With the corn and soybean harvest winding down in Iowa, many farmers are turning their attention to fertilizer.
Roughly 25% to 50% of the total commercial nitrogen applied to fields across the state occurs in the fall, according to a survey led by the Iowa Nutrient Research & Education Council. But to prevent nutrients from flushing into streams and rivers, agronomists say the type of fertilizer and timing are key.
Richard Roth, a nutrient management specialist with Iowa State University Extension and Outreach, said the primary motivation for farmers to apply fertilizer in the fall is to spread out the workload.
Spring rainfall and muddy fields can narrow the window for applying fertilizer and planting seeds.
“Our goal when we apply [anhydrous ammonia] in the fall is to do it when the soil temperatures are below 50 degrees and on a downward trend,” Roth said. “The reason why we do that is because when you get below 50 degrees, microbial activity in the soil really drops off.”
When the soil is warm and moist, microbes convert ammonium into nitrate, the form of nitrogen that most plants need to grow. But without living roots to absorb nitrate, it can easily move with water through layers of soil into the water table or field tile drainage. From there, nitrates can accumulate downstream, contributing to harmful algal blooms and increasing health risks in sources of drinking water.
Roth recommended farmers monitor the Iowa Environmental Mesonet’s soil temperature maps to decide when to apply fertilizer in the fall. The Nitrogen Fertilizer Application Consultation Tool can help in figuring out optimal rates based on soil data, weather conditions and anticipated planting dates.
He said farmers should also pay close attention to soil moisture when applying fall fertilizer to ensure the soil closes over it. Roth also encouraged the use of additives that disrupt some of the enzyme activity of the microbes that help convert ammonium into nitrate.
“It’s an added layer of protection so that we can help keep even more of that nitrogen in the ammonium form,” Roth said. “It is an added cost, but I think it’s a tool that everybody that's making fall applications should at least consider.”
What about cover crops?
Cover crops, like cereal rye, clover, oats and hairy vetch, can absorb excess nitrate in the soil and curb runoff until the corn or soybean crop are well established.
“They're probably the most easily implemented and generally one of the lower costing options for us to implement on farm to help reduce nutrient losses,” Roth said.
Cover crops also help prevent topsoil from blowing or washing away and build organic matter in the soil while supporting earthworms, beneficial microbes and mycorrhizal fungi.
Programs under the Iowa Department of Agriculture and organizations like Practical Farmers of Iowa offer incentives or help offsetting the costs of cover crops. But researchers say each type has its pros and cons and different management requirements.
Cereal rye, the most planted cover crop in Iowa, easily establishes in the fall and is good at suppressing weeds. However, it can compete with cash crops for nutrients and moisture if it’s not terminated early enough.
Clover and hairy vetch are nitrogen-fixing legumes, which can decrease the need for fertilizer in farm fields. But they need some heat and moisture in the fall to grow and survive over winter, Roth said. His current research includes testing different clover species as a cover crop with corn to narrow down the best planting dates.
