Conservation practices such as bioreactors and cover crops are helping reduce nutrient loss from farm fields, Iowa Soybean Association (ISA) environmental experts reported last week.

The ISA collected and analyzed more than 2,500 water samples last year from 375 edge-of-field sites, mostly tile drainage outlets. Preliminary data show the average nitrate concentration leaving farm fields was about the same as in 2016 but lower than the two years prior to that, Tony Seeman, ISA environmental research coordinator, reported last week at the annual ISA Farmer Research Conference in Des Moines.

Nitrate concentrations in water from fields with cover crops averaged 9.5 mg/L, Seeman said. The data also showed an average 43 percent reduction in tile water nitrate concentrations after flowing through a bioreactor, he said. Some bioreactors reduced nitrate concentrations by as much as 60 to 70 percent.

“That shows us when we put bioreactors in, they do reduce the concentration,” said Seeman. “We see the performance.”

Nitrate concentrations at the majority of sites, 58 percent, were between 10 to 20 mg/L. Just 10 percent of sites were deemed to have “high” nitrate concentrations averaging 20 to 30 mg/L. The remainder, 32 percent, had low or very low nitrate concentrations below 10 mg/L.

Value of targeting

A small segment of the sites — 16 percent — accounted for 50 percent of the total nitrate loss measured by the ISA sampling, Seeman said.

“What this tells us is targeting matters,” he said. “These are the fields we should be looking at for edge-of-field management.”

Nitrate concentrations and loading peaked in the spring, when tile flow increased due to higher rainfall totals.

“Time of year matters,” Seeman said. “When we’re thinking about (nitrate) load, we want to look at those tiles that run a lot. When there’s a lot of water moving, there’s a lot of nitrate moving.”

The highest nitrate concentrations were measured in the fertile soils of the Des Moines Lobe, encompassing much of north-central Iowa, which averaged 15 mg/L.

Nitrate concentrations in the Iowan Surface land form (northeast Iowa) averaged 11.3 mg/L, and the Southern Iowa Drift Plan had average nitrate concentrations of 11.0 mg/L.

In general, the highest nitrate concentrations came from soils with high organic matter, but there were some surprises, said Adam Kiel, ISA water resources operations manager.

“Some places we would expect were high, were not,” he said. Conversely, some parcels with good conservation practices still had above average nutrient losses, he said.

“You have to test to know where your high nitrate concentrations are,” he said.

Interestingly, there was no strong relationship between nit­­rogen rates and nitrate concentrations in water, Kiel and Seeman said. Weather and field-to-field variability have proven to be more important factors than nitrogen rates.

“That really challenges our mindset and forces us to think about what we can do to improve water quality,” said Kiel. “We hope to help farmers think about what they can do to improve water quality without sacrificing production.”