Cheap biofuel feedstocks take a toll on soil health
Biofuel experts are already looking beyond grains for cheaper feedstocks such as straw or corn stover. But there is a price to be paid when you remove biomass that normally goes back to the soil.

By Tom Button

Posted September 13, 2007: It can take as little as three years to effectively burn your way through most of the 50 to 100 tonnes of humified organic matter in a typical acre of Ontario's corn producing soil. Once you burn through that organic matter, however, it may realistically take three lifetimes to build it back up.

That imbalance has Canadian soil specialists shaking their heads in disbelief at some of the bio-energy concepts being touted as representing the brave new world of renewable energy.

"If it burns up your organic matter, there's nothing renewable about it," snaps a worried Keith Reid, soil and fertility specialist for the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA).

Those concepts, based on removing whole-plant top growth from the field and not putting any organic residues back, have the potential to zap the productivity out of today's soils almost as quickly as turning off a light switch. But they aren't the only plans that have soil scientists on the alert. Even ethanol and biodiesel proposals that rely on corn and soybeans will prove damaging to Ontario's long-term agricultural productivity if high prices lure more acres into monoculture and away from sound rotations, the scientists warn.

"There's no free lunch," says Ed Gregorich, soil scientist for Agriculture and Agri-Food Canada at Ottawa. "Organic matter is the key to soil productivity. If we allow it to be depleted, there might be a short-term financial gain but we'll be faced with compaction, poor soil structure and a whole suite of other long-term soil quality issues."

In fact, it turns out that in order to maintain any hope of sustainability, most soils have very little cushion. Now, a team of OMAFRA extension specialists including Kevin McKague, Adam Hayes and Christine Brown, has taken a first provisional stab at calculating how much cushion exists.

editor's NOTE:
U.S. readers note that one metric tonne equals 1.1 U.S. tons, and one hectare equals about 2.5 U.S. acres. This means the carbon recharge rate cited here that is necessary to maintain Ontario soil productivity of 3 tonnes per hectare is about 2.2 tons per acre.

The OMAFRA team based their calculations on existing research showing it takes 3 tonnes of carbon per year to simply maintain the organic matter content of a hectare of moldboard-plowed corn soil. (Carbon is the building block of organic matter). Less carbon is needed in no-till soils, in part because of lower erosion rates, but also because the mere turning over of plowed soils exposes more organic matter to atmospheric oxygen, where it undergoes chemical processes similar to combustion. Even so, a no-till field still needs a carbon injection of about 2.1 tonnes per hectare per year.

The OMAFRA team calculates that Ontario could only afford to divert 2.7 percent of its corn stover to off-farm processing, assuming that the spent stover isn't returned to the field after processing. Even in years with growthy crops, such as 2005, the province could only export 9.5 percent of its stover.

With no-till, the stover available for harvest is higher, but if growers hope to be sustainable in the long-term, they'd be limited to selling a quarter of their corn residue, the team says.

For other crops, the outlook is even more restricted. Not surprisingly, there's essentially no room to export soybean top growth, since soybeans produce so little organic material. More surprising, though, is the team's conclusion that Ontario also cannot afford to export wheat straw off-farm for energy processing. That's because 450,000 tonnes are taken off for straw, and that's already stretching the limits.

"We need to look at the soil as a finite resource," McKague says. "We haven't got a lot of organic matter to spare."

Indeed, soil specialists generally applaud Ontario's grain farmers just for being able to hold onto current organic matter levels, following rapid declines in the '70s and '80s when many farms gave up their livestock and therefore had no manure source, and when they also aggressively moldboard plowed.

Progress has stalled, however. McKague points out that while about half of Ontario's cash-crop soils are managed with conservation tillage, less than 20 percent of cornfields are no-tilled, and that number hasn't been growing.

"We won't be doing any favors for anyone, least of all ourselves, if we damage that equilibrium just to grow energy crops," McKague says. "None of us are against using farms to grow energy. That's not what we're saying. But we do need to get it right, using minimum tillage as much as possible and not taking off more bio-materials than the soil can sustain."

Like McKague, Reid thinks farmers need to draw the line for themselves. "We're already seeing signs that some of these promoters are either underestimating, or they're completely ignoring the impact their plans are going to have on organic matter," Reid says. "Fortunately, the farmers are asking tough questions, and they're going to have to keep on asking those questions."

Indeed, Gregorich says the best outcome for everyone—farmers and consumers alike—may be to actually increase the amount of organic matter in our soils. That would have the double advantage, Gregorich explains, of boosting soil productivity and lowering the level of climate-warming carbon dioxide in the atmosphere.

Fortunately, Gregorich adds, scientists in the past decade have vastly improved their ability to provide advice about soil organic matter. Detailed studies are unraveling its complex nature, delving into the roles of each of its myriad components instead of treating organic matter, as in the past, as a single uniform substance.

"Organic matter is going to be a critical issue, one of the critical issues," McKague says. "Society is saying it needs us to help diversify our energy future, but we also need to look after the future of our soils."