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Ash Back

The ash left after burning crop residues for energy can be recycled as fertilizer. Biomass ash retains many nutrients, especially phosphorus and potassium.

This spring, AURI will evaluate ash’s effectiveness as corn fertilizer. Ash from biomass reactors in Benson, Winnebago and Little Falls, Minn., will be applied to corn test plots at the University of Minnesota’s Southern Research and Outreach Center in Waseca.

“One of the criticisms of using biomass for energy is that you also remove nutrients along with the plant residues,” says Al Doering, a scientist at AURI’s coproduct lab in Waseca. But using biomass ash for fertilizer “returns the nutrients and micronutrients to the land.”

Many ag-processing facilities generate ash, which is often spread on cropland. “But every kind of ash is a little bit different,” says U of M soil scientist Jeff Vetsch, who will lead the research trials.

So far, there’s little experimental evidence demonstrating how field crops respond to biomass ash, he says. “That’s why we are doing these trials. We want to see how available the nutrients are compared to conventional fertilizers. We expect to show growers that ash waste is a valuable source of fertilizer, and that there are no negative effects on crop production.”

Biomass ash nutrients vary with feedstock and combustion method. In general, though, agricultural ash contains about 200 to 300 pounds of phosphorus (P) and potassium (K) per ton, Vetsch says. Ash also contains important micronutrients, such as sulfur and zinc, but very little nitrogen. Ash’s high pH may provide a small liming effect, as well, Vetsch adds.

The upcoming research trials will evaluate ash produced by turkey-litter incineration, wood-chip gasification and distiller’s syrup combustion — biomass energy technologies now in commercial use by Minnesota companies.

Ash from each source will be chemically analyzed for nutrient content, then applied to test plots of corn following soybeans. Two different application rates will be evaluated, equivalent to 50 and 100 pounds per acre of phosphate, Vetsch says.

Corn’s response to ash will be compared to conventional P and K fertilizers. Besides comparing plant growth and grain yields, corn-tissue samples will be analyzed for P and K concentration and uptake. Soil samples will be taken before, during and after the growing season to assess mineralization — the decomposing of organic nutrients into forms that plants can use.

Keeping it together

The second research phase will focus on ash agglomeration methods.

Biomass ash is fine and light and can be applied with a commercial air spreader or spinner-type spreader. But Doering says compressing ash into pellets would greatly improve handling. Pelletizing would also allow ash to be incorporated into commercial fertilizer blends. AURI will collaborate with Bepex International, a Minneapolis particle processing company, to evaluate ways to densify the material, Doering says.

Biomass ash supplies are becoming available while conventional fertilizer prices are surging. Phosphate fertilizer’s price, for example, rose 40 percent between April 2005 and April 2007, according to the USDA Economic Research Service.

As biomass gasification develops, Vetsch says, waste ashes “may become very economical sources of fertilizer.”