Growers have believed that crops will get the carbon dioxide they need from the surrounding air, but environmental scientist and Woods End Soil Lab founder Will Brinton says that's no longer the case.

"Things have changed over the years. Soils have fallen so low in organic carbon that they no longer produce enough carbon dioxide to feed vigorously growing plants," he says.

In a recent No-Till Farmer webinar, Brinton addressed the importance of measuring carbon dioxide activity in soil as part of a new approach to soil fertility. He explains that it’s important to realize that natural carbon dioxide levels in the air are actually controlled by soil because it originates there — microbes hold and release organic matter from the soil as carbon dioxide. In fact, carbon scientists agree that topsoil is the greatest storehouse of available global carbon.

“In pre-industrial times, soils were naturally rich in humus, the decomposed plant and animal matter essential to our soil’s health,” Brinton says. “Thus, the amount of carbon dioxide coming out of the ground — right under our crops — was more than sufficient to meet all our plants’ photosynthesis needs. Now that agricultural soils have become so carbon-depleted, however, plants need to access more of their carbon dioxide from the air rather than the soil. We don’t know the full extent of this soil biology yet, but some crops may now be limited from lack of carbon dioxide, which ensures full growth.”

According to Brinton, carbon dioxide needs can be as high as 400 pounds per acre per day. 

“In a race against time and population growth, we imagine that more fertilizer will produce more crops and feed more people,” he says. “But this equation is far too simple — and may even be dangerous.”

A recent scientific paper shows that in spite of increased fertilizer usage, soils are steadily declining in organic nitrogen (N) reserves, which is held by humus. Long-term plot studies confirm that humus has also declined in agricultural soils by more than 50% in the last century. Brinton uses an analogy to explain what’s happening: “We’ve depleted our savings account, and the interest we’ve benefited from is no longer there.”

Brinton says to fix this is to alter the way yield response is measured by paying attention to the background carbon dioxide and organic N fertility. 

“These have been factored out of earlier studies. Correcting this omission is critical to assuring soil health and high-yielding crops. The 2015 International Year of the Soil seems to be the perfect time to address this issue,” he says. “If we don’t start accurately measuring soil carbon biology, I fear we will continue to ignore it in soil management. We just can’t keep starving soil microbes of their food.”