Healthy soil and sustainable growing

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Top of the crops
USDA scientist: Some crop residues may be too valuable for biofuels
Posted by Ron Steenblik (Guest Contributor) at 6:55 AM on 17 Jul 2008
http://gristmill.grist.org/story/2008/7/16/112158/855

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.

Kennedy again:

    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.