USDA scientist: Some crop residues may be too valuable for biofuels
Converting crop residues into cellulosic ethanol sounds to many people like a good idea — certainly better than using food crops themselves. Yet according to respected USDA soil scientist Ann Kennedy, the stems and leaves left over after crops are harvested may have more value if they are left on the ground, especially in areas receiving less than 25 inches of precipitation annually.
That includes most of the United States (click on link to see map) west of the 100th meridian, which runs roughly from Bismark, S.D. through Laredo, Texas.
To regular readers of Gristmill, this probably does not sound like news, but to others it may.
According to Kennedy (the full story can be found at ScienceDaily), a USDA Agricultural Research Service soil scientist and adjunct professor of crop and soil sciences at Washington State University:
“With cultivation, organic matter tends to decline in most places around the world. In the more than 100 years that we have been cultivating soils in the Palouse [the wheat-growing region of Eastern Washington, Northern Idaho and Northeast Oregon] we have lost about half of the original organic matter.
In terms of organic matter content, “according to Kennedy, soils in the Palouse should contain about 3.5 percent organic matter. In most farm fields in the region, however, it is now closer to 2 percent.”
As explained in the article, organic matter provides crop nutrients, improves the water-retention capacity of soil, and contributes to the formation of soil clods that help prevent wind erosion. Generally speaking, more moisture encourages more vegetation, which is the feedstock for the microbes that break down residue into organic matter.
A lot of people think residue is part of organic matter. But that is not correct. Organic matter is well-decomposed plant material and microbes. It is black and rich and gives soil its dark color.
The tillage system used to prepare the soil for planting is crucial to the conversion of residue to soil organic matter. As explained by Kennedy, “in no-till (direct seed) or one-pass tillage systems … at least a ton of residue per acre per year is needed to build soil organic matter over time.” In minimum tillage systems, decomposing roots, as well as the residues left over after harvesting, add to the soil’s organic-matter content. In no-till research plots at the Palouse Conservation Field Station, Kennedy found that the percentage of organic matter “increased from 1.9 percent to 3.6 percent over the course of 20 years.”
Kennedy thinks than one of the problems of multiple tillage is that it mixes the soil and residues too well — essentially, over-feeding the microbes. The microbes as a result consume the incorporated residue too quickly, releasing most of its carbon it into the air as CO2. Or, as Kennedy puts it, cultivated soil is like a “pig out” for microbes, who, in a very real sense, develop indigestion.
Thus, for the long-term health of the soil, leaving residue on the soil surface works best.
“It will tend to stay around longer, and the microbes will slowly invade it and convert it into organic matter with less lost as carbon dioxide,” said Kennedy.
As for proposals to harvest crop residues for the production of biofuels, Kennedy notes that
“You could remove the extra residue, but it still provides surface cover and will eventually become organic matter; this residue layer is especially important if you rotate with low-residue crops legumes and canola.”
Harvesting residues, in short, requires farmers to find other ways to increase the amount of organic matter in their soils. “‘We need to constantly replenish organic matter — so removing valuable residue, especially in areas with low rainfall, may not be the best practice’,” says Kennedy.
This is clearly vital research, especially given plans to produce massive amounts of cellulosic ethanol — in part from crop residues — in the future. However, the continuation of this research now looks uncertain. According to a report in Wednesday’s Capital Press, the WSU land and water conservation unit, for which Kennedy works, is one of many such units nationwide that are potential candidates for closure.