Straw residue too valuable to use for biofuels production

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PULLMAN, Wash.– Wheat growers should think twice before harvesting crop residue for cellulosic ethanol production, said Ann Kennedy, a USDA-Agricultural Research Service soil scientist.

“In the more than 100 years that we have been cultivating soils, we have lost about half of the original organic matter,” she said.

“Organic matter provides nutrients crops need; it holds water and contributes to aggregation.”

Large aggregates, or soil clods, help prevent wind erosion.

Makeup

Soil is made up principally of mineral particles, organic matter and microorganisms that break down residue into organic matter.

The percentage of organic matter in a given soil varies naturally from region to region, depending on climate, soil disturbance, moisture and vegetation.

Organic matter may not be what you think.

“A lot of people think residue is part of organic matter,” she said, “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.”

Needed residue

Kennedy, whose current research is examining the composition of cereal crop residues and the amount of residue needed to maintain soil quality, said that in 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 these minimum tillage systems, the intact and slowly decomposing roots also add to organic matter. In fields with multiple tillage passes, every bit of residue is needed and even then, organic matter may not increase

She increased the percentage of organic matter in no-till research plots at the Palouse Conservation Field Station from 1.9 percent to 3.6 percent over the course of 20 years.

Mix it up

Tillage may mix the soil and residue too well, in essence over-feeding the microbes. The microbes will consume the incorporated residue too quickly and release most of it into the air as carbon dioxide.

“It is like going to an all-you-can-eat restaurant every day and eating too much,” she said.

“You cannot adequately metabolize all the food you ate. Cultivated soil is like a ‘pig out’ for microbes.”

In her trials, she was able to increase organic matter up to about 3.2 percent when the soil was tilled with a one-pass chisel.

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,” she said.

Could excess crop residue be baled off for biofuels?

“You could remove the extra residue,” she said, “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. We need to constantly replenish organic matter.”

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