits - more if you do the accounting as I've outlined above.

Growers have burned crop trash, often in the fields where it is produced, since the dawn of agriculture. They may not have had a formal understanding of the benefits, but they had a practical grasp. They knew how, but the why of it sometimes resorted to spiritual and supernatural explanations. I have advocated that researchers pay more attention to practitioners and seek to understand their tacit wisdom. Rather than dismissing such wisdom as superstition, translate it into the language of science. Progress could accelerate. We have a communication problem.

I'm not sure how it pencils, but understanding that the calcium silicates in ashes reacts with CO2 in the air as well as the ground, sequestering it as limestone (calcium carbonate) gives us a different perspective on agricultural burning. Does subsequent draw down negate the emissions of burning? There are still aerosols to consider, but it may be that the CO2 released cycles back into the soil through chemical rather than photosynthetic processes.

The task can be restated as one of seeking ways to profit from the heat of burning rather than concern for emissions. When wastes are burned in fields the heat does no useful work, but in a co-gen plant it can make electricity. If the energy to collect and haul the wastes, and then haul and spread the ashes, it not greater than the power generated then we profit. I suspect that there is a net loss rather than profit, one that may be reduced by distributed on farm systems. And I suspect that making biochar with low temperature pyrolysis and using the gases to make electricity, fuel and fertilizer in on farm systems would be better still.