A Growing Possibility Fertilizing crops with solubles leftover from ethanol processing may work if the timing is right

By Dan Lemke

Waseca, Minn. — It looked like an experiment using a syrupy ethanol coproduct as fertilizer had failed.

When University of Minnesota soil scientist Gyles Randall looked out at his corn test field this spring, “the plots with commercial fertilizer were green.” But those spread with syrup solubles from ethanol processing “were yellow. They looked sick.”

But a funny thing happened on the way to harvest. When the corn started to silk — about mid July — it greened up and looked better. By September, the solubles test plots at the U of M Southern Research and Outreach Center perked up and looked good.

“That made us think that perhaps the syrup was a source of slow-release nitrogen,” Randall says.
 

AURI scientist Alan Doering came up with the idea of using the thick liquid syrup for fertilizer. Since ethanol plants are always looking for ways to add value to their coproducts, he wondered about the syrup’s nutritive value. Tests by several independent laboratories revealed high enough
levels of nitrogen, potassium and phosphorous to warrant field trials. One thousand gallons of solubles contained about 73 pounds of total nitrogen — almost 100 percent in the organic form.
 

“The solubles looked good after the lab analysis, but the only way to find out for sure was to test it in the field,” Doering says.

Six treatments, replicated four times, were planted last May, one with no added nitrogen to serve as the control, two containing different levels of commercial fertilizer and three treatments with syrup applied in incremental rates.

Harvest data collected this fall show the solubles had an effect, though not as much as chemical nitrogen. Ammonium and nitrate forms of commercial fertilizer are almost immediately available to plants. Organic nitrogen, including solubles and livestock manure, take time to mineralize or break down so plants can use them.
 

Comparing the response rates of solubles and commercial fertilizer, Randall found “only about 21 percent of the nitrogen in the syrup was actually available,” compared to 100 percent for fertilizers like urea.

Test data revealed that plots spread with 3,000 gallons per acre of solubles yielded about the same harvest as plots with 60 pounds of commercial fertilizer.
 

While yield results showed modest success, Randall and Doering expect the syrup would provide more available nitrogen if applied, like manure, in the fall and incorporated during post-harvest tillage. “There should be better mineralization,” Randall says. “It’s potential as a spring-applied fertilizer appears limited, but it may be much better fall applied.”

One advantage is the relatively low cost of using syrup as fertilizer. However, transportation and
application costs may impact its economic feasibility.
 

“Each producer would have to look at how close they are to the source, and how much it would cost to transport and apply (syrup) … to decide if it was an option for their operation,” Doering says.
 

While nitrogen is the most sought after component, Randall is also testing soil treated with solubles for potassium and phosphorous. Those results are expected later this winter. ■