Early indicators show that cover crops are indeed improving water quality and soil health. But, agronomists say, it will take time to determine the extent of soil health gains from cover crops.
"Because soil health improvements can take a lot longer to see in our carbon rich soils, the kind of indicators we use can take longer to show change," says Sarah Carlson, cover crop director for Practical Farmers of Iowa (PFI).
Tom Kaspar with the U.S. Department of Agriculture’s National Laboratory for Agriculture and the Environment in Ames says that right now there is a lot of different groups that are trying to document the effect of cover crops on soil health. A lot of these are on-farm efforts.
"A number of studies across the country have documented nitrate losses either to leaching or ground water or lateral flow that eventually ends up in surface water," Kaspar reports.
One study near Ames is measuring nitrate levels coming out of tile lines on fields planted to cover crops. It has documented a significant reduction in the quantity and concentration of nitrates in tile drainage water.
Carlson said peer-reviewed data show an average reduction of 30 percent in nitrate loss in a field with cover crops as compared to fields not planted. On the phosphorous side, the cover crops shows a 25 percent reduction, she said.
Carlson also observes that when cover crops are added to a system, it usually indicates there will be a trend toward a reduction in tillage. Tillage practices alone can have the biggest effect on phosphorous movement. Just reducing tillage (like converting to no-till) can dramatically reduce phosphorous movement, she said.
Kaspar said that for farmers measuring nitrate and phosphorous levels in tile water, such as farmers using test kits provided by the Iowa Soybean Association, will see a year lag time before a difference is noted.
Though improvements in overall soil health may not be as easy or as quick to determine, there is one main and early indicator that things in the subterranean world of Iowa farm fields under cover crops is improving: earthworms. A PFI study showed an increase in earthworm middens by 32 percent on fields that had cover crops.
According to Carlson, another study conducted by PFI was a Haney soil health test of long-term no-till cover crop strips that had been in place for eight years. The Haney test measures biological and physical variables not just chemical variables.
"At the six-year mark, we did not see any changes in the Haney test, but we did see an increase in earthworm middens. That tells us something is changing and their habitat is healthier. We feel that over time, like at the 12-year mark, we will see improvements in the Haney test."
Studies show that Iowa had 592,000 acres of cover crops on corn/soybean acres in 2015-2016. It is believed that before 2006 there were only 25,000 acres.
Kaspar said there is still a long ways to go on cover crops. "We need about 12 million more acres of cover crops to really tackle water quality issues across the state. We have a good trend, but we need a lot more farmers doing cover crops. That’s all part of the nutrient reduction strategy."
Meyer is a freelance reporter in Garrison.