2025 August Stewardship Advocate

2025 August Stewardship Advocate

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Dispelling Some Myths about Non-Point Sources of Nutrients

The following article contains excepts from research reports written by Dr. Andew Margenot, soil biogeochemist and associate professor at the University of Illinois Urbana-Champaign. Iowa Corn, the National Corn Growers Association and Illinois Nutrient Research & Education Council (NREC) are some of the multiple funders of this independent and academic research. Background
Dr. Andrew Margenot and his research team at the University of Illinois initiated a research project in the Walnut Creek watershed in Bureau County located in northeastern Illinois with the objective of quantifying and allocating the contributions of sediment from streambeds and streambanks to nutrient loading in Walnut Creek. This research follows on the heels of a meta-analysis in which they found little to no research in the central cornbelt that evaluated the role of tile drainage and surface runoff in nutrient loading into streams. This 12,000 acre watershed, 70% of which is in a corn-soybean rotation and 60% tile-drained, was chosen as a highly representative catchment and - most importantly - due to the willingness of 11 farmers to collaborate in the study with the support of Bureau County Farm Bureau. A few months in into the study, initial results evaluating "how much" of the total phosphorus (total P) in streams was due to agricultural practices have provided preliminary insights. First, some background: currently, non-point agricultural allocations in Illinois and many other cornbelt states are calculated by subtracting non-point and urban sources from total phosphorus exported (see figure 1 for an example from the Illinois Nutrient Loss Reduction Strategy). This assumption by definition over-estimates the contribution of agricultural practices on the amount of P loading into streams and other waterways because some amount of that P is coming from non-agricultural sources such as streambanks or streambed sediments. But just how much remains unclear - and is the focus of the project, using Walnut Creek as a case study.

Figure 1. Illinois Nutrient Loss Reduction Strategy estimate of phosphorus export.

Methodology

Previous work (2022-2025 seasons) in Bureau County showed negative phosphorus balances across 11 fields for all years. This means that more phosphorus was removed from the cropping system via harvest of the corn and soybean crops than was applied as fertilizer (i.e., fertilization is under maintenance rate) - and yet, P was still lost, highlighting that "overfertilization" is not simply the reason why we see P (and nitrate-N) leaving fields. Similarly, the researchers have found Illinois-wide balances for phosphorus being negative since 1990, including the last five-year average (2017-2021). Yet, a 35% increase in phosphorus loss was measured in the last five years at the state scale of Illinois. The research hypothesized that other sources of phosphorus and to a lesser extent nitrogen were contributing significantly to these losses. To estimate of the contribution of agriculture, stream-beds and stream-banks on nutrient export, Dr. Margenot and his research team positioned monitoring equipment in five points in Walnut Creek to measure the link between the sediment-derived phosphorus, tile discharge and stream dynamics, including sediments from the streambed and from eroding streambanks.

Preliminary Results

Results for P loading in the stream indicate substantial contributions from non-fertilizer P sources, as half or more of the dissolved P being in organic form. This means that we may be overestimating the proportion of phosphorus derived from agricultural practices, and underestimating sediment contributions.At multiple timepoints in the summer of 2025, total N in the stream was 95% or more in non-nitrate forms. This points to organic N, derived from organic matter via leaching or sediment erosion from stream banks and stream beds, can be an appreciable contributor to total N loading and meaning it's not nitrogen fertilizer coming out of tile drains.

Next Steps

• Continue monitoring streambank erosion measurements, and measuring loading of nitrogen and phosphorus to the stream with erosion.
• Sampling streambank soils as well as streambed sediments to measure how much and what type of nitrogen and phosphorus is stored, and could be released, to help explain observed nutrient concentrations in the stream.
• Continue steam turbidity monitoring, to quantify the "suddenness" of sediments in streams that tend to drive the majority of phosphorus losses over time.

Look for future Stewardship Advocates for more results and insights from this research.

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Rusty Olson is a 5th generation farmer on his family farm in Hancock County. He began his farming career in the early 2000s while still in college at Iowa State University.

Upon returning full-time to the farm, Rusty tapped into his entrepreneurial spirit by renting several farms and running a custom harvesting and spraying business. As the farming operation grew, Rusty chose to focus on his farm more. "I began to let some of the sideline businesses go and focus more on production agriculture," shared Rusty. "This led me into organic row crops, hay and small grain production."

"The farm is now a 50/50 conventional and organic and we plant fall and spring cover crops to maintain vegetative growth 10 months of the year. We utilize several practices for weed control in the organic fields," shared Rusty. "We have also found that precision technology has been beneficial in improved efficiency and accuracy in our field operations. Parallel farming operations of organic and conventional pose challenges to keep equipment clean and separated, so we are slowly and strategically transitioning more acres to certified organic acres each year."

Rusty plants 34,000-35,000 kernels per acre on conventional corn acres and bumps up the planting rate in organic fields to 36,000-38,000. Corn hybrids are typically a third 95-99 day, a third 100-105 day and a third 106-113 day. The final stand and yield goals are the same in both operations.

Rusty uses a combination of hog manure and other nitrogen sources on conventional acres. The organic acres typically receive spring applied hog manure and occasionally raw chicken litter or pelletized chicken litter. He has added a continuous manure analysis system to the application process that automatically adjusts rate and speed of the application equipment to maintain a steady and consistent nutrient value "as applied" per acre.

When asked about changes he has made in 2025 and plans to make in 2026, Rusty explains "In 2025, we began to transition an additional 80 acres to organic. In 2026, we will begin transitioning 300-400 acres to organic production. We have purchased a high clearance machine to focus on biological late season foliage applications on the organic corn."Rusty is busy off the farm too. He is on the board of directors of the Organic Farmers Association and currently serves on the Iowa Corn Growers Association (ICGA) board of directors, representing District 2 in north central Iowa. Regarding his role with IGCA, Rusty says it is beneficial to have a different perspective in the organization and he can represent an alternative form of crop production. "Sustainability in farming operations should be the goal of all farmers, no matter what type of production they pursue. My goal is to represent all farmers and do what is right and proper to ensure stability and profitability in corn production. When farmers are successful, agriculture is strong and so are rural communities."

Upcoming Events:

Throughout September (numerous topics): ISU Extension Calendar of Events

Throughout September: 2025 Field Days - Practical Farmers of Iowa

September 3: Edge-of-Field Batch and Build and Water Quality Field Day ; Mark Schleisman Farm, Lake City, IA

September 3: 2025 INI Field Day; Bloomsbury Farms; Atkins, IA.

September 10: Cover Crop Drone Seeding and Water Quality Field Day; Woodbine, IA

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