Learn runoff lessons from Rock County

By: Abby Augarten
Published in Agriview

University of Wisconsin-Discovery Farms is currently monitoring water quality on two farms in Rock County. We began monitoring one of the farms in 2015 and are ready to share some preliminary data.

The farm runs 4,000 acres of corn and soybeans, milks 170 cows and finishes 200 Holstein steers each year. The management system focuses on precision agriculture and rotational strip tillage to control soil loss – strip tilling in the spring prior to corn planting, and no-tilling soybeans.

Our UW-Discovery Farms edge-of-field surface-water-quality dataset has taught us that the first step to controlling phosphorus loss is to reduce soil loss.

Although soil losses have decreased at the Rock County farm, phosphorus loss remains similar – 1.08 pounds per acre in Rock County average vs. the 1.02 pounds per acre Discovery Farms average. For farms that have little sediment loss, phosphorus loss is mostly in the dissolved form. It happens while the soil is frozen. Surface-runoff data from the farm reconfirm that once soil loss is controlled, the next step to reducing phosphorus loss involves fine-tuning nutrient timing and placement.

Decrease phosphorus concentration in the top inch of soil to decrease the loss of dissolved phosphorus during winter runoff. Greater concentrations of phosphorus at the surface are common in systems where nutrients are routinely surface-applied without any tillage to mix nutrients into the soil profile. We observed greater phosphorus concentrations in the top inch of soil at the Rock County farm; phosphorus is surface-applied during soybean years. Delivering nutrients below the soil surface can decrease the amount of phosphorus vulnerable to surface runoff, as long as soil loss doesn’t increase as a result of the incorporation.

That is especially true when there is increased wintertime runoff due to increases in precipitation and weather conditions that impact snowmelt. In January and February 2018, increased precipitation and favorable snowmelt conditions were present at the Rock County farm. During those two months an average of 1.4 pounds per acre of phosphorus were lost in wintertime runoff.

We can’t control the weather or predict the amount of wintertime runoff before the season starts, but we must take the opportunity to decrease phosphorus available at the surface through management during the rest of the year. Timing and method of nutrient application is critical. Fields that have a fall application of nutrients that are not incorporated can show elevated concentrations of both phosphorus and nitrate in snowmelt runoff.

That was witnessed at the farm. A broadcast and not incorporated fall-nutrient application increased nutrient loss from subsequent winter surface runoff. Fall nutrient applications on the surface are at a greater risk of running off during the first runoff events while the soil is frozen. Incorporating or applying nutrients close to when the plant will begin using them or when soil can infiltrate precipitation water can reduce dissolved phosphorus losses.

There has been a lot of speculation that planting a cover crop can retain some nutrients in the soil to potentially decrease losses. The two sites in Rock County are part of a research initiative to measure the potential reductions of nutrient losses by planting cover crops. This past fall one of the basins was seeded with rye. This year will be the first of three years that UW-Discovery Farms measures the impact of cover crops on surface runoff and nutrient losses. Stay tuned for upcoming information and results.