Breaking the cycle of “agricultural landfill”: can cellulosic ethanol offer a path to more no-till farming?

Could diverting agricultural residues for cellulosic ethanol solve the problem of tilling agricultural residues into the topsoil — and help provide a path towards pristine soils not seen since the days of prairie sod?

DuPont says, amidst the focus on renewable fuels, cellulosic ethanol’s potential contribution to sustainable agriculture can get overlooked.

“The secret to soil health,” says DuPont’s John Pieper simply, in speaking with the Digest, “is that all the things above the ground should really stay above the ground, and all the things below the ground should stay below the ground. That’s Nature’s way, how it was intended.”

Pieper should know, after more than 20 years with Pioneer Hi-Bred working with growers, and after years as a grower himself. “The negative is tillage, it can kill the soil structure, as all the microbes and the ecosystem that build up in the ground are tossed around and the soil structure is disturbed. What you have is basically an agricultural equivalent to residential landfill. Things are not where they are supposed to be.”

As Marion Owen, author of Chicken Soup for the Gardener’s Soul observed, “Healthy soil is chock-a-block FULL of living things such as plant roots, animals, insects, bacteria, fungi and other organisms. It’s a jungle down there…Roto-tilling destroys the network of fungal hyphae that gives soil structure. This includes the mychorrhizal network that is so important to plants.”

The rising yields problem

Pieper pauses. There’s a problem. “But the farmer has to get some of the biomass off the field at the end of the season, in most areas. Some of that material is needed, but the more corn we produce by increasing yield, the more corn stover we’re producing, and the grower has to do something with it, and for many growers that means tilling it back into the ground.”

Pieper has a point. Nature had much lower productivity in mind when she came up with corn. Just a few generations ago, yields were a couple of dozen bushels per acre, and the biomass left on the soil after harvest would have been well short of a ton per acre. But now, yields are rising, and biomass is rising along with the corn.

This year, corn yields will average 172 bushels per acre, according to USDA, and that means something like 3 tons of corn stover biomass on the field, after harvest. In areas of Iowa where the productivity reaches 240 bushels per acre, there can be four tons of stover per acre, or even more.

There’s no one we know that believes that an infinite amount of biomass left on the field is a good idea for soil health and farm productivity. There’s some disagreement about exact amounts. But somewhere around 1 ton per acre is emerging as a consensus. And that’s where cellulosic biofuels can come in handy. You see, one of the reasons all the biomass is left on the field, and one of the reasons why growers are ultimately tilling, is that there’s no good economic use-case for the biomass.

How much can be removed?

Today, it’s changing. Growers are realizing payments in the $70 per ton range for biomass, before harvest costs. Plus, improvements in soil productivity and consequent savings in fertilizer costs. That market is bringing biomass off the field, and ultimately encourages no-till farming, since there’s no biomass that needs to be “landfilled”.

How much, 20 percent, 30 percent?

“We’d very much like to get away from percentages,” says Pieper, “because it is really a finite amount, and as yields change the percentages change. So it is better to think about how much needs to be left on the field, in tons per acre, and then the rest can be lifted.”

In a recent Digest story we looked at optimal acres, but Pieper encourages growers to think not only in terms of optimal acres, but feasible acres.

“In looking at the data,you see continuous corn being the best for cellulosic, because you have that biomass every year, but corn-soy rotation will work just as well in the corn years. So, while continuous corn acreage can be thought of as “optimal”, there are more acres in corn-soy rotation and they are very feasible for a biomass harvest program.

“We think in terms of 40 million tons of biomass, or enough to support 100 biorefineries, and as much as 2.8-3 billion gallons of ethanol. Now, of course there are sources beyond corn stover, such as the woodbasket, that will take you much farther. But there is plenty of opportunity with corn stover and it’s there and the economics work.”