[midjji]

Assuming the soil type and waterlevels are sustainable, the carbon capture would be proportional to the land level increase over the bedrock plus farm runoff. I would outright discard the latter as its more of environmental hazard than a benefit, but it is also bound by the upper limit of particulate matter the rivers can sustainably move even if we discard all the other problems. Its a coarse model to be sure, but there is no problem at all to measure this effect for the only large scale carbon capture currently in use. Landfills (10% carbon) can be visibly seen growing in height year, and do so despite the same kind of runoff as farms. It seems highly unlikely that farms can reach zero carbon emissions while powered by coal and oil, but the carbon captured in the ground should be measurable, or I suspect its largely a pleasant sounding lie.

The problem with simply weighing it isn't the cost, ore processing facilities already do this at scale. The problem is the cost of fertilizers and that you need heat it up to turn it into mostly coal first, or the methane generated by large scale decomposition after dumping it in a pile somewhere will be far worse than the carbon captured. Heating and transportation could be solved by nuclear and electric vehicles respectively, but its still a rather bad idea. Not to say that biological carbon capture cannot work, there are good ways to do so.