WEIGHING THE IMPACTS OF TILE DRAINS ON NUTRIENT LOSSES

Data shows how nutrient losses and water quality differ in fields with or without tile drainage.

by Lindsey Hartfiel and Laura Paletta

The use of tile drainage is becoming more popular in Wisconsin. Installing a tile drain system can be a great tool to dry soil out faster, improving timeliness of field operations throughout the cropping season. However, how do tile systems influence water movement off of the field, and what are the water quality implications?

The University of Wisconsin-Madison and Minnesota Discovery Farms Programs on-farm research has included many fields that have tile drainage systems installed. With this data, comparisons between field surface runoff to tile discharge can be made to understand the differences in nutrient losses and dynamics. It is important to be aware of any changes in hydrology or negative impacts tile systems might create.

UW-Madison Discovery Farms has edge-of-field monitoring stations through a partnership with the United States Geological Survey that calculate total runoff while collecting samples to be analyzed for sediment and nutrient content for surface runoff. Minnesota Discovery Farms uses a similar system for edge-of-field monitoring from surface runoff. For tile drainage monitoring, both programs use a similar system to document total tile flow volumes and to collect water quality samples.

Runoff and soil loss

Surface runoff sees a handful of large events each year, whereas tile drains are typically running more often with smaller amounts. Discovery Farms observed eight days as the median annual number of days of flow for surface runoff at any given site. In tile systems, runoff was seen 174 days out of the year.

Data also show that surface runoff occurs on fields with tile when tile lines are flowing at maximum capacity. This suggests that runoff on field surfaces occurs during large precipitation events when the soil is fully saturated or when tile lines cannot keep up with the large amount of water.

As seen in Figure 1, surface runoff typically has higher annual soil loss compared to tile systems. The erosion potential of surface runoff is much greater due to the impact of raindrops as well as the power of water moving over the soil surface, dislodging the soil particles. Tile drainage can reduce the amount of surface runoff by limiting the length of time the soil is at saturation and, therefore, will lower soil loss.

Phosphorus losses are less

Compared to surface runoff, phosphorus losses are reduced in tile system flow. Both particulate phosphorus (bound to soil particles) and dissolved phosphorus (in the water solution) interact with precipitation at the soil surface. Phosphorus generally does not move down into the soil profile with water, meaning most phosphorus losses occur at the soil surface. With more runoff on the surface (as discussed above), we can expect elevated phosphorus losses.

Figure 2 shows a higher amount of total phosphorus lost via surface runoff compared to tile flow. Total phosphorus losses from surface runoff had median concentrations approximately 11 times higher than in tile systems.

Nitrogen is more worrisome

Nitrogen is a larger concern in tile systems than in surface runoff (Figure 3). From data collected by both the Wisconsin and Minnesota Discovery Farms programs, over 90% of the nitrogen in tile systems is in the form of nitrate. Nitrogen in the form of nitrate is very mobile in water and moves easily through the soil profile. Excess nitrate can contribute to eutrophication in downstream water bodies, causing algal blooms.

Water quality considerations

Being aware of the condition of your tile drainage system as well as soil conditions are critical when it comes to limiting the water quality risks associated with tile systems. During wet periods of the year, such as early spring, check known tile systems to ensure they are functioning properly.

In older or deteriorating tile systems, tile blowouts can occur where a weak spot has opened. Eventually, the soil around this weak spot will collapse and the tile blowout can be identified from the soil surface (as shown in the photo). Being aware of the condition of your tile drainage system allows for timely repairs, as blowouts can act as a direct conduit for unfiltered surface water into tile systems.

Other considerations for tile systems focus around nutrient management decisions. The soil condition prior to nutrient application is a critical consideration to lower the water quality risks with tile drainage. Avoid nutrient applications during high soil moisture conditions when tiles are flowing. During these conditions, a liquid manure application would add to the soil moisture, causing nutrients to be more susceptible to move into the tile system.

On the other hand, avoid nutrient applications on excessively dry soils. If your soil has visible cracks on the surface, nutrients have a direct pathway into the tile system or could be flushed in with the next rain event.

Utilize visual field assessments and tools such as Wisconsin’s Manure Advisory System (www.manureadvisorysystem.wi.gov) to guide your decision-making process.

Soil test phosphorus (STP) levels can also be used as an indicator of water quality risks in agricultural systems. Looking at Figure 4, the STP levels in the top one to two inches of soil are well correlated with the dissolved phosphorus levels leaving the field in both tile systems and surface runoff. Control and maintain STP at optimum levels to reduce the amount of phosphorus leaving the field. continued on following page >>>

There are conservation practices that can also be included in a farming system to lower the water quality risks. Add cover crops, use the “4Rs” for nutrient applications (right rate, source, placement, and timing), and follow a nutrient management plan to further reduce losses.

At the edge-of-field, engineered treatment options can be added to intercept tile drainage to reduce nitrate concentrations. These include denitrification bioreactors, saturated buffers, and controlled drainage. On fields without tile systems, utilize grassed buffer strips or grassed waterways to limit soil and nutrients losses. 

IN FEBRUARY 2023, the Discovery Farms program lost an important member of their team. Eric Cooley, Director of Discovery Farms, passed away after a courageous battle with cancer. He will be remembered for his kindness, positivity, and passion for agricultural water quality, with tile drainage being a favorite interest of his. The Discovery Farms team is dedicated to honoring and building off the legacy Eric created for water quality monitoring and education.

The spring window for manure application can be unpredictable, and a delay in manure application can lead to a delay in planting. Rather than risking yield reductions due to late planting, a growing trend is to apply manure using a drag hose on newly planted corn or soybeans.

In a recent edition of the Buckeye Dairy News, Ohio State University extension field specialist Glen Arnold offered advice for in-season manure application. He shared that 10,000 to 12,000 gallons of dairy manure on newly planted crops will not hinder germination. In fact, it can actually provide additional moisture to encourage crop emergence. Arnold cautioned that the field must be firm enough to support the drag hose so that it does not create piles of dirt that deeply bury the seeds.

For corn, manure can be applied twice. The first application should be immediately after planting. The second application could take place a few weeks later, anytime up to the V4 stage of growth. Arnold noted that less damage will be caused by tractor tires when the corn is shorter.

A five-year study in Ohio showed that corn can be sidedressed or topdressed with liquid manure through the V4 stage without yield loss. Meanwhile, if corn was at the V5 growth stage, 60% of the plants broke off when manure was applied via a drag hose, and even though the plants regrew, they did not develop fully productive ears. For soybeans, it must be applied before emergence and preferably a few days within planting. Arnold indicated that once plants emerge, manure can kill the plants until they reach the V3 stage. At that point, they are strong enough to handle damage caused by the manure and the drag hose.

Manure can also be applied to growing wheat. While one application at 10,000 to 12,000 gallons per acre does not provide enough nitrogen to maximize grain yield, Arnold said it will spur crop growth and can lead to better wheatlage.

Arnold pointed out that fields where manure is left on the soil surface unincorporated will not capture as much nitrogen. Collect manure samples during the application process to keep track of what nutrients were applied.